B EMM’S SIXPEMX

LIBRARY, Mo. 7

NUTRITION U5 DIETETICS

By E. P. CATHCART

M.D., D.Sc., F.R.S.

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\ /J. /V. o ^?yVvC^*iVi DEAKIN UNIVERSITY LIBRARY

FROM THE COLLECTION OF

PROFESSOR W.G.K. DUNCAN

DATE DUE


NUTRITION AND DIETETICS

Our Food and the Uses we Make of It

By E. P. CATHCART

C.B.E., M.D., D.Sc., F.R.S.

Gardiner Professor of Physiological Chemistry in the University of Glasgow


iy53<*>S73

CONTENTS

CHAPTER

I. Introduction - -

-

PAGE

- 3

11.

Proteins - - -

-

- 6

hi.

Carbohydrates - -

-

- 20

IV.

Fats - - *

-

- 26

V.

Accessory Substances -

-

- 32

VI.

Salts - - *

-

- 40

VII.

Water - - -

-

- 51

VIII.

Quantitative Problems -

-

- 55

IX.

General - - -

-

- 72

Bibliography - -

-

- 80

First -published 1928

NUTRITION AND DIETETICS

CHAPTER I

INTRODUCTION

Man literally “ is an edifice of viands erected by his teeth,” and man as a whole is intensely interested in the nature of the building material which he passes through his dental mill. The majority of men further do not hesitate to lay down the law on dietetics—how the consumption of this article of food is right and proper and the consumption of that article is little less than suicidal. And yet with all there is much ignorance of the fundamental facts which lie behind the science of nutrition, if one can venture to call nutrition a science when so much yet remains obscure.

In order that life may proceed, it is essential that the organism receive an adequate supply of nutriment. The organism differs from the man-made machine in many ways, but perhaps the most marked of all differences is that the fuel supplied for the generation of energy in the machine is solely of use as a potential supply of energy, whereas in the case of the living organism the material supplied as food serves not only as a source of energy, but as material which can be utilised by the organism for growth and the repair of tissue wear and tear. It is essential, then, that attention should be given, not only to the quantity, but to the quality of the material supplied.

4 NUTRITION AND DIETETICS The demand for food material by the living organism is unceasing, although the amount required during the course of the twenty-four hours varies with the demands on the organism in the form mainly of external muscle work. The workshops of the living body—the cells—never shut down night and day, year in, year out; so long as life lasts the activities of the living cell, although they may vary in intensity, never cease. This endless activity, which characterises the protoplasm of the living cell, may be called the manifestation of life itself. The whole of man’s well-being, of his fitness as a member of the race, is solely dependent on the balance of these cellular activities. And what is the nature of this all-important living protoplasm? We do not know. All we can venture to say is that it is a complex substance in which protein is a prominent ingredient. In the words of Sir Michael Foster, “ He (the biologist) may speak of protoplasm as a complex substance, but he must strive to realise that what he means is that it is a complex whirl, an intricate dance, of which what he calls chemical composition, histological structure and gross configuration are, so to speak, the figures; to him the renewal of protoplasm is but the continuance of the dance, its functions and actions the transference of figures.” In other words, when we speak of cellular activity and all that it connotes, we must realise that we have to deal with a dynamic, and not a static, state of affairs. The changes which are constantly taking place in this complex of protoplasmic activity are grouped together under the general title of metabolism (from the Greek metabole— change). As the name indicates, it simply means exchange. Physiologists have, however, quite properly divided up these metabolic changes into two phases— that of building up and of breaking down, into anabolism and catabolism. The struggle between these two phases is constant throughout life; at one moment the anabolic phase is in the ascendant and the next the

catabolic. There is a constant striving towards an equilibrium which is never attained. During the early years, in which growth is a prominent feature, the anabolic phase is most pronounced, but, as the years go on, this ascendancy becomes less and less marked.

But physiologists proceed even further in their analysis, and speak of the metabolism of energy and the metabolism of matter. This dichotomisation into energy and matter, although it may be most convenient for purposes of study and description, is a purely fictitious separation, as it is impossible, in fact, to differentiate between the two varieties. The metabolism of energy cannot take place, so far as we know, without an accompanying metabolism of matter, nor can the reverse process occur. And yet, for purposes of treatment of the subject, it has been found advantageous to adhere to this separation and to discuss the problem of nutrition separately from its qualitative and quantitative aspects, which, in practice, is roughly the corw sideration of the problem from its material and energetic aspects. We must, before proceeding to the finer division of the subject, consider briefly the nature of the materials which compose the ordinary diet.

The average diet, if it is to fulfil all its functions, is a mixture of a number of foodstuffs.

It will be found, however, if these various materials which are utilised as foods be analysed chemically, that, despite the wide variety of origins, the majority can be reduced to a matter of six components, which have been termed the proximate principles. These are :

A.

Proteins

Carbohydrates

Fats


B.

Accessory substances

Salts

Water.

It will be noted that these principles have been arranged in two groups, not, however, because they vary in value as components of the dietary as a whole.

Group A—protein, carbohydrate and fat—may be regarded as the fuel stuffs, the sources of energy for the organism, whereas Group B, although they may play an intimate part in the utilisation of energy from the fuel stuffs, play no part, as far as we are aware, in the actual supply of energy.

In the following chapters the nature and the value of the various essential constituents of a diet will be briefly considered, first from the qualitative and then from the quantitative aspect. It may be stated now that it is impossible to dogmatise on the subject of dietetics; we know much—sufficient, perhaps, to enunciate general principles—but not enough to lay down hard and fast rules. The “ factors of safety ” of the body, so far as food is concerned, would seem to be extraordinarily high.

CHAPTER II PROTEINS

4'

The proteins are a series of substances of very varying composition, but they are all characterised by containing within the molecule, nitrogen—many also contain sulphur and some phosphorus—in addition to carbon, hydrogen, and oxygen. They occur in all living matter and are well exemplified in lean meat, blood serum, and white of egg. Although the examples cited are all drawn from the animal kingdom, vegetable materials also contain proteins.

The proteins are not simple substances of uniform composition; there are many varieties of proteins, and the different tissues of the body vary both in the amount and the nature of their protein content.

The molecule of protein is a very complex one, and, when decomposed by chemical treatment, usually by boiling with acid, it yields a series of simpler chemical bodies known as the amino acids. These amino acids are derivatives, for the most part, of simple fatty acids, in which one or more of the hydrogen atoms in the chain are replaced by the amino (NH2) group. As an example, we may cite one of the simplest of the amino acids, alanine, or, to give it its full chemical name, aminopropionic acid. Propionic acid itself—a fatty acid—has the formula

H H O

I    I    S

H—C—C—C

i    I    \

H H    OH

When one of the hydrogen atoms is replaced by the amino group, thus—

H NH, O

I    I    ^

H—C—C-C

!    I    \

H H    OH

aminopropionic acid, or alanine, is formed. Some of the amino acids—and they would seem to be amongst the most essential—are more complicated, as they contain the benzene ring in their molecule, and hence they are known as aromatic amino acids.

The number of amino acids which are contained in a molecule of protein varies, ranging from only four in the simplest type of protein to seventeen or eighteen in the most complex. This variation in structure is not only of interest from the chemical aspect, but it plays a very large part in the variation in value of the various proteins as foodstuffs. The proteins, from a nutritional point of view, vary markedly in value. It would seem that we could divide the proteins roughly into three classes: (a) perfect, (b) imperfect, (c) deficient. The perfect proteins may be classed as those which contain all or most of the amino acids in such proportion as to suffice for nutrition without necessitating the consumption of excessive amounts. The imperfect proteins are those in which there is a deficiency, but not an absolute lack, of one or more of the essential amino acids, or it may be even only an ill-balance of the amino acids. The deficient proteins are those in which some of the essential amino acids are completely missing. The differences of which we have been speaking might be graphically represented as follows, where the different letters of the alphabet represent different amino acids:


PROTEIN MOL.

PROtEIN moL.

prot IN MOL.

It is obvious that by the ingestion of large amounts of the imperfect protein the deficiency in the amount of amino acid “ T ” might be made good, with, however, at the same time some waste of the increased amounts of the ingested amino acids represented by the letters P, R, O, E, I, N, M, O, L, whereas in the case of the deficient protein no increased ingestion could ever make good the deficiency of “ E.”

Before the average protein can be utilised as a food by the body, it must undergo digestion.

Table I.

PARTIAL PERCENTAGE COMPARISON OF THE AMINO ACID CONTENT OF VARIOUS PROTEINS

_ V Ni

-“I

Gliadin

Wheat.

Gelatin.

Casein* ogcn of Milk.

Milk

Albumin.

Fish

Muscle

Protein.

Chicken

Muscle

Protein.

Ox Muscle Protein.

Glycine ..

O

0

25*5

°*5

o'4

0*0

0 7

2‘X

Alanine ..

I3'3

2*0

8-7

i*9

2‘4

+?

2'3

37

Leucine ..

19*6

6-6

71

97

14*0

10*3

11'2

xi 7

Glutamic Acid

26’2

437

S'?

2*8

12'9

io’i

16*5

15*5

'lyrosine ..

3*6

3‘5

O’OI

6*5

t *9

2*4

2*2

2 *2

Histidine ..

o’8

3‘4

0*9

2*8

2’6

2‘6

2‘5

i*8

Lysine ..

0

°’9

5'9

7-6

9*9

7*5

7*2

7’6

Tryptophan

0

X'l

0

2*2

+

+

+

+

Digestion simply means that the insoluble complex protein as it exists in food, especially cooked food, must be broken down into a relatively simple soluble form by the action of different ferments, or enzymes, which are secreted into the stomach and the small intestine. The process of breakdown is not, so to speak, of an explosive nature, where the protein complex is immediately disrupted to its constituent amino acids. It is, on the contrary, relatively slow, and it passes through a series of stages. The ferments which are responsible for this breakdown of the protein molecule are three in number : (i) pepsin, which is secreted by certain glands in the stomach; (2) trypsin, which, formed in the pancreas, is poured into the small intestine; and (3) erepsin, which is secreted by certain glands in the small intestine itself.

By proper mastication the ingested protein is reduced to smaller fragments, which permit of more rapid attack by the ferment pepsin. Acting in the presence of the hydrochloric acid of the gastric juice, the pepsin converts the insoluble protein into a soluble form; it is then further broken down to substances called proteoses, which, in part at least, undergo a further partial disintegration into peptone. At this stage, under normal conditions, digestion in the stomach ceases, and there is passed on from the stomach to the small intestine, in small amounts at a time, the acid chyme which is a mixture of proteoses and peptone.

When the acid chyme reaches the intestine, its acidity is neutralised by the alkaline intestinal secretions, so that the trypsin, and eventually the erepsin, may have the optimal conditions necessary for the further splitting of the proteoses and peptone. These substances are converted, first, into what is called polypeptide form—that is, into substances which are compounds of two or more amino acids—and, finally, the majority, although not all, of these polypeptides are converted into the simple amino acid form.

Although this outline gives a very abbreviated account of the course of the breakdown of protein, such as would appear normally to take place during digestion, it is not to be inferred that all protein must of necessity be reduced to the amino acid form before it can be utilised by the body, as there is very good evidence that, no matter how long digestion by means of the ordinary ferments goes on, there is always a certain amount of the protein left in polypeptide form. Further, there is evidence that, under certain conditions, absorption of undigested protein—egg albumen and serum, for example—can occur. But there is no evidence which would suggest that this absorption of unchanged protein takes place normally. The majority of investigators to-day believe that the main absorption of protein is in the form of amino acids. The absorption of the amino acids from the lumen of the intestine takes place into the small bloodvessels which surround the walls of the intestine. The blood, thus laden, passes into what is called the main portal bloodstream, and is therefore conveyed, in the first instance, to the liver. The blood, still rich in amino acids, then passes on to the heart and is thus eventually sent to every tissue in the body. The amino acids are then taken up from the blood by the various tissues according to their several needs. A certain number of these amino acids are required for the repair of tissue wear and tear, but the great bulk of the ingested amino acids sooner or later undergo, in their turn, decomposition. They are de-aminised—that is, the amino group NH2 is removed from the fatty acids. The amino group would seem, for the most part, to be converted into an ammonium compound which is very toxic to the organism, and is therefore rapidly converted, in its turn, into an innocuous compound called urea. This formation of urea apparently takes place chiefly in the liver. The urea, in its turn, is conveyed by the bloodstream to the kidney, and is there excreted. The deaminised fatty acid probably undergoes various changes, and its energy is eventually liberated by its complete combustion. Again, although this account gives the commonly accepted course of events under normal conditions, it cannot be held either to be a complete or, perhaps, even a true picture of the different phases of the metabolism of protein. It is true it may roughly represent the catabolic aspect, but it leaves the infinitely more complex anabolic phase quite untouched.

Modern research has shown, however, that when the amino acid has undergone deaminisation it does not of necessity follow that the nitrogen-free residue undergoes immediate combustion. Indeed, there is very good evidence that under appropriate experimental conditions this nitrogen-free residue may he converted into sugar. There is, on the other hand, little or no evidence available which permits us to say definitely what happens, after deaminisation, to the residue in the normal organism where the supply of food is sufficient for the immediate needs.

Although stress has rightly been laid on the formation of urea as an end-product of the metabolism of protein—it forms under normal conditions about 80 per cent, of the nitrogen excreted in the urine—other nitrogenous compounds are also formed. As the result, for example, of the metabolism of one of the conjugated proteins—nucleo-protein—although part of the nitrogen may ultimately be converted into urea and be so excreted, another part is converted into what are known as the purin bodies, and appears, for the most part, in the urine as uric acid.

If our knowledge of the breakdown, or catabolism, of protein is very imperfect, the information available about the building up, or anabolic, processes is, to say the least of it, but fragmentary. We possess a few clues, but no certain knowledge. It is, of course, most obvious that the anabolic phase is even more important than the catabolic, but our means available for attacking the problem are not very effective. We know that if a person be fit and well he maintains his weight and his strength; we know further that if he has suffered from some exhausting disease and has become emaciated, during convalescence he not only puts on weight, but his muscular power returns. We are also well aware of the fact that if a man goes into training for some athletic event and takes systematic exercise, he not only becomes capable of greater muscular effort, but his muscles actually increase in bulk. We have, then, good evidence that under certain conditions, at least, the body has the power of increasing the anabolic phase of protein metabolism.

The problem looks a simple one, but, as a matter of fact, it has proved one of the real conundrums of metabolism. One might have argued that if protein retention takes place, the fact would be indicated by the excretion of nitrogen being less than the intake, that if, for example, 15 grams of nitrogen had been ingested in the form of protein, less than this amount would be excreted in the urine in the form of urea and other end-products of protein metabolism. It is true that under certain conditions this is the case, but, curiously enough, neither is the diminished output of nitrogen very marked when anabolism might be presumed to be active, nor is an increased output a very conspicuous-feature when, on the other hand, catabolism might be presumed to be the predominant phase of metabolism. The whole tendency of the intake and output is to balance, in other words, to enter into a state of nitrogen equilibrium. This means that if the amount of nitrogen ingested in the form of protein be carefully determined and the intake kept constant, and if the output from day to day in the form of end-products be equally carefully estimated over a period of days or weeks, it will be found that the output just equals the intake. It might be presumed, then, that the nitrogen excreted was the product of the nitrogen ingested in the form of protein, that ingested protein after absorption into the tissues is immediately dealt with and its nitrogen, apparently so much waste matter, at once excreted. Or, of course, it might be assumed alternatively that the nitrogen excreted represents so much effete tissue nitrogen, the place of which in the organism is taken by the freshly ingested protein nitrogen. The explanation of this very puzzling phenomenon of nitrogen equilibrium is not known, but it can be asserted that the nitrogen excreted in any twenty-four hours is not wholly the product of the protein nitrogen ingested during this period.

The debate on what actually occurs in the organism when food, not protein alone, is ingested, has been of long standing, and formerly led to much acute polemic. The point at issue is simply whether, when such a foodstuff as protein is ingested, it becomes an integral part of the living protoplasm of the cell before metabolism can take place, or whether the cells can bring about the metabolism without first incorporating the food material. The point at issue may be, perhaps, better illustrated by the use of a simile. If we assume that the active living cell mass be represented by the flame of a jet of gas in a gas-fire, we may represent the enhanced combustion by adding finely vaporised oil to the gas before actual combustion or by spraying finely divided oil on to the lit flame. Although the available evidence is very faulty, it would appear that the majority of investigators incline to the view that the food material need not become an integral part of the living protoplasm before metabolism occurs.

Whatever be the explanation of nitrogen equilibrium, the evidence is in favour of the view that within a given period of time the nitrogen excreted does not wholly represent the metabolism of the protein ingested within that time. It has been shown beyond all manner of doubt that if, when a subject is in a condition of nitrogen equilibrium, there be added on a single day an increased amount of nitrogen in the form of protein, this extra ingested nitrogen is not all excreted on the day of ingestion. The extra output is spread over a period of time, according to the nature of the protein taken in, varying from three to six days.

Further, if, when a subject is in a state of nitrogen equilibrium with an intake, say, of io grams of nitrogen, the intake be suddenly increased to 15 grams, the output does not rise to a corresponding height on the first day. The new level of nitrogen equilibrium is only reached and maintained after three or four days. The reverse action takes place in nitrogen equilibrium when the nitrogen intake has been definitely lowered. All three types of experiment point in the same direction—namely, that there is always a period of lag in adjustment. This means, of course, that there is a certain capacity—it may be, and probably is, only temporary—for storage of protein or protein degradation products in the tissues. The astonishing thing is that the capacity for storage is so limited.

As regards the form in which such storage takes place, it is difficult to find experiments which will give irrefutable evidence. Many investigators believe that the storage, such as it is, occurs in the form in which the bulk, at least, of the nitrogen seems to be absorbed from the lumen of the intestine—viz., as amino acids. Certain experimental work would seem to point in this direction, but, on the other hand, there is a considerable body of evidence which indicates that the material retained is in a more complex form than amino acids.

It was stated earlier that the majority of proteins contain sulphur in the molecule in addition to nitrogen. This sulphur compound, just like the nitrogen, undergoes metabolism, and, as the result, it appears in the urine in certain well-defined and analysable forms. Indeed, when occasion arises, the metabolism of protein may be followed almost equally conclusively by the study of the sulphur intake and output. When the intakes and outputs of both nitrogen and sulphur are determined, a certain amount of light is thrown, by a study of the ratio of the two outputs, on the actual course of metabolism within the living cells. It has been found, for instance, in those experiments in which an extra amount of protein has been superimposed for a single day on a subject in nitrogen equilibrium, that the rate of excretion of the sulphur does not synchronise with that of nitrogen. This means, of course, that the N: S ratio in the urine is disturbed. It has been found from repeated experiment that the output of sulphur precedes the output of nitrogen, and one may infer that there is some selective activity shown either in the rate of the catabolism of the protein or its component parts, or in a difference in the capacity for retention. One might not, perhaps, be able to maintain that such a disturbance of the nitrogen : sulphur ratio was sufficiently notable to refute the hypothesis that retention takes place in the form of amino acids. But if this partial study of the N: S ratio be, so to speak, completed by an investigation of the effects on the ratio when anabolism, and not catabolism, is in the ascendant, further light is thrown on the problem. In this case it is found that the retention of sulphur precedes the retention of nitrogen. If tissue selectivity of individual amino acids were the answer to our problem, then we are faced with the paradoxical position that this selectivity varies with the phase of metabolism in operation. On the other hand, the theory, that the retention occurs in some complex form in which the sulphur-containing substance is the nucleus, is difficult to accept, in view of the fact that during the process of catabolism the output of sulphur precedes that of nitrogen. Such evidence as is available would seem to point to the fact that the retained material is in a more complex form than a simple amino acid. For instance, if, instead of superimposing protein during a period of nitrogen equilibrium, one of the simple amino acids be utilised, it is found that there is practically no retention of nitrogen.—that is, nitrogen in equivalent amount to that ingested in amino acid form appears in the urine within twenty-four hours.

Arising out of this problem, the question may well be asked : Does the nature of the ingested protein influence in any way the nature of the tissue protein? Several attacks have been made on this problem, many of them of a most ingenious nature. In one of the most conclusive of these experiments, where active regeneration of blood protein was known to be taking place, the sole protein present in the diet was of peculiar chemical composition—viz., gliadin, the chief protein obtained from flour, which is very rich in the amino acid, glutamic acid. Gliadin contains about 40 per cent, of this acid, whereas the blood proteins only contain about 8 per cent. The experiment showed most conclusively that although regeneration of the blood proteins took place, feeding with gliadin had not modified their composition in any way.

Other experiments with the same end in view, but carried out in different ways, have led to the general conclusion that irrespective of the nature of the protein fed, provided it contains the requisite amino acid, no matter in what proportion, the protein formed in the tissues is the protein proper to the particular animal. There may, it is true, be some temporary disturbance of balance, but the end result seems to be beyond all question.

There is another important fact which would seem also to be beyond all doubt—namely, that with the exception of the simplest of all amino acids, glycine or aminoacetic acid, and perhaps of the one which stands next to it, alanine or aminopropionic acid, the amino acids required by the organism must be supplied preformed in the protein fed. This means, of course, that the higher organisms have, in practice, little capacity for synthesis of amino acids. Hence, it follows that deficient proteins—that is, proteins which lack completely some of the essential amino acids, can never be utilised as the sole source of nitrogen for the tissues. We know, for example, that when certain of the amino acids are not present, life cannot go on, but there is either no evidence at all, or evidence which is difficult of interpretation, in the case of other amino acids. Many experiments have been carried out in the attempt to illuminate this most interesting field. The following experiment of Mendel and Osborne is one of the most illuminating. Hopkins had shown that the protein obtained from maize, zein, which was known to be defective as regards its content of aromatic amino acids, was useless as a sole source of protein, but that if its deficiencies were made good, it would suffice. Mendel and Osborne examined the problem further, and in the end showed most conclusively that if certain deficiencies were made good, not only might life be maintained, but growth would actually occur. Two amino acids were proved to be involved, one 18 NUTRITION AND DIETETICS called tryptophan and the other lysine. They showed that if a diet were designed, perfect in every other way except that the sole protein was in the form of zein, rats fed on the mixture would die unless the diet were changed. When a certain amount of tryptophan was added in addition to the zein, rats fed on the mixture did not die; they continued to live, but did not grow. When, in a third series of feeding experiments, they added still another of the amino acids missing from zein —viz., lysine, the rats fed with the mixture containing zein + tryptophan + lysine not only lived, but grew.

Zein + Tryptophan + Lysine.

Death.

Maintenance.

___________—-^

Growth.

Other experiments were carried out with other proteins which were deficient in still other amino acids, and results in every way similar to that detailed above were obtained.

Another type of experiment, in which one at least of the essential amino acids—tryptophan—was destroyed by prolonged boiling of the protein with a strong mineral acid, led to tne same conclusion. It was found that when an acid-hydrolysed protein, suitably prepared, was used as the sole source of nitrogen in a diet, life could not be maintained, whereas a protein hydrolysed or completely digested by means of a ferment provided a perfectly good substitute for protein in the food. Examination showed that the acid-digested protein solution contained no tryptophan. When a proper amount of this amino acid was added to the acid digest it became an effective substitute for protein, and if tryptophan were removed from the ferment digest and the residue utilised as the foodstuff, it was found to be no longer an effective substitute for protein.

The evidence, then, that tryptophan must be regarded as one of the really essential amino acids is beyond all question.

One of the interesting and astonishing results of the series of experiments of Mendel and Osborne was that although, as a result of feeding rats on a defective diet, growth might cease, yet the capacity to grow remained latent and apparently unimpaired even after over 500 days—that is, after more than three-quarters of the average duration of life of a rat. It is very evident that the “ hereditary inertia ” of the living protoplasm is a most important factor in the regulation of the cellular activity of the organism.

Another question which may well be asked is whether the organism can obtain any of its necessary nitrogen and sulphur in forms other than complex proteins or, ultimately, amino acids. So far as the higher mammals like man are concerned, there is no satisfactory evidence that any nitrogen ingested in forms other than the amino grouping can be utilised by the tissues. In the elemental forms of life, like yeast, there is no doubt that the cells can utilise nitrogen obtained from inorganic sources, and from this source build up protein material. But as life becomes complex, this capacity of using elementary forms of nitrogen seems to disappear. The extraordinary thing is, that although the element nitrogen forms about 80 per cent, of the atmosphere, it is not available in this form as a source of building material. It is true that plants like the leguminosa: have the capacity of utilising elemental nitrogen, but even they do it at second hand through the agency of bacteria, which form nodules on the root basis of the plant. The hosts accommodate the bacteria but ultimately make them pay in full; the bacteria, which have built up complex compounds, utilising the elemental nitrogen, are destroyed, and the plant utilises the bacterial products for the building of its own protein material.

It would also seem to be demonstrated that sulphur, which is to be utilised for building purposes, must also, for the most part at least, be supplied in complex form.

The evidence available thus points to the fact that the organism is very selective, indeed limited, in the choice of the material which is to be utilised for building up of tissue, and thus the source of its nitrogen supplies is more or less stricdy limited to protein. There is also the evidence that proteins, although in many respects they may bear a strong family likeness, differ very materially in structure. There is good reason to believe that the differences between proteins derived from even closely allied species are much finer than a mere variation in content of amino acids. When it was shown many years ago, as the result of very careful analysis, that the chemical composition of the principal protein of milk—caseinogen—obtained from a wide variety of animals agreed so closely, it was generally assumed that caseinogen, no matter what its source, was a substance of uniform composition. More recent work has, however, shown quite conclusively that, although the ordinary chemical analytical data agree perfectly, there are undoubtedly well-marked intra-molecular differences of structure which differentiate caseinogens of varying origin. Similar differences have been found between the “ white ” (albumin) of hens’ or ducks’ eggs. (See also p. 68, Chapter VIII.)

CHAPTER III
CARBOHYDRATES

The carbohydrates comprise the group of sugars and starches, and are relatively simple compounds of carbon, hydrogen, and oxygen. So far as nutrition goes, these substances are of primary importance as they form the great bulk of the average diet. Although, of course, a certain amount of carbohydrate is required for repair, the main purpose is to supply energy—that is, to act as a fuel.

Except for a very small fraction which is present in meat and other products of animal origin, the great bulk of the carbohydrate consumed is derived from vegetable sources, either in the form of starch, as in flour and kindred materials, or in the form of sugars of various kinds. The starches are chemically referred to as polysaccharides, as the starch molecule is a compound of a large and unknown number of the simplest units known as monosaccharides. There are three of these monosaccharides, which are of importance from a physiological standpoint—viz., dextrose (or glucose or grape sugar), galactose, and laevulose (or fruit sugar or fructose). The first is widely distributed in nature, being found in the seeds, leaves, and other parts of plants, and, along with laevulose, it is found in sweet fruits and honey. The second, galactose, occurs in combination with dextrose as sugar of milk in ordinary milk, and the third, laevulose, as already stated, is found, along with dextrose, in sweet fruits and honey. There are also three double sugars—that is, compounds of two monosaccharides—of physiological importance. They are cane sugar, lactose or milk sugar, and maltose. Cane sugar is found distributed very widely in the vegetable kingdom, as in sugar cane, beetroot, sweet fruits, bananas, pineapple, etc. It is a compound of one unit of dextrose and one of laevulose. Lactose, or milk sugar, which is a compound of dextrose and galactose, is found in the milk of all animals to the extent, for instance, of about 5 per cent, in cows milk, and between 6 and 7 per cent, in human milk. So far it has not been found in plants. Maltose, which is a compound of two units of dextrose, is found in plants, and is formed, for example, in considerable amounts during the germination of barley. Inter-22 NUTRITION AND DIETETICS mediate between maltose and the complex starches is a series of substances known as dextrins.

As regards the digestion of carbohydrates, it is a question simply of the conversion of the complex polysaccharides into monosaccharides. So far as the starches are concerned, this digestion is commenced in the mouth, as the saliva, with which the food is mixed on mastication, contains a starch-digesting ferment—a diastase—which converts starch into dextrose and maltose. As a matter of fact, although the diastase is secreted in the saliva and mixed with the starchy food in the mouth, little or no digestion of the starch actually takes place there. The ferment exercises its main activity after the food reaches the stomach. Digestion proceeds in the stomach until the food mass is thoroughly permeated with the acid gastric juice. When this has happened, the action of the diastase ceases. There is no ferment capable of digesting carbohydrates secreted in the stomach, although it is possible that the acid of the gastric juice may account for a small further breakdown, as carbohydrates are readily hydrolysed by mineral acids. In due course the stomach contents pass on into the small intestine, and here they meet with a series of other ferments, which eventually bring about the complete reduction of the various carbohydrates to monosaccharide form. The pancreatic juice secretes a diastase which converts any unchanged starch or dextrin into maltose, and formed in the intestine are ferments which convert the disaccharides into their constituent monosaccharides, cane sugar into dextrose and lævulose, lactose into dextrose and galactose, and, finally, maltose into two molecules of dextrose.

Absorption of these simple monosaccharides takes place, as in the case of protein, for the most part into the bloodstream, and, as in the case of protein, the absorbed material first passes to the liver. ^ certain amount of the carbohydrate, if need be, may be utilised throughout the body, as there must always be a small

amount of dextrose in the blood, and this is kept wonderfully constant, about 0 • 1 per cent. As the result of the ingestion of carbohydrate, particularly in the form of dextrose, there is shortly afterwards an increase in the demand for oxygen from the inspired air, and a still greater increase in the output of the waste gas, carbon dioxide in the air expired from the lungs. (See The Body in this Series.) The ratio between the amount of oxygen (02) utilised and the output of carbon dioxide (C02) cjiffers from that which exists when the organism is without food. The study of the changes in the C02/02 ratio, which is also called the respiratory quotient, has suggested to some researchers that much of the carbohydrate which has been taken in has undergone combustion. Such an inference is probably not wholly correct. It is believed that the alteration in the respiratory quotient gives an index of many other metabolic changes besides that of simple combustion. It is probable that some of the carbohydrate is at once utilised, but the rest is built up again either into animal starch or glycogen, and is in part stored in the liver and in part in the muscles. Another part of the absorbed sugar is converted into fat and is stored in this form. If an excessive amount of sugar has been given in a single dose, absorption from the intestine is not inhibited, with the result that too much sugar circulates in the body and neither the glycogen nor the fat storage methods can cope with it. When such an accident occurs, the sugar, in spite of its nutritive value, is treated as a foreign and dangerous material, and is carried by the blood to the kidney and excreted. In the case of the carbohydrate which undergoes combustion, as it is built up out of carbon, hydrogen, and oxygen, there are no specific end products for excretion by way of the urine. The combustion is complete, the end products being carbon dioxide and water.

As stated at the outset, carbohydrate is commonly regarded as the fuel par excellence of the body. It is

certainly the fuelstufi which is most readily available. The work of Hill, Meyerhof, and others has rendered it very certain that carbohydrate, possibly in the form of glycogen, is to be regarded as the immediate source of energy for the contraction of muscle. But the importance of carbohydrate for nutrition is not solely confined to its capacity for supplying an easily available form of energy. It plays an intimate role in many, if not all, of the metabolic processes which occur in the organism. It is, for instance, very intimately associated both with the metabolism of protein and of fat. In the case of protein, it can be shown to be most active in sparing tissue breakdown. It has been found, for instance, that if a man’s diet be confined solely to carbohydrate, the excretion of nitrogen in the urine falls to a very low level. Further, it has been shown that the retention of nitrogen is most marked when carbohydrate is present in the diet in sufficient amount. There is no unequivocal evidence which permits of a definite statement as to the exact relationship between the metabolism of protein and carbohydrate. The evidence of the influence of the carbohydrate on metabolism is not, however, confined merely to its effects on the total output of nitrogen. .It has been shown quite conclusively, when the output of the various end products is considered, that the influence of carbohydrate on metabolism is a much more intimate one than the mere study of the total nitrogen output would indicate. For instance, as already stated, on a normal mixed diet, urea nitrogen forms about 80 per cent, of the total nitrogen excreted, but in some experiments, where carbohydrate predominated in the diet, the output of urea formed only about 40 per cent, of the total nitrogen. The output of uric acid is also definitely influenced by the amount of carbohydrate present in the diet.

In the case of the influence of carbohydrate on the metabolism of fat, the results are much more dramatic.

If carbohydrate be removed completely, or almost completely, from the diet, or if the body cannot metabolise carbonydrate, as is the case in the disease known as diabetes, a condition known as acidosis ensues. In acidosis, the combustion of fats is faulty; they are not converted during metabolism, as they should be, into carbon dioxide and water; the metabolism ceases before these end products are reached. The result is that certain abnormal acid products circulate in the body, although in part excreted, which bring about further disorganisation in the already overtaxed tissues, a disorganisation which may lead to fatal results unless dealt with. When a condition of acidosis has been induced experimentally by the withholding of carbohydrate, it can be immediately caused to disappear by the addition of a comparatively small amount of carbohydrate, preferably in the form of a simple sugar, to the diet. In the case of diabetes, the object of using insulin, obtained from the pancreas, is to enable the body to utilise carbohydrate. In diabetes, it is not a case of the absence of carbohydrate which is responsible for the acidosis, but, owing to some metabolic disturbance connected with the lack of the active secretion—insulin—of the pancreas, the tissues can no longer utilise carbohydrate, even when present.

The question may well be asked : How, then, do peoples maintain their health and strength, who, like the Eskimo and others, live in countries where grain and other carbohydrate-producing plants are not available as sources of supply? It is quite true that all observers agree that the diet of the Eskimo, judged on other dietary standards, is very deficient in carbohydrate, and yet, as a race, they are well nourished, healthy, and capable of standing great physical strain and effort. So far as observations go, the Eskimos do, in the summer, obtain a certain amount of carbohydrate from berries and mosses, but mostly they have to rely on the animal starch-glycogen which is present

26 NUTRITION AND DIETETICS in the skin of young whales and in the liver, muscles and blood (as sugar) of the animals they kill for food. There is a certain amount of evidence, as already stated (p. n), that, when required, the tissues can convert a certain amount of the deaminised residue from amino acids into sugar. It is possible that this is a normal event in the metabolism of the Eskimo, and certainly it has been shown that only a relatively small amount, as compared with the total intake of foodstuff, of carbohydrate need be present to inhibit the genesis of acidosis.

CHAPTER IV
FATS

The ordinary fats are a group of substances which, like the carbohydrates, are built up out of carbon, hydrogen, and oxygen, but the oxygen percentage present is much smaller than that in carbohydrate, and the hydrogen and oxygen present are not in the proportion to form water. The molecule formed is, however, much larger. They are compounds of certain of the higher fatty acids, more particularly of palmitic, stearic, and oleic acids, with glycerol (glycerine). The consistency of the various fats or oil depends simply on the particular admixture, as two of the fats, palmitin and stearin, are hard solids, whereas the remaining one, olein, is fluid. Hence it follows that hard tissue fats, like the tallow obtained from beef and mutton, consist mainly of palmitin and stearin, whereas lard and human fat, which are much softer, contain more olein. Oils like olive oil, for instance, contain chiefly olein. Fats which are utilised as foods are drawn from both animal and vegetable sources, although in the average mixed diet most of the fat must be of animal origin. Fats are very widely

distributed in Nature, both among plants and animals. In plants, although they are, perhaps, found in largest amounts amongst the seeds, they are also components of roots and fruits, whilst in the animal kingdom, in addition to the fat laid down in ordinary adipose tissue, every individual tissue and organ contains fat in varying amount.

In addition to the ordinary fats, a series of interesting substances, which form important components of living cells, is known to exist. These substances are grouped together under the title of the lipoids, or lipins. Two well-marked groups of these bodies have been differentiated—the lecithins, phospholipines or phosphatides, and the cerebrosides or galactolipines. The first group, the lecithins, which seem to exist in the cell in some form of loose combination with the proteins, contain, in addition to their fat nucleus, phosphoric acid and a basic nitrogen-containing substance called choline—i.e., lecithin is a fatty substance containing both phosphorus and nitrogen. Egg yolk contains about 10 per cent, of this phosphorised fat, and liver and blood about 2 per cent. The other group, the cerebrosides which contain nitrogen but no phosphorus, are found mainly in brain and nervous tissue, although they may be present in small amount in other tissues. Closely allied to these substances is another—cholesterol—which seems to be present in varying amount in all cells, and is particularly abundant in nervous tissues. This substance, whose function is unknown, has in recent years assumed a position of considerable interest, if not of importance, in connection with the most recent work on vitamins, to which reference will be made later.

The course of digestion of fat may again be reduced to the simple statement that the process resolves itself into the splitting of the neutral fat into its constituent fatty acids and glycerol. The fats are neither attacked in the mouth nor normally in the stomach. When they 28 NUTRITION AND DIETETICS reach the small intestine they come into contact with a fat-splitting enzyme—lipase—which is secreted both by tne pancreas and the glands of the small intestine. The bile, which is formed in the liver and poured out into the intestine, although it contains no ferment, assists both in the action of the lipase and, acting as a solvent, in the subsequent absorption of fat. After the neutral fat is broken down into its constituent fatty acids and glycerol, both components are absorbed, the glycerol as glycerol and the fatty acids probably partly in the form of fatty acids and partly in the form of a soap—i.e., fatty acid combined with a basic substance like sodium or potassium (such basic substances are present in quite marked amounts in the alkaline secretions both of the pancreas and the intestine). Very soon after absorption into the villi which line the intestinal canal (see The Body, p. 22), the fatty acids and glycerol reunite to form neutral fat again. This neutral fat passes on, not, like the protein and carbohydrate digest products, into the bloodstream, but into the series of lymphatic channels called the lacteals. These lactcals eventually lead back to the main lymphatic channel, the thoracic duct, which opens finally into a large vein at the base of the neck. In this way the absorbed fat passes direct into the bloodstream, the liver being avoided. It is a curious and interesting fact that, chemically speaking, the most inert of the three proximate principles sidetracks the liver, whilst the other two labile series of products have to run the gauntlet of this, in the metabolic sense, remarkably active organ.

The question as to why neutral fat in the intestine should apparently be completely split into its constituent fatty acids and glycerol, and why immediately after absorption into the epithelial cells of the villi they should be resynthetised into neutral fat again, is both interesting and intriguing. Part of the explanation may be that the necessity of preliminary splitting will prevent the absorption of other substances which have a physical similarity to fats and oils, substances like paraffin oil, petroleum, vaseline, etc., and which, although they may become finely emulsified in the intestine, cannot be split by lipase into substances capable of absorption. Such substances as those mentioned have neither food nor fuel value in the organism. Another explanation may be that the preliminary splitting may allow of a certain amount of alteration in the structure of the absorbed material, a function of considerable value where the fats of the food, of very varying sources of origin, have eventually to be converted into a type of fat peculiar to the specific organism. T here is a certain amount of evidence which would support the hypothesis that an alteration of chemical structure takes place. There is good evidence, in the first place, that if glycerol be deficient in amount it can be made good by the organism; thus, when fatty acids themselves were fed in place of neutral fat, it was not the fatty acids alone which were found in the lacteals, but neutral fat. It has also been shown that when a hard fat like mutton fat, with a high melting-point, was fed, the fat absorbed into the lacteals was softer—i.e., it had a lower melting-point. There is some evidence, then, that olein can be added during the process of resynthesis in the intestine.

It is, however, not to be inferred from such observations that the villi do not absorb, so to speak, foreign fat from the lumen of the intestine and finally deposit it from the blood in the tissues when large amounts of unusual fats or oils—s.g., linseed oil—are given in the food. As a matter of fact, probably the most vital factor in bringing about differences in the absorbed fat from the fat in the food is that the three common fatty acids—palmitic, stearic, and oleic—differ in the ease with which they are taken up. Oleic acid would seem to be most completely absorbed, palmitic fairly readily, and stearic acid least readily.

After the fat reaches the blood, our knowledge of its subsequent history is somewhat obscure, but part of it at least is carried to the ordinary fat depots like the adipose tissue and is there deposited. Another part, probably very variable in amount, undergoes immediate metabolism. The information available as to how the fat is utilised in the body is very scanty.

Although fat may be synthetised in the body from carbohydrates and from the nitrogen-free residues of the amino acids, the great bulk of the fat laid down as adipose tissue is not of synthetic origin, but has been laid down from the fat taken up into the bloodstream as the result of absorption. But there is a great lack of reliable information both as to how the fat travels in the blood, how it comes to be deposited as neutral fat in the tissues, and finally as to how it is later mobilised from these depots when it is required for metabolic purposes.

A certain amount of real evidence suggests that there is a very close interrelation between the neutral fat as absorbed and the lipoid (phospho-lipine or phosphatide) content of the red blood corpuscles, which seems to follow the rate of absorption of fat from the intestine. The amount of lipoid gradually increases, and then slowly returns to its fasting level when the absorption of fat ceases. Whether or no this formation of lipoid is associated with intra-molecular changes in the fat taken up, or what particular end is served by the conversion, is quite unknown. Nor is it at all clear where the supply of phosphoric acid and the nitrogenous basic compound requisite for the formation of the lipoid body comes from.

With regard to the other question as to how the fat, after deposition in adipose tissue as reserve fat, is mobilised and put again into circulation, no definite answer can be given. There is no doubt whatever about the main fact that fat laid down in the tissues is of the nature of a reserve, as when required, for

instance during starvation, these supplies can be, and actually are, readily utilised. Unfortunately there is no experimental method available which will permit us to follow the migration of the fat from the reserve depots to the site of metabolism. There is no clue even to that most dramatic mobilisation of fat in the mammary glands during lactation. There is no reason to doubt the commonly accepted belief that the fat of the milk is brought by the blood to the mammary gland and is not actually produced by that gland, but there seems to be no increase in concentration of fat in the blood. In other more abnormal conditions large amounts of fat can be made to appear in the liver with an increase in the mobilisation of the reserve fat. But how these accelerations in the migration of fat are brought about is still a mystery.

Fats do not merely exist for the purposes of storage or mobilisation; they are reserves of food and fuel. The question as to how they are actually metabolised in the organism must now be briefly referred to. They, like the other food materials, yield their stores of energy by a process of oxidation. But what are the intermediate stages? The fatty acid chain, in contradistinction to those of amino acids and sugars, is a very long one, that of stearic acid, for instance, consisting of 17 CHj, groups. The process by which such a long, unwieldy chain is broken down in the organism is supposed to be due to two processes, one of which is called ft oxidation and the other desaturation, (i oxidation is an oxidation process in which two carbon groups are detached from the long chain at a time, and desaturation is a process by which the chain is weakened by an alteration of its molecular bonds. No matter what the nature of the intermediate processes, the end products of metabolism, provided it runs a normal course, are, as already stated, carbon dioxide and water.

CHAPTER V
ACCESSORY SUBSTANCES

In the year 1912, as the result of a long series of experiments, Professor F. G. Hopkins suggested that, in addition to the ordinary components of a diet—viz., protein, carbohydrates, fats, salts, and water, which at that time were accepted as the complete and final admixture, some other material was required, and he termed the unknown substance or substances accessory food factors. He had found that a group of young rats, when fed on a basal diet composed of purified casein, sugar, lard, and salts (ash from oats and dog biscuits), given in sufficient quantity and apparendy satisfying all physiological requirements so far as the proximate principles were concerned, did not thrive. Another similar group of young rats, on the same basal ration, who received in addition 2 to 4 cc. of milk daily, grew, on the contrary, in a perfectly normal fashion.

Other experimental work, for instance that of Eijk-man and others in Java, showed that a scourge of the East, a disease known as beri-beri, was caused by the lack of something in the diet. They found that pigeons fed on polished rice developed the disease, whereas those fed on unpolished rice, that is, rice in which the grains still retain the germ and sub-pericarpal layer, did not develop the disease. Later it was shown that if white polished rice were used, the something missing could be supplied by an extract made from the rice polishings. Still later it was shown that the missing something could be supplied by certain beans. Some years later this problem was again investigated by Funk, who suggested that the name “ vitamin ” be applied to these essential substances, the presence of which in a diet are necessary to prevent the onset of certain diseases like beri-beri, scurvy, rickets, etc., as well as to others which were, for instance, essential for growth in contradistinction to mere maintenance. Many investigators, among them Osborne, Mendel, and McCollum with his co-workers, took up the study of this problem, with the result that to-day five accessory food factors or vitamins are now generally accepted. The designation of the various vitamins by letters of the alphabet, instituted by McCollum, has been adhered to; thus we speak of vitamins A, B, C, D, and E.

Apart from the recent work of Rosenheim and Webster on vitamin D, it may be said that little or nothing is known of the chemical nature of these very elusive accessory factors. Further, apart from D and B, no material of a high degree of potency has ever been isolated by the finest of chemical technique, although a specific colour reaction has been described which seems to be associated with the presence of A. We are, therefore, confronted with a series of active substances, whose activity and even existence can only be postulated when they are absent. It is rather a negative method of analysis but is the only one, unfortunately, which is available. In view of the many difficulties of assessment of the amounts and activity of the various substances, it is not to be wondered at that there has been much diversity of opinion. Moreover, when it is impossible to determine in an accurate fashion the range of action of a series of relatively unknown substances, it is perhaps not a matter for surprise that many and varied virtues have been ascribed to them. Many people resemble the ancient Athenians in their erection of altars.

What, then, can be accepted about these various accessory factors?

Vitamin A.—Until 1913, it was generally believed that all fats had essentially the same value as foodstuffs. It was subsequently shown that cod-liver oil, butter fat, and the fat of egg yolk contained some

2

34 NUTRITION AND DIETETICS factor essential for the proper nutrition and growth of the rat, which was not present, for instance, in lard and olive oil. It would seem, moreover, that if it be absent from, or present in insufficient amount in ¿he diet, a peculiar disease of the eyes called xerophthalmia results. Incidentally, it may be remarked that David Livingstone, in an account of his hardships in Africa as long ago as 1857, referred to a peculiar eye condition which resulted when his men were restricted to a diet of sugarless coffee, roots, and meal—a diet which would be naturally very deficient in fat. On account of its close association with fat, this vitamin is often referred to as fat soluble A.

As regards its source of supply, the outstanding one is cod-liver oil. It is also found in marked amount in butter, milk, egg yolk, and in several kinds of vegetables, like spinach, carrots, etc. It has been shown, in the case of milk products that their vitamin A content depends upon the diet of the cow, and that the content of vitamin is highest in those fed on green pasture. Vegetable oils are generally deficient in A; therefore, the majority of margarines made from vegetable oils have a low vitamin content. Formerly it was thought that there was some relationship between the vitamin and the yellow pigments which are often found associated with fat, but later investigation has shown this relationship to be a purely chance one.

Repeated attempts have been made to isolate A, but so far without success, although one investigator has claimed that he has isolated a substance, closely related to cholesterol (see p. 27), which he maintains is identical with A. As already mentioned, a colour reaction has been described by Rosenheim and Drummond which the authors believe to be due to vitamin A. At any rate, the reaction is given when A is expected to be present and is absent when there is no A, as tested, for instance, by the feeding of animals. At present the only reliable quantitative test for the vitamin is by

means of feeding experiments or biological assay, as it is called.

Whatever be the nature of this vitamin, all the evidence goes to show that it is a relatively stable substance. In the absence of air or oxygen it will stand heating to a high temperature—it is still present, for instance, in the yolk of a hard-boiled egg. Cod-liver oil can be heated to a high temperature without destroying the vitamin, but if, at the time of heating, air is bubbled through the oil, the vitamin is ultimately completely destroyed. It is also stated that exposure to light eventually destroys the vitamin.

Vitamin B.—This was the first vitamin to be discovered. It is the absence of this vitamin which is believed to be responsible for the onset of the polyneuritis (inflammation of nerves) characteristic of the disease bcri-beri. Loss of the co-ordinating capacity of the muscles is one of the most striking features of polyneuritis. It must be understood, however, that certain investigators in the Far East do not accept the view that the absence of vitamin B is the sole cause of beriberi.

This vitamin, so far as our present knowledge goes, would appear to be the most widely distributed of all, as it seems to be present in all natural foodstuffs. It is particularly rich in yeast. Certain manufactured food products like polished rice, white wheat flour, sugar, etc., lack the vitamin. Many attempts have been made to isolate vitamin B, which is soluble in water, but until recently it has not been obtained in a really high degree of concentration. Two Dutch investigators have managed, however, to obtain a product which possesses active curative properties when given in very small doses, but so far its chemical nature has not been elucidated, although there is a certain amount of evidence that it is basic in nature. It is also by no means certain whether the properties which have been ascribed to vitamin B should not be divided between two or more vitamins acting together, as it seems to have some relation to growth as well as its anti-neuritic properties.

It is a peculiarly stable vitamin, being much more resistant to breaking down than either A or C. It can stand exposure to high temperatures provided the solution be acid, whereas in alkaline solution it is eventually destroyed. The ordinary processes of cooking and canning do not apparently interfere to any extent with its activity.

No test of the presence of vitamin B is known except that of the biological assay or feeding test.

Vitamin C.—There is a fair degree of unanimity amongst the majority of modern investigators that the disease known as scurvy is due to the absence of vitamin C from the diet. So long ago as 1753, Dr. James Lind, in his Treatise on the Scurvy, pointed out that this dreadful scourge of seamen could be successfully combated by the use of fresh fruit juice. He also recommended as a practical method of preventing the onset of scurvy, the giving of cress, which might be grown on board the vessel on layers of wet cotton wool. He laid stress on the uselessness of dried vegetables as antiscorbutics. Captain Cook later, by the adoption of similar measures, kept his crews free from scurvy. In some of the more recent Polar expeditions most interesting observations have been made. Thus Stefansson found that the men who lived chiefly on reindeer and other meat, very often in an uncooked state, escaped scurvy, whereas those who consumed mainly the preserved foods succumbed. The same is true as regards the men in other expeditions. Those who lived on fresh bear meat, often for long periods, escaped, whereas the members of the expedition who lived on the preserved and tinned foods carried on the ships, in spite of the fact that they were given a ration of lime juice, contracted scurvy. Scurvy is a disease which is not confined to ships. It is liable to occur, and has occurred, in beleaguered garrisons. In the Crimean War, for example, the Turkish Army was nearly destroyed by it.

It is very evident, then, that fresh foods like meat, vegetables, and fruits possess properties which do not exist in the same foodstuffs when preserved. But modern research has clearly shown that although dried cereals, like peas and beans, do not contain the vitamin in an active state, yet when they are germinated they develop marked antiscorbutic properties. The dried pulses are soaked in water and, after draining away the water, the peas, beans, etc., are allowed to remain in a moist condition with access to air and germinated for twenty-four to forty-eight hours according to the environmental temperature. After cooking for not longer than fifteen minutes they form an admirable substitute for fresh 'food material, and, in the event of scurvy having broken out, these germinated. pulses act as powerful curative agencies.

Fresh fruits and fresh vegetables may be regarded as the main sources of this vitamin. Orange juice and raw tomato are particularly rich. Other valuable sources are cabbage, peas, lettuce, spinach, turnip, and lemon juice. Raw milk also contains vitamin C, but is not very rich in it, and milk which has been pasteurised contains still less. The pasteurisation of milk gets rid of the bacterial contamination and does not otherwise interfere with its food value; the deficiency in the vitamin can readily be made good by the addition of, say, a little orange juice. Whole milk powder, which has been pasteurised in the absence of air after drying, is said to be equal in antiscorbutic power to the raw milk from which it was made.

Vitamin C is the most readily destroyed of any of the vitamins, especially by exposure to nigh temperatures and to oxidation. Hence it follows that in the great majority of dried or preserved or canned food preparations, vitamin C is either absent or present in very deficient amount.

So far no one has managed to obtain this vitamin in a very highly concentrated form, although some investigators nave claimed that they have obtained concentrated preparations by the adoption of special methods. The only test available for the quantitative determination of the amount of C present in any foodstuff is by feeding experiments.

Vitamin D.—Experimental work had roused the suspicion that there might exist a specific substance in a material like cod-liver oil, which would account for its well-marked and much-valued properties of combating the disease known as rickets. These anti-rachitic properties were at first ascribed to the presence of vitamin A. It was shown, however, that vitamin A might be destroyed and yet the anti-rachitic properties remain intact. A fourth vitamin, vitamin D, was accordingly postulated, and the merits of cod-liver oil as a preventative or curative agent in rickets is now commonly ascribed to this vitamin. It was soon found that the anti-rachitic substance was associated with some special substance present in cod-liver oil which was not one of the ordinary fatty acids. Later experiments demonstrated that this vitamin was somehow related to cholesterol or some nearly allied substance. It was found that when an ordinary preparation of cholesterol was exposed to ultra-violet light, the previous inactive substance developed anti-rachitic properties. This result did not follow when a highly purified specimen of cholesterol was employed. Finally, owing to the skill and patience of Rosenheim and Webster, it was discovered that the substance which was influenced by the ultra-violet rays, and which developed marked anti-rachitic properties, was a nearly allied congener of cholesterol—viz., ergosterol. Rosenheim and Webster have demonstrated that this material, properly irradiated, can exert a curative action in the most minute of doses.

Vitamin E.—Experiment had shown that rats, which were brought up on certain synthetic diets containing relatively pure proteins, carbohydrates, fats, a proper mineral salt mixture, and a supply of vitamins A and B, although they remained apparently healthy, sooner or later became sterile. To account for this sterility the absence of a special fertility factor, or vitamin E, was postulated. The sterility could be cured or prevented by the addition of certain natural foodstuffs to the diet. The natural foodstuff which would seem to be richest in this vitamin is the oil of the wheat-germ. Other sources are oats and a number of oils like olive oil, cotton-seed oil, peanut oil, etc. Milk products apparently contain but small amounts of E.

As is the case with A and D, and in contradistinction to B and C, E is a fat soluble vitamin. It would seem to be a remarkably stable one, even in the presence of a high degree of heat. Nor does it seem to be sensitive to oxidation, but is said to be destroyed by excessive exposure to ultra-violet light. Its nature is quite unknown beyond the fact that it is present in the non-saponifiable fraction of the oil.

The foregoing account gives a brief resume of some of the salient facts about the highly elusive substances known as vitamins. It is probable that many of the results given in the modern literature will turn out to be completely erroneous on the score of faulty dieting. It is almost certain that the vitamins themselves will all eventually be found to be substances of definite chemical constitution. Dealing as we are with relatively unknown substances with high degrees of activity, and being at the same time woefully ignorant of many other aspects of dietetics, we are in danger at the present time of ascribing properties and functions to an increasing series of unknown factors, and of postulating the presence of such or other unknown factors before we have exhausted the potentialities of the known.

CHAPTER VI
SALTS

It is very obvious that as a body is not wholly composed of proteins, carbohydrates, and fats, but contains also a supply of mineral salts and water, a supply of these substances is essential in any balanced diet. The mineral salts of the body are not confined merely to the bony framework, although the bulk may be there; the salts are found in varying amount in every tissue and fluid in the body. Without salts, cell life is impossible. Their presence directly or indirectly controls all the metabolic processes.

The most important of the mineral elements which are present in the body are sodium, potassium, calcium (lime), magnesium, iron, phosphorus, sulphur, chlorine, and iodine. There are also present traces of other elements. When tissue material, or the body as a whole, undergoes combustion, these mineral substances are left as the “ ash.” These various mineral substances combine either with the oxygen of the air to form oxides, or, in some cases, with one another, as, for example, the combination of sodium and chlorine to form sodium chloride, or common salt. Such compounds are the forms in which the mineral substances are mostly found in the body. As all these substances are continually leaving the body in the excretions, obviously it is essential that a supply be always available in the food.

Nor must it be imagined that the body can wholly protect itself from excessive salt loss, a loss so great that abnormal symptoms may supervene. Thus it has been shown that men who, in the course of their employment, are subjected to excessively high environmental temperatures, as is the case in deep workings in mines, firemen in stokeholds of the old type, and at furnaces, naturally perspire very freely. Men working under these conditions are very subject to cramp (miner’s, firemen’s cramp), a condition which is frequently accentuated when the men drink freely of water to allay their thirst. Recently it has been found that the condition is due to an excessive loss of sodium chloride from the body, carried away in the sweat, and that the appropriate cure, and fortunately also preventative measure, to adopt is to give the men a dilute solution of salt to drink.

The fluid tissues of the body, the blood and lymph, are, so to speak, essential tissues in that without their aid no other tissue could receive its meed of nourishment or get rid, either, of the products which it contributes to the general welfare of the cell community or of the waste products which are formed in the course of metabolism, and which must be excreted if cell life is to continue. It is therefore one of the important functions of these mineral constituents to maintain the faintly alkaline reaction of the blood and lymph. As the result of the various metabolic changes taking place in the cells, there is a constant production of acid material, chiefly from sulphur and phosphorus, which requires to be neutralised by the different basic substances like sodium and potassium, and probably also by calcium and magnesium. There is, it may be remarked, a further safety factor which becomes operative when the production of acids is excessive—viz., some of the ammonia which is formed as the result of the deaminisation of protein (see p. 11) is utilised for its basic properties to help in the neutralisation of the acids. The kidney plays, of course, a very important rôle in keeping up the balance of the various essential mineral constituents in the blood, as it exercises a marked selective action in what material it will eventually discard in the urine. The ultimate basis of the delicate mechanism which determines the balance of acidity and alkalinity of the blood is thé presence of compounds which result from the union of basic substances and carbonic and phosphoric acids—viz., bicarbonates and phosphates. The nature of the food consumed does, of course, influence markedly the relations which exist between acidic and basic substances in the blood. Table II. gives some idea of how the different foodstuffs contribute salts which are either predominantly acidic or basic. As a matter of fact, the effects of the administration of food, so far as its acidic or basic properties are concerned, is reflected more markedly on the urine than the blood itself.

_ regards the distribution of the mineral material in the tissues, it is calculated that about five-sixths of the total ash of the body is found in the bones. Fresh bones on analysis show an ash content of about 35 per cent.; about 84 per cent, of this ash is in the form of calcium (lime) phosphate, 7-5 per cent, as other calcium salts with, in addition, certain other salts such as those of magnesium. The bones are computed to contain approximately 99 per cent, of the total calcium, about 70 per cent, of the total magnesium and 75 per cent, of the total phosphorus in the body. Although the bones seem to be very firm and resistant, it must not be imagined that they are mere inactive structures. They function as active storehouses, and, when the need arises, the body as a whole can draw upon them for such mineral constituents as they contain. When, for instance, there is a demand for lime and phosphorus, the supply in the food being deficient, the bony framework gives the necessary supplies even to its own detriment. The bones, indeed, may yield up so much of their mineral constituents that they become soft, and can no longer function efficiently as a framework.

The remaining one-sixth of the mineral constituents found in the body are not distributed uniformly throughout the remainder of the tissues. The muscles, which, of course, form the great bulk of the active tissue of the body, contain, for instance, more potassium than sodium, the ratio being of the order of 5 or 6 to 1; further, they arc said to contain three times as much magnesium as calcium. They also contain marked amounts of phosphorus. The blood which bathes all living tissues is, on the contrary, richer in sodium than potassium, and is poorer in phosphorus than muscle. Even when the tissue is of one and the same organ, there are differences found in the mineral composition. Thus, the grey matter of the brain is stated to be slightly poorer in calcium and magnesium than the white matter and the grey matter is much poorer in phosphorus; whereas both grey and white matter would seem to have about the same amounts of sodium, potassium, and iron.

That the mineral constituents of the tissues play an important part in the digestion of the various foodstuffs has long been recognised, as without the hydro chloric acid (HC1) of the gastric juice and the alkaline salts, like sodium carbonate, in the intestinal digestive juices, the utilisation of the food ingested would be impossible. But that the actual content of the diet in mineral matter was equally important was not so firmly grasped, although many of the older experimenters had shown that diets, which lacked or were deficient in all or certain mineral constituents, led to faulty nutrition. The ill-effect of salt-free diets is not due solely to a resultant faulty digestion or utilisation of the various food materials; the lack of these mineral substances leads to a whole train of curious nervous symptoms such as poor appetite, sweating, lack of energy, sleeplessness, and, if long continued, will bring about actual death. There is, unfortunately, a great shortage of data available in connection with mineral metabolism, largely due to the difficulties in carrying out the experiments and in the actual analytical technique. The result is that our information regarding the actual requirements of the various

mineral constituents is not very definite. It may be remarked, however, that there is apparently some close connection between several of these mineral constituents—i.e., if one is omitted from a diet the results, obtained may be also influenced by the disturbance in the metabolism of another mineral besides the one which has been deliberately removed. It would seem that of the three mineral constituents which are most likely to be deficient in a diet—viz., calcium, phosphorus, and iron—there should be a daily intake of about i gram of calcium, about 1-5 gram of phosphorus, and as regards iron, about 15 milligrams per day. In the case of children, according to the calculation of Shferman, the diet should contain 0-25 grams of calcium, 0-48 grams of phosphorus, and 0-005 grams of iron for each 1,000 calories of food ingested. These figures must be held to represent minimal intakes, at least, in the case of calcium, as it has been shown that in order to obtain maximum storage the calcium intake must be three times greater than the amount stored.

As regards the importance of some of these mineral constituents, certain facts are available.

Sodium (Na) and Potassium (K).—These two alkali metals may be taken together. They are present in all organs and tissues. It has been noted that those tissues which are, on the whole, most active functionally and which are rich in cells have a higher ratio of potassium (K) to sodium (Na) than the tissues of supporting structures or the body fluids. In the ordinary mixed diet the ratio is reversed—i.e., there is more sodium than potassium, due largely to the fact that relatively larger amounts of sodium chloride or common salt are added, mainly as a flavouring agent.

Calcium (Ca).—Calcium or lime salts are usually found in foodstuffs like milk, vegetables, and cereals, not as inorganic, but as organic compounds, although everyone consumes daily in drinking water a variable

amount of calcium in inorganic combination. Calcium is not only required for the building up of the bony supporting structures; its presence is essential, so far as we know, in every living cell. It also plays a role in the coagulation of two of the body fluids—blood and milk. In so far as it is essential in the clotting of blood, it is one of the active protective agents which the body possesses to prevent loss of valuable tissues. The presence of calcium salts in the blood is also essential for the normal working of heart muscle. Calcium is in some way also associated with the excitability of the nervous system. It would seem that the withdrawal of calcium leaves the nerves in a state of excessive excitability. In this connection there is apparently some very close connection between the calcium metabolism and one of the glands of internal secretion—viz., the parathyroids. When this hyper-excitability is artificially induced, it is claimed that the injection of calcium or magnesium salts allays, whereas the injection of salts of sodium or potassium intensifies the degree of excitability. A close correspondence between the metabolism of calcium and phosphorus has also been shown to exist particularly in relation to bone formation. There may also be some intimate relation between the amounts of sugar and calcium in the blood. Under certain conditions of ill-health or disease and in old age, there is a marked tendency for lime to be deposited in the lining membrane of the bloodvessels, so that they may in the end become hard and brittle, and are readily ruptured.

Magnesium (Mg.).—Up till the present, very little attention has been devoted to the metabolism of this mineral constituent, and very little is known about the effects which may follow its withdrawal from the diet. The fact that the amount of magnesium present in milk is low rather suggests that the needs of the organism for this substance are not very great. As regards its distribution in the tissues, it has been found that

PUKU4 UNIVERSITY URRAftY

there is about nine times less Mg in bone than Ca, whereas in muscle and nerve the amount of Mg is about double that of Ca. If the statement be true that Mg is just as effective as Ca in allaying nervous hyperexcitability, it would seem that Mg, under certain conditions, might be able to function as a substitute for calcium.

Phosphorus (P).—This element is found in a wide variety of forms in the animal body, both organic and inorganic, and, speaking generally, it would seem to be, of all the mineral constituents, the one most universally required by the tissues of all types. In inorganic combination it is found chiefly in the form of potassium and calcium salts of phosphoric acid. In organic form it is built up into complex compounds in special proteins like the nucleo-proteins, phospho-pro-teins, and some of the lipoids. There are also compounds of phosphorus with carbohydrates, which are apparently of primary importance.

There is present in muscle and essential for muscular activity a compound of sugar and phosphate (hexose diphosphate). This substance has been shown to have a very intimate relationship to the production of lactic acid, a substance which is always found in and which may be responsible for the contraction of muscle. After the contraction is over the hexose diphosphate is apparently largely reformed. This recombination seems to be helped by the presence of calcium. Harden and Young were the first to show that in the relatively simple series of chemical changes which take place when sugar is converted into alcohol through the agency of yeast, the formation of a hexose diphosphate was one of the essential stages. It has been claimed by certain investigators that the capacity of man for the performance of muscular work can be enhanced by the administration of a certain amount of phosphate.

It has proved extremely difficult even to obtain a vague estimate of the phosphorus requirements of the body, largely due to our ignorance of the availability of the different combinations of phosphorus. There is still considerable doubt as to whether phosphorus given in inorganic combination is wholly available. In the opinion of some investigators it is not available at all when given in inorganic form, although, on the other hand, it has been claimed by Osborne and Mendel that, in certain of their experiments on growth, phosphorus, when given solely in inorganic form, was perfectly effective. Sherman has calculated that an intake of about i • 5 gm. phosphorus per day is required in the diet of an adult.

Iron (Fe).—This mineral constituent enjoys a unique position. The presence of iron in the red colouring matter of the blood (haemoglobin) permits this substance to be the carrier of the essential oxygen to the tissues. It is calculated that in the blood of an adult there is present approximately 3 grams (about one-tenth of an ounce) of iron. As there is no appreciable store of iron in the body corresponding with that of calcium and phosphorus in bone, it is essential that constant supplies be available from without. Fortunately the daily loss of iron from the body is not great, and it has been calculated that a daily intake of about 15 milligrams will suffice.

Iron is, of course, found in other tissues in the body besides the blood. It is present in the liver, spleen, bone marrow, and muscles. It would seem also to be a constant constituent of nucleo-proteins, which are the special type of proteins found associated with the essential nuclein of the cells. It may, indeed, therefore be, in minute amount, a constituent,of all cells. Some of the fruits and vegetables, especially those rich in chlorophyll, like spinach and cabbage, are fairly rich in iron. Some of the cereals, like oatmeal, and legumes, like peas and beans, are also relatively rich in iron, as is also egg yolk, but milk and

48 NUTRITION AND DIETETICS white of egg, on the other hand, are poor in this mineral.

Iron is present in these various foodstuffs, as a rule, in a complex organic form, so that apart from such iron as is taken in drinking water, and in some medicinal iron preparations in inorganic form, most is ingested in organic combination.

So far as the importance of iron is concerned, its relation to haemoglobin and blood overshadows all other activities. Hence, if the supply of iron in the food fails to equal the excretion, a diminution in the hemoglobin follows sooner or later, which, in turn, will lead to a condition of anaemia. It would seem from many experiments that iron given in an inorganic form cannot be freely utilised for the production of haemoglobin, although in some way it acts apparently as a stimulus to this production. The bulk of the evidence available supports the idea that haemoglobin is derived in the main from iron in organic combination. Although iron is so essential for blood formation, at the very period when one would have inferred that blood formation was very active—viz., in the early months of life, we are faced with the strange anomaly that the sole nutriment of the infant is milk, which is exceptionally poor in iron. It would seem the difficulty is met during intra-uterine life by the young storing iron in the body, particularly in the liver, and that it is from this personal store that the shortage of iron in the milk is made good. In view of this shortage of iron in milk, it is advisable to let young children have as free a supply as possible of fruits and vegetables which are rich in iron.

Iodine (7).—The need of iodine for the normal functioning of the body has in recent times been strongly emphasised. So far as is generally known at present, iodine in the body is related solely to one of the glands of internal secretion—viz., the thyroid. In the thyroid it exists in a most complex organic form, to which the name of thyroxin has been given. Unless this substance be secreted in sufficient amount, a condition of goitre develops, characterised by a swelling of the thyroid gland in the neck and with a series of bodily changes and a dulling of the intellect. When secreted in excessive amount, it over-stimulates the metabolism and induces a series of nervous and other changes. When there is a shortage of iodine in the food or water supply, as occurs in certain districts, the incidence of goitre is very high. The condition may be cured or prevented by the addition of an appropriate amount of iodine or iodine compounds, such as potassium iodide.

As regards the content of the various foodstuffs in mineral constituents, the following table gives the accepted values for some of them. In general it may be stated that the carbohydrate rich foodstuffs are on the whole poor in minerals. Milling removes most of the mineral-containing material. A point of very considerable importance is whether the mineral constituents are present in such proportions as to render foodstuffs acidic or basic. If it be remembered that the body requires a faintly alkaline medium for metabolism, it is obvious, unless undue strain is to be put on the organism, that the food given in a mixed diet should tend towards the basic side. The cereals and meat have an excess of acid radicles, as has also egg yolk, whereas cow’s milk, like human milk, vegetables and fruits, is predominantly basic. Porridge, then, has its corrective in milk, and meat in vegetables.

Table II.

ASH CONSTITUENTS IN GRAMS OF SOME FOODS IN PERCENTAGE OF THE EDIBLE PORTION (FROM SHERMAN) Basic.    Acidic.

Food.

Calcium.

Magnesium.

Potassium.

Sodium.

| Phosphorus.

Chlorine.

Sulphur.

Iron.

Almonds ..

239

•25t

'741

*019

•465

'037

•160

•0039

Apples .. ..

’007

■008

‘127

‘OIX

‘012

•005

*006

‘0003

Bananas ..

•009

‘028

•40 X

■034

'O3I

'125

010

’0006

Barley (pearl)..

*020

(•070)

C241)

(■037)

*x8x

C016)

(■«*>)

(‘0020)

Beans (dry) ..

*l6o

'156

i *229

•097

'471

■032

'215

‘0070

Blood (average)

*008

‘004

*075

‘261

•03 X

*280

■137

*0526

Bread (white)..

•027

*023

*io8

C394)

'093

(•607)

'105

‘0009

Cabbage ..

■045

‘015

•247

‘027

*029

*024

*066

‘con

Carrots .. ..

•056

‘021

*287

*IOI

*046

■036

‘022

‘0006

Cheese .. ..

■931

'OS?

‘089

’606

•683

•880

■263

'0013

Dates .. ..

*065

*069

‘6ix

■055

‘056

*228

*070

*0030

Egg white ..

‘015

*OIO

*x6o

’156

*0x4

■155

•2x6

'0001

Egg yolk .

l37

*ox6

•1x5

*075

'524

■094

•166

‘0086

Figs (dried) ..

‘162

•071

■96«

*046

*ii6

' '043

*056

•0030

Lemons ..

•036

‘007

’175

*004

*022

‘002

•on

’0006

Lentils (dry) ..

*107

*101

•877

*062

■438

•050

•277

‘0086

Lettuce ..

'043

*0x7

'339

*027

*042

*074

*014

‘0007

Milk (whole) ..

*120

0X2

’M3

'on

'O93

*I06

■034

‘00024

Oatmeal ..

'069

*1X0

'344

*062

‘392

*069

‘202

0038

Onions .. ..

*034

*Ol6

•.78

‘016

■045

‘02 X

*070

’0006

Peas (dry) ..

•084

‘149

'903

‘104

*400

'035

*219

*0057

Potatoes ..

*014

‘028

•429

‘02 X

*058

•< 38

•03O

‘0013

Raisins ..

’06 *

■083

*820

x33

13*

‘082

•051

‘0021

Rice (white) ..

‘009

‘033

*070

‘025

*096

'054

1X7

*0009

Spinach . .

'067

*037

'774

I25

•068

•074

*038

‘0036

Tomatoes . .

‘on

0X0

’275

‘010

‘o 26

•034

■OI4

‘OOO4

Per

100 gr

ams pr

otein t

here is

in

Meat (average)..

*058

•118

1-694

*421

i‘078

■37s

i*X46

‘0150

Fish (average) ..

•109

■133

1*671

'373

1*148

•528

1*119

*0055

CHAPTER VII WATER

The final substance on the list of essentials is water. Perhaps owing to the fact that in this climate of ours water is at times too abundant, there is a general lack of appreciation of its virtues. Water, if one considers alone the amount present in the body, must be considered of all constituents the one which is most essential. But apart from the mere bulk of water present, life would be impossible in every cell if water were not present in sufficient amount. A man may live for many weeks without food, but life is only a matter of days in the absence of water, and death takes place long before anything approaching desiccation occurs; indeed, there is only a loss of about io per cent, of the body water content. Water forms about 60 to 65 per cent, of the body weight of man, and in some of the lower living creatures it amounts even to over 95 per cent, of the total weight. The amount of water which is present in the body of the higher animals is, to a certain extent, inversely proportional to the fat content. The age of the organism also plays an important role in variation in the water content, the younger the animal the greater the proportion of its weight in water. The water is not evenly distributed throughout the body, nor is it by any means found chiefly in the blood and lymph. Elaborate calculations of the percentage distribution in the human body have been made, and it has been found that of the total water content of the body, about 50 per cent, is present in muscle, about 13 per cent, in the skeleton, about 5 per cent, in the blood, and only 0 6 per cent, in the kidneys, and 0-4 per cent, in the spleen. Of course, the greater part of the water is not found in the free state, but as water incorporated in the protoplasm of the cell. All life processes may then, with propriety, be said to be ultimately referable to changes, which take place in solution.

Water is the most wonderful fluid which exists in the physical world; indeed, it may be said to hold quite a unique position. Owing to its various physical properties, it plays an all-important role in the control both of the environmental and body temperatures, allowing, for instance, in the body of large changes in heat formation with but small changes of body temperature. Unless the body temperature be maintained at a relatively constant level, the constancy of the metabolic activities could not be kept up. Water is of especial value in getting rid of heat from the body by the evaporation of sweat. No other fluid known can fix so much heat during evaporation, hence it follows that with the loss of a comparatively small amount of fluid there is a relatively large loss of heat. On its chemical side, too, water possesses many unique properties. As a solvent, no fluid can compare with water. Not only can it hold a wonderful variety of substances in solution, but whilst in solution these substances undergo very little chemical change. Although one is perhaps inclined to look upon water as very inert, there is good evidence that it plays anydiing but a passive role in certain chemical combinations. The fact, too, that in watery solutions the degree of ionisation is high, renders possible a most mobile chemistry. Finally, as its surface tension is high, it probably accounts for a number of the phenomena which characterise the changes occurring in living cells of both plants and animals, such as that of adsorption, which permits of an uneven distribution of the various substances in a solution. Incidentally it may be remarked that the exact chemical form in which water exercises its activities is by no means clear.

All the water that is present in the organism does not come from some source external to the body, but is in part formed or liberated in the tissues during the course of metabolism. According to certain calculations, about 16 per cent, of the water excreted is derived from this metabolic source.

The literally enormous turnover, the simple translocation, of water in the body, quite apart from excretion, is not commonly appreciated. The active surfaces of organs like the intestine, lungs, and other mucous surfaces must be kept moist, and wherever movement occurs, like that in joints and tendon sheaths, fluid must be present. Such requirements arc, however, insignificant when compared with those of the digestive tract, whose normal functioning makes extraordinary demands on the fluid content of the body. The total turnover of fluid during the course of ordinary digestion is large. It is calculated that in the course of the day in the whole digestive tract not less than 6 to 9 pints of fluid are poured out from the various glands, where it mingles with water taken in with the food, and yet, apart from the odd 3 or 4 ozs. of water which are present in the average faeces, it is reabsorbed into the system.

Very large quantities of water can be lost from the body, when required for protective purposes, in the form of sweat. At rest, the average loss per diem by way of the skin amounts to approximately 33 ozs. When work is done, the loss may be very great. Losses, chiefly due to water, of body weight of 14 lbs. in 1 hour 10 minutes of football, 5! lbs. in a 22-minute boat-race, and lbs. in a 3 hours’ marathon race, have been recorded by reliable observers.

Whether it be the normal loss which goes on even during complete rest, or the excessive loss which follows muscular work in the effort to get rid of the increased heat, this constant loss of water has to be made good by ingestion from without. Fortunately, in

the majority of civilised countries there is, as a rule, no difficulty in obtaining a sufficiency of fluid.

There is an old question which constantly crops up —viz., should fluid be taken with meals? The whole of traditional doctrine is against such a practice. All kinds of arguments have been adduced reprobating the consumption of fluids with meals, but not a single convincing piece of experimental evidence has emerged. Indeed, the experimental evidence which does exist, obtained, it is true, on normal subjects, would go to show that, instead of drinking with meals leading to disaster, the contrary is true. Hawk, for instance, as the result of many experiments, goes so far as to say that the drinking of a reasonable volume of water with meals will promote the secretion and activity of the digestive juices, and the digestion and absorption of the ingested food.”

There is no doubt that the drinking of reasonable amounts of water is helpful in other ways than those merely dietetic, particularly so where there is a tendency to high blood pressure. In the course, perhaps, of ordinary cellular metabolism, and certainly during the time the food material is in the digestive tract, due probably in the main to bacterial action, a number of noxious products may be formed which, when absorbed, may in part account for high blood pressure. It would appear that the consumption of large volumes of water leads to a diminution in the bacterial content of the intestine, and it has been clearly demonstrated that such a consumption is followed by a definite fall in blood pressure, due, in all probability, to the washing out of the system of the toxic material.

CHAPTER VIII

QUANTITATIVE PROBLEMS

The energy expenditure aspect of nutrition must now be considered, bearing in mind, of course, what was said earlier that, although this phase is treated separately in actual fact, it is quite inseparable from the metabolism of material. Here we deal with the quantitative aspect of the problem, as it is from the determination of the total energy output that we are able, in turn, to determine the necessary intake to cover the loss.

The standard unit of measurement utilised here is the Calorie. It must be grasped at once that there is no real food virtue in Calories. They are simple units for the measurement of heat. We cannot live on Calories, but without the use of such a unit it would wellnigh be impossible to state the problem and to deal practically with the results. It is quite true that if a good mixed diet is quantitatively sufficient—i.e., has a sufficient caloric value—qualitatively it will probably be all right.

The heat unit in question, the large Calorie, is the amount of heat which is required to raise one kilogramme (2-2 lbs.) of water through C., or, more accurately, to raise the temperature of one kilogramme of water from 16° to 17° C. As all forms of energy tend sooner or later to degenerate to heat, and as heat is very accurately measurable, heat units form a most serviceable method for computations. By the use of appropriate methods we can convert both the energy of income (food) and of expenditure into heat units. This ability permits of the presentation of a proper balance sheet.

In the first place, the determination of the caloric value of the foodstuffs will be discussed—i.e., the determination of the potential value in energy of our maintenance resources. The method employed is that of the Bomb Calorimeter. In this apparatus a known amount of the footstuff is burnt completely in an atmosphere of oxygen, the heat liberated is taken up by the water in which the bomb is immersed. This causes a rise in the temperature of the surrounding water, and from this rise the amount of heat liberated by the combustion of the foodstuff in question can be calculated. A large series of these measurements has now been made, with the result that certain figures are practically universally accepted. These figures are:

Calories for i Gram Material.

Protein ...    ...    ...    41

Carbohydrate ...    ...    4-1

Fat......... ...    9-3

The fact that caloric values and food values are by no means synonymous terms may again be emphasised by drawing attention to the fact that many materials, like coal and strychnine, for instance, may have a readily determined caloric value, but even the most misguided of parents would hesitate before trying to bring up their offspring on a diet composed of these materials. Further, even supposing the material be of the nature of a foodstuff or a component of foodstuffs —such a material, for example, on the one hand as gelatine or cellulose (a regular component of all vegetable food materials) on the other—the fact that the material in question has a caloric value of standard type affords no criterion of its biological value or capacity in the organism.

Just as the energy value of a foodstuff can be stated in terms of Calories, so the energy expended by the organism can also be stated in similar heat units. When we consider the actual daily energy output of the organism, which has to be made good by the ingestion of an adequate supply of food, we may roughly divide this into (1) the energy liberated as the result of the mere running of the organism, the internal work, so to speak, and (2) the energy which is related to the production of external muscular work, either during the performance of manual labour, the daily walk to and from the place of business, or in the taking of exercise in the form of golf, tennis, etc.

The fact that we can thus express the transformations of energy during intracellular activity in terms of heat units permits of the most varying types of physical work being compared and assessed in terms of Calories. We can therefore, given certain necessary conditions, compare, for instance, the daily energy output of such divergent forms of employment as that of a postman and a tailor, a painter and a bus-driver, and so on. These assessments may be done in either of two ways, which have been called Direct and Indirect Calorimetry.

The direct method of determining the energy expenditure is to enclose the subject in a special chamber, so designed that all the heat given off from his body can be accurately measured. As a rule, at the same time, the amount of Oxygen utilised by the subject, and the amount of Carbon Dioxide he gives off, are also determined. This method of estimating the energy output is very accurate, but the apparatus is costly, the determination exceedingly difficult, and there are, moreover, very definite limitations as regards the type and nature of the work, which can be carried out in a chamber of limited size.

Recourse is had most frequently nowadays to the method of Indirect Calorimetry, in which reliance is placed on the accurate measurement, under definite conditions of the amount of Oxygen (O,) utilised, and the amount of Carbon Dioxide (CO,,) eliminated in a given time. The volume of Oxygen used in unit time (say one minute) is multiplied by a caloric value factor, which has been determined experimentally, and varies slightly with the ratio CO? excreted /02 used. (See p. 23, Chap. III.) The result obtained is a statement of the energy expenditure of the organism, expressed in Calories in unit time.

The reason why such a method permits us to measure and state the metabolism in terms of Calories is that the Oxygen utilised by the tissues is Oxygen which, in the various metabolic processes, plays a part analogous to that which it plays in the burning of coal, say, in the fire-box of a locomotive. It is, of course, well known that combustion of coal will not take place without a free supply of air (Oxygen), and it is equally true that internal combustion or metabolism in the tissues will also not go on without a similar supply of Oxygen. There is good evidence to show that although the ratio COJO,, cannot be wholly ascribed to combustion processes, it does give some clue to the type of foodstuff which is being burned in the tissues. Provided the technique of the determination is attended to with scrupulous care, and full attention is given to the many difficulties and pitfalls, this indirect method does give perfectly reliable results.

Obviously, as human beings are of very different physiques, it would be, for purposes of comparison, of very great advantage if the energy transformations could be expressed in terms of some physiological unit. Formerly it was the custom to use the body weight of the subject as the physiological unit, and the results were stated in Calories per kilogramme body weight.

It is very manifest that body weight may be made up in different ways; it may be due in the main, for example, to muscle, bone, or to fat. Muscle is a particularly active metabolic tissue, whereas fat is quite inert. It is the common custom now to correlate the metabolism with the surface area of the body. It has been found that a good approximation to this area may be made by the use of a formula determined by two

American researchers, D. and E. F. Du Bois, in which die body weight and the height of the subject are used as the basis. More recent work has shown that if the factor of age be also utilised, there is some increase in accuracy.

The method of estimating the energy expenditure having been discussed, we must now consider it in greater detail, especially with regard to the factors which influence it. It is very obvious, when we come to study the problem, that the really fundamental value for all the calculations of the energy requirements of the organism must be, of necessity, that which is related to the mere running of the body when in a state of complete rest—i.e., the metabolic rate at its lowest level in the waking state. This lowest rate has been called the basal or standard metabolism, and is attained when the subject is warm, at rest in bed, and twelve to fifteen hours after the last meal—i.e., in the postabsorptive state. Under these conditions it has been found from a long series of determinations that the basal metabolism of men between the ages of 20 and 50 is approximately 40 Calories per square metre per hour. lit it be assumed that the surface area of the average man (in Britain) amounts to 1 - 7 square metre, this would give a value of 68 Calories per hour, or approximately 1,600 Calories per diem.

Certain factors are well recognised as influencing this basal metabolic rate. Sleep, for instance, brings about a reduction which may exceed 10 per cent., whereas the taking of food may cause a rise of from 16 to 20 per cent. The temperature of the environment may also exercise an influence; a cool climate and exposure to open air stimulates the metabolism. The state of the health, the state of the nutrition, and the nature of the diet of the subject are all influential, but all these factors sink into insignificance alongside that of muscular work, which is the most potent of all stimulants of the metabolic rate, the extent of the 60 NUTRITION AND DIETETICS increase being more or less proportional to the amount of work done. It must be remembered, however, in this connection that in practically any occupation there are periods when the muscles are in a state of contraction, and therefore increasing energy output from the body, without, however, the production of external, useful, or assessable work.

The amount of external muscular work done is calculated either in terms of kilogrammetres or foot pounds (i kilogrammetre = 7 • 233 foot pounds)—i.e., the amount of energy expended in raising 1 kilogram through 1 metre vertical distance, or 1 pound through 1 foot vertical distance. Accurate determinations have shown that these work units can be converted into heat units, that, for example, approximately 427 kilogram-metres are the equivalent of one Calorie. Repeated experiment on subjects has shown further that the mechanical efficiency with which the work is done is, in general, about 25 per cent. This figure means that only one-quarter of the energy liberated from food can be converted into external work. Thus, if work equivalent to 25 Calories is done, available energy in the form of food must be provided equivalent to 100 Calories. In some types of muscular work the efficiency may be less than 25 per cent.

The problem of how much work the average man performs in the course of his working day is one which has been frequently attacked. Long before the refined laboratory methods now available were utilised, many attempts were made to state in a numerical form an average day s work. The results put forward varied in the wildest possible fashion from several millions to tens of thousands of foot pounds. Since the introduction of the modern methods a few attempts have been made to find a solution of the problem. It would seem that the average workman, engaged in manual labour, has a daily (8 hours’) output of between 80,000 and 90,000 kilogrammetres, which means, when these

values are converted into heat units, that there is an energy output in the form of external muscular work of about 200 Calories per diem.

It is, of course, well recognised that literally enormous amounts of work may be performed by the human body on occasion, but not as a daily routine. One individual who was carefully studied had an output equivalent to 125 Calories per hour for over four hours.

The foregoing consideration of the basal metabolism and of the probable expenditure of energy in the form of external muscular work, permits of the formulation of an approximate estimate of the daily energy expenditure of the average man (vide p. 59). The day for purposes of assessment may be divided into three periods of 8 hours each-—sleep, free time, work.

As regards sleep, this may be rated at about 10 per cent, less than the basal metabolism, therefore for the average man about 62 Calories per hour. Wor\ is assessable as the basal metabolism (68 Calories per hour) plus the increment due to an average day’s work at, say, 80,000 kilogrammetres, or 200 Calories multiplied, if we assume a mechanical efficiency of 25 per cent., by 4. Finally, as regards the assessment of energy expenditure during free time, it may be admitted that it is wellnigh impossible to lay down any hard and fast rule. Generally it may be stated that the expenditure is inversely proportional to the severity of the work during the work period. There is certainly no common factor. It is suggested, however, taking everything into consideration, that the provision for the average man would be more than adequate, if it be assumed that the energy expenditure during the time at the man’s own disposal is equal to 175 per cent, of the basal—i.e., one allows that during this time he expends more than half of the energy he expended in doing his day’s work.

If the actual experimental data and the above assumptions be utilised, we may express the total daily energy output in Calories as follows:

Calories

496

*>344


8 hours’ Sleep at 62 Calories per hour ......

8 hours’ Worf( at 68 Cals, (basal) per hour +

200 Cals, x 4 (for work increment) ......

952


8 hours’ Free Time at 68 Cals, (basal) +175 per cent..........

2,792

But the man does not work 8 hours 7 days a week. If we assume he works a 48-hour week, his daily needs, week in week out, must be adjusted accordingly. The weekly balance-sheet would work out as follows:

Calories.

56 hours’ Sleep, 496 Cals, x 7...... 3,472

48 hours’ Wor\, 1,344 Cals, x 6    ...    8,064

64 hours’ Free Time, 952 Cals, x 8...    7,616

19,152

This gives an average daily expenditure of 2,736, which may, on the basis of our calculations, be regarded as maximal, as, for instance, it is highly improbable that the average man’s expenditure of energy on the Sunday is anything like 175 per cent, of his basal.

If we take it that the average expenditure is approximately 2,700 Calories per diem, it would not suffice if the man purchased a similar number of Calories in the form of food to cover this loss. There must be taken into account the refuse and waste, which must occur, and the losses during cooking, etc. Obviously the loss will vary with the nature of the food consumed. It would be very small in the case of bread, but quite appreciable in the case of roast meat and foods cooked

in fat. As the result of much observation, it is at present customary to allow a difference of 10 per cent, between food purchased and utilised. Hence, it would be necessary to purchase 10 per cent, more Calories than the actual expenditure. It follows, then, that to cover the loss of 2,700 Calories, just under 3,000 Calories should be provided. This value agrees closely with those obtained by Voit and Rubner, and with that of 3,000 adopted by the Interallied Scientific Food Commission during the late war, but it is somewhat below that put forward by Atwater.

As regards the food requirements of women, there is a more or less general agreement that for the average woman, due mainly to her smaller amount of muscle or active metabolic tissue, the demand is about 17 to 20 per cent, below that of the average man.

The problem as to the proper amounts to allocate to children of different ages is very difficult. Many investigators have attempted to lay down standards— family co-efficients—using the requirements of man as a basis. There has been no general agreement on the point. Most of the earlier investigators made use of the co-efficients of Atwater, but the Interallied Scientific Food Commission utilised those suggested by Lusk. Lusk’s figures are as follows :

Man ............

I

00

Woman ............

0

83

Adolescent Male, 13 up. ...

I

00

„ Female, 13 up.

0

83

Child, 10-13 .........

0

83

„ 6-10 .........

0

60

„ 0-6 .........

0

5°

The method of expressing the food requirements of each member of a family as a proportion of those of a man, allows one to assess how far any family diet is adequate, and also permits of a comparison being made between the requirements and the diets of families of varying size. Thus, the man value of a family consisting of father, mother, and four children aged 13 (boy), n, 9, and 7 would be as follows:

Man ...

......... 1 ■ 00

Woman ...

......... 0-83

Boy, 13 ...

......... 1 • 00

Child, 11 ...

......... 0-83

» 9 •••

......... 0 • 60

» 7 •••

......... 0 • 60

4-86

Some consideration must now be devoted to the optimum proportions, in which the various proximate principles, protein, carbohydrate, and fat should contribute to the total requirements of the body, both for the purposes of maintenance and the supply of energy.

Carl Voit, many years ago, laid down, as the standard of the requirements for a man engaged in moderately heavy work, 118 grams1 protein, 500 grams carbohydrate, and 56 grams fat, yielding in all 3,055 Calories. If these values be converted into a percentage distribution, they are 67 per cent, of the Calories from carbohydrate, 16 per cent, from protein, and 17 per cent, from fat. There has been very considerable debate upon this standard, and most actively with reference to the amount of protein present.

How much protein should a diet contain? Opinion may be roughly divided into those who advocate a reasonably high intake, and those, more limited, perhaps, in number, but more vociferous, who maintain that salvation in health lies in a low protein content.

Chittenden, for example, as the result of a most

1 oz. = 28-35 grams.


interesting series of experiments, came to the conclusion that the high intake of protein advocated by Voit was both extravagant and dangerous to health. He believed that an intake of 40 to 50 grams was fully adequate to satisfy all protein needs. On the other hand, the experience of many other investigators has led them to advocate a relatively high intake, of protein when long-continued, hard, muscular work has to be carried through, especially under conditions of low environmental temperature. A survey, for example, of the dietaries of modern armies in the field emphasises this point. In certain experimental tests on soldiers which were carried out to determine the optimum protein content, it was concluded that, although possibly the allowance was more than ample, the soldiers did well on a diet of a total caloric value of 3,400, which contained 190 grams of protein—i.e., approximately 23 per cent, of the Calories were derived from protein. But an equally definite conclusion was also reached that a diet which contained 145 grams of protein with a total caloric intake of 3,500—i.e., approximately 17 per cent, of the caloric intake being derived from protein—was as low as it was advisable to go.

An extremely interesting survey of the composition of the diets of many of the races in India was made by McCay. He obtained evidence indicating some direct relation between the virility of the race and the consumption of protein. This work, on a much smaller scale, was confirmed by the observations of Campbell at Singapore.

There seems also to be a more or less general corn sensus of opinion on the part of those who train athletes, that they get their best results from a diet rich in protein. And there is the very widely held belief of the ordinary “ man-in-the-street,” with no pretensions to, or knowledge of, academic controversy, that if he has to do hard work he must have an ample supply of protein in the form of meat.

There would seem to be no doubt about the general opinion of the special virtues of protein. How much protein, then, does the average man, who takes an overage mixed diet, without giving any thought to its chemical composition, sufficient to satisfy his immediate needs, consume in the course of a day? Several ■Investigations on the point have been made, and it may be concluded that in this country he will be found to ■have a daily intake of between 90 and 100 grams of protein. Docs he really require to consume this amount; do his tissues demand it; or does he merely consume this quantity either because the dishes available yield it, or because it satisfies his tastes and appetite? It may be, as has been suggested, that deductions drawn from the dietary habits of generations of peoples in all quarters of the globe are fallacious, as the intervention of different factors of unknown value or importance may well have vitiated the observations. But, as Sir William Roberts emphasised long ago, these long-established food customs cannot be thrust to one side and completely ignored, as they represent “ the fruit of colossal experience accumulated by countless millions of men through successive generations.” I think we may take it that a diet which contains about 100 grams of protein, or a little less, may be regarded as an approximately normal one for the average man. But this leaves unsettled the question whether, when much muscular work is to be done, an increased amount of protein is demanded or is necessary.

Does muscular work increase the demand for protein, and if the demand be met, is it necessary? The scientific evidence, such as it is, does not lend much support for such a view. In the majority of experiments, at least, there is perhaps evidence that active muscular work does give rise to some, although, as a rule, slight, increase in the output of Nitrogen—i.e., some slight increase in protein catabolism. Let it be noted that the general demand is for animal protein,

QUANTITATIVE PROBLEMS 67 and not for protein in general. The demand is for meat, and not for peas, beans, and other vegetables rich in protein. This, of course, raises the question whether meat has other properties, besides its content in protein, which lead to the demand. There is a certain amount of evidence to show that, when muscular work is carried out on two diets, one containing and the other lacking meat, the physiological cost of the work to the organism is lower on the meat-containing diet. One, at least, of the chemical properties which distinguish meat from vegetables, for instance, is the possession of a special series of substances called extractives. It would appear that these extractives do possess a certain stimulating action, and it is possible that part of the real value of meat lies in the fact that it is a natural mixture of protein and extractives. How these extractives act it is difficult to say; they may exercise some influence on general metabolism, may play the part of some kind of catalyst or facilitator of activity, or it is conceivable that they may bring about a better utilisation of the protein element. There is, at any rate, in all probability some special virtue in the natural meat protein with its normal complement of extractives, as in certain experiments, in which extractives alone were added to a non-meat but protein-containing diet, the results were not so satisfactory as those in which meat itself was given.

This finding naturally leads to the question as to whether proteins drawn from various sources have the same or a varying value to the organism, whether they differ in their biological value. Over eighty years ago, long before anything was known practically about the chemical composition of the proteins, one of the early well-known Continental workers on the subject wrote :

“ The lion is so fierce and so powerful, not only because it is built as a lion, but also because lion’s blood flows in its vessels. Lion blood is not derived from twigs or roots, but from meat. The cow is as 68 NUTRITION AND DIETETICS quiet as she is, not only because she is built as a cow, but also because quiet grass or hay blood flows in her vessels. . . . Therefore, he who has too much energy should eat vegetable matter like the cow, and he who has too little energy should eat meat like the lion.”

There is experimental evidence now available which demonstrates clearly that considerable variation between the biological values of proteins does actually occur. If we set aside the extreme cases of those proteins like zein, gelatine, etc., in which essential amino acids are totally missing, there are still left a large number of proteins, many of them being of vegetable origin, in which, although no essential amino acid may be completely lacking, yet the percentage distribution of the different amino acids within the molecule may be so unbalanced as compared, say, with the composition of muscle protein, that they may, as a result, be of low biological value. If, in view of the fact that muscle is the preponderating active metabolic tissue in the body, we take it as the standard, it may be assumed that the more closely the ingested protein approaches in composition muscle protein, the smaller will be the amount required to make good the wear and tear. The experimental work of Thomas, although on some grounds it may be open to objection, does point to the existence of considerable variation in biological value. He determined the amounts of tissue protein nitrogen which could be spared by the ingestion of ioo grams food protein nitrogen. The values he obtained were, in round figures, as follows :

ioo Grants.

Milk

it

,,

it

>1 IOO

Fish

tt

tt

It

II 9»

Rice

> 1

11

88

Potato

tt

>>

It

11 79

Pea

11

tt

II

» 56

Flour

it

it

II

11 40

Maize

it

II

11 30


Ox flesh protein nitrogen can spare 105 grams tissue protein nitrogen

Although these figures may be only approximately correct, they probably give the relative values of the different proteins; hence the quality of the protein ingested must be one of the marked factors in determining the level of the protein intake. The practical conclusion to be drawn is that a good mixed diet is the « one which is most likely to provide the supply of necessary protein in the most economical fashion. Reference to the table on p. 9, which gives the practical chemical composition of sundry of the principal proteins, would seem to afford adequate reason for the above-noted variation in biological values. Attention should be especially directed to the composition of Zein (from maize) and Gliadin (from flour) in comparison with the composition, for instance, of the protein from Ox Muscle.

As regards the intake of carbohydrate, there is little to be said, for apart from a few races, such as the Eskimo, inhabiting lands where crops cannot readily be raised, the great bulk of the food consumed by the average man of average income is of a carbohydrate nature. If the Voit standard be accepted as more or less representative of Western European dietaries, carbohydrate yields 67 per cent, of the total caloric requirements, whereas in the dietaries of many Eastern races, it may yield over 80 per cent, of the total. In consequence of its relatively low cost in practice—in Eastern countries, at least, and, generally speaking, it is true for the rest of the world—it will be found that the poorer the consumer the greater the amount of carbohydrate eaten. Bread is indeed the staff of life. This consumption is not a matter of choice, but of compulsion. The following dietaries of Bengalis, taken from McCay’s work, afford an interesting view of the variation in the source of Calories as the result of

income:

is a marked diminution in the amount of energy derived from carbohydrate sources. The marked change, despite a warm climate like that of Bengal, takes place in the consumption of fat. The total caloric intake in the four groups runs as follows:

Percentage Distribution of Calories in Bengalese Diets

I.

II.

III.

IV.

Cultivators

Middle

Middle

Well-

Class (<i).

Class (b).

to-do.

Protein ...

8

9

12

12

Carbohydrate

81

71

52

41

Fat ......

10

20

35

47

It will be noted that,

as the income increases

, there


I., 2,390; II., 2,310; III., 2,350; and IV., from 2,900 to 3,400.

Finally, the amount of fat which an adequate diet ought to contain must be discussed. The amount of carbohydrate which can be ingested is limited by man’s powers of digestion and metabolism. Bulk for bulk, fat provides more than twice the amount of energy provided by either protein or carbohydrate. It is, if one may so put it, a concentrated form of fuel. As has already been noted, some investigators have gone so far as to say that fat is not an essential constituent of any diet. Although it is quite true that carbohydrate can be readily converted into fats, in practice all diets contain a certain amount of fat, even although in some— as, for example, the Japanese—the fat content is very low. Indeed, one of the most difficult of all procedures is to render a diet fat-free. It has also been shown that foodstuffs, completely freed from all fats and other lipoid material, will not serve to maintain life. Further, as fat is the vehicle of the fat soluble accessory substances which are necessary for the well-being of the organism, the provision of an adequate supply of fat in the diet is not only dietetically convenient, but is essential. According to some recent calculations, the dietary, in order to afford the requisite supply of these accessory substances, should contain about 50 grams of fat in the form of milk fat.

It may be true that carbohydrate is to be considered the main fuelstuff of the body, but it is a curious and interesting fact that when very hard muscular work is to be done, an increase in the fat content of the diet is craved. Thus, in the dietaries of lumbermen and others engaged in hard work in the open air, the fat content of the diet may account for some 40 per cent, of the total Calorics. This value of fat as a fuel food was emphasised many years ago by Frankland, as the result of his observations on navvies engaged in the construction of a railway. He records : “ I well remember being profoundly impressed with the dinners of the navigators employed in the construction of the Lancashire and Preston Railway; they consisted of thick slices of bread surmounted with massive blocks of bacon in which mere streaks of lean were visible. These labourers doubtless find that from fat bacon they obtain at the minimum cost the actual energy required for their arduous work.” Lawes and Gilbert also pointed out that those engaged in hard muscular work selected meat rich in fat, in preference to lean meat.

But even in the case of men training for heavy athletic events it would seem to be the case, if Finnish wrestlers can be cited as examples, that these men select diets particularly rich in fat. Thus, Lavonius, who critically studied two of these athletes, found that the sources of their Calories were, respectively, 17-6 and 17-5 per cent, from protein, 47-6 and 44-8 per cent, from fat, and 34 • 8 and 37 ■ 7 per cent, from carbohydrate.

It may be that a good layer of adipose tissue may at times be an encumbrance, but, on the other hand, it is 72 NUTRITION AND DIETETICS possible that men who are called upon to perform arduous and continuous work are better equipped when they have a good, but not too abundant, store of fat in the body. It is stated that Lord Roberts, after his forced march on Candahar, expressed the opinion that men with a good store of fat stood the strain best.

CHAPTER IX
GENERAL

When we come to consider the practical or applied aspect of the dietetic question, we are immediately faced with innumerable difficulties. General experience from the consideration of the many different schemes and methods of feeding, which have been propounded by a succession of writers, both by those with special knowledge of the problem and by a much larger number of those whose previous experience and training did not entitle them to dogmatise, would seem to show that the capacity of the living organism to withstand dietetic ill-treatment is very high. Common sayings have often the knack of summing up the concentrated wisdom of the ages, of wisdom gained in the hard school of experience. If the palm has to be awarded to any one saying in virtue of its general approximation to the truth, then, in my opinion, it may be awarded to “ One man’s food is another man’s poison.” We have only to consider in the most casual fashion the number of people one meets who hold the most diverse views, who propound ideas and schemes of nutrition of diametrically opposite natures, and who, moreover, seem, so far as all outward and visible signs go, to thrive on the particular combination which they advocate, to make us hesitate at coming to any definite con-

elusion. It would seem that the nervous system has a much greater influence over somatic or bodily activity than is commonly supposed to be the case. It is true that Pavlov has demonstrated that the nervous system plays an all-important role in the secretion of certain, at least, of the digestive juices (see later, under “ Appetite ”), but there is very good evidence that the influence of the nervous system on metabolism generally extends much further than its influence on mere secretion. The view has even been put forward by one well-known worker in the field of nutrition that there is actually a well-defined centre in the brain which exercises a controlling influence on metabolism. He has produced a certain amount of evidence which apparently supports his conclusion.

Be the control what it may, the fact remains that humanity, as a class of living creatures, can survive and maintain its health under conditions of nutrition which may seem to run absolutely counter to our individual ideas of what is right and proper in a diet.

In the following table there is given a list of some of the commoner foodstuffs with their approximate contents, in round figures, of protein, carbohydrates, and fat, the caloric value per ounce, and their approximate caloric substitution amounts. It will be noted that food materials like butter, meat, and fish contain no appreciable amounts of carbohydrate, whilst a composite meat product like sausage may. The figures for the various meat products must be regarded as average figures; there is a marked variation in the distribution of protein and fats in the different “ cuts.” Thus, in the case of beef, in lean rump steak the protein amounts to about 12 per cent., and fat about 4 per cent., whereas, if we take the average of the whole hindquarter, there is about 8 per cent, protein and 14 per cent. fat. It is very evident from these figures that meats in general must be looked upon as rich sources of fat. Cheese, a most valuable foodstuff, is rich both in protein and fat, but poor in carbohydrate. Fish, on the other hand, contains only moderate amounts of protein and fat. Milk is relatively poor in all three principles. When we consider the cereals and pulses, it will be noted that many of diem contain more protein than do the meats, but, though rich in carbohydrate, they are poor in fat, the product richest in fat being oatmeal. Peasemeal contains nearly double the amount of protein as compared with oatmeal, but, on the other hand, it is about five times poorer in fat. Vegetable products like potatoes, carrots, turnips, etc., are poor in all the proximate principles. (See Table III.)

Although these values are given, and despite the disparity in the amounts, it must not be imagined that because vegetables and fruits are poor in percentage amount in protein, carbohydrate, and fat, they must be looked on as poor value from a dietetic standpoint. It was emphasised earlier that mere caloric content is no criterion of the value of a foodstuff, so it may now be said tnat the presence in abundant or small amount of protein, carbohydrate, and fat, although they may be essential, does not of necessity say the last word about the biological value of any particular constituent of a diet. Vegetables and fruits are rich in the various accessory substances and salts, which are truly essential in the diet. Although it is true that other food products—like meat and oatmeal, for example—also contain salts, reference to Table II. will show that the percentage distribution of the salts present varies in marked degree. The aim is to get a good balance of salts, so that the ash is neither too acid nor too alkaline. As already pointed out, meats tend to be rich in acid radicles, and the natural correctives are the vegetables, rich in alkaline salts, which are commonly consumed with the meat.

The fact, then, that proteins vary markedly in their chemical make-up (p. p) and in their biological value, and that there is this marked difference in the distribu-

Parts per 100* fapp»-ox.).

J)

V s?

|j

V u

Foodstuff.

0

X 0 « U

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0 c

E C

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is-

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Meats :

Beef .. .. ..

Mi

l6

65

5

325

Pork .. .. ..

12

3°

94

3 s

329

Bacon .. ..

9i

55

155

2j

349

Sausages .. ..

18

20^

75

4 J

337

Fish (as purchased): Herring .. ..

to

7

36

9i

342

Haddock .. ..

12

15

23

345

Cod .. .. ..

o

>7

20

340

Mackerel .. ..

II

6

29

nj

333

Salmon .. ..

15

M

5°

6i

325

Kippers .. ..

M

I I

46

7

322

Dairy Products:

Milk......

3i

5

3i

19

17(8.ozs.)

323

Cheese .. ..

26

3

35

126

2#

i4

346

Butter .. ..

i

84

222

334

Margarine .. ..

S5

223

ti

334

Kggs......

11

10

41

S (4 eggs)

328

Cereals, etc. :

Bread .. ..

7

48

65

5

325

Flour .. ..

13

74

I h

105

3i

341

Oatmeal .. ..

12

7°

8i

1T7

2?

322

Peasemeal .. ..

22

64

IO4

3i

338

Peas (split) .. ..

20

63

Ï

99

3*

322

Beans ^haricot) ..

18

65

X

97

3i

339

Lentils .. ..

20

64

i

99

3i

322

Sago......

8s

102

3i

331

Tapioca .. ..

SS

'

102

3i

331

Rice .. .. ..

6

SO

i

IOI

3i

228

Sugar ... ..

IOC

”3

3

339

Golden Syrup ..

}

76

S9

334

Honey .. ..

/t

St

4

324

Jam and Marmalade

i

69

81

4

324

Vegetables and Fruit :f

Potatoes .. ..

2

18

23

M

322

Carrots .. ..

I

9

-

12

2?i

33°

Turnips .. ..

I

4

6

54

328

Onions .. ..

I

IO

M

24

33<5

Green Vegetables ..

I

6

_

7

54

32S

Fruit, fresh (aver.)

i

8

10

33

330

Fruit, dried (aver )

60

1

72

4i

324

* Balance of weight mainly made up by water, to a lesser extent by salt content. In process of ccoking, water content of meats, fish, etc., diminishes, that of Cereals, pulses, etc., is increased.

t Vegetables and fruits all contain very small amounts of fats and oils.

76 NUTRITION AND DIETETICS tion of the various inorganic constituents, offers a perfect valid reason why, for the majority of mankind, a good mixed diet, in which the foodstuffs are drawn from as wide a variety of sources as possible, will offer the best and most economical method in the end of affording nutriment suited in every way to the organism and its protean needs.

Although stress has been laid on the digestibility of the various foodstuffs, attention must be drawn to the fact that a diet in which the components are capable of complete absorption cannot be considered a sound diet. Every diet must contain a certain amount of residue or “ ballast.” From the point of view of actual nutriment, this material is without value, but from the health standpoint it is a necessary constituent. Unless this ballast be present, the movements of the intestine do not take place in the steady, regular fashion which is essential. This is the reason why a milk diet is so liable to lead to constipation. The delayed evacuation, which is the characteristic phenomenon associated with constipation, gives the bacteria, which, even in health, swarm in the intestinal tract, a better chance of acting on many of the products of digestion, and of producing a series of products which, when absorbed from the intestinal canal into the body, act as toxic or poisonous matter. It is the presence of these toxic substances in the blood and tissues which accounts for the general depression and many of the other symptoms associated with constipation.

Two other factors in dietetics are of primary importance, and to both but little attention is given by many; these are purchasing and cooking. No matter how abundant the supplies of food available, unless the money be properly expended the yield per penny spent may be inadequate. This is particularly true when the income is low. It has been our experience, as the result of repeated dietary studies, that one of the most prominent contributing factors towards defective

and deficient dietaries is not so much the inadequacy of the income as its faulty expenditure.

The purchase of prepared food, in spite of the diminution in waste, gives, speaking generally, a poorer yield of the essentials per penny spent than the purchase of food in its natural form.

The other factor is cooking. Probably more food is wasted by faulty cooking than in any other fashion. It is relatively easy to deal with selected foodstuffs of good quality, but it is a real art to convert foodstuffs of poor quality into palatable and nutritious dishes; and yet it is on this art, and this art alone, that a sound dietary may be constructed in an economical fashion. The purposes of cooking are :

1.    To render the unpalatable palatable and agreeable to the eye.    _

2.    To render mastication easier and to facilitate

digestion.    _    ,

3.    To destroy micro-organisms and certain noxious products.

It is not merely that cooking in its various methods renders the various food substances more open to the attack of the different digestive ferments. Although it is undoubtedly true that coagulated or cooked albumin is more rapidly digested than raw, that many of the vegetable materials have their essential food materials enclosed in firm cellulose walls, which are cither broken down or rendered permeable by cooking, other changes take place which not only render the food more readily digestible, but more desirable. In other words, the food is rendered more appetising. Appetite and desire for food are frequently despised as being too closely akin to the sin of gluttony. But the fact is that appetite plays an important role in digestion, as it is one of the most effective stimuli for the secretion of saliva and gastric juice. And anything which contributes towards a free flow of the digestive juices is a powerful adjuvant in digestion.

It has been definitely proved that many stimuli, other than actual contact of the food with the mouth, will evoke the flow of the digestive juices. The sight and the smell of food, and, under appropriate conditions, even the thought of food, are effective stimuli. Further, it has been shown that food, when properly set out in an attractive fashion on a clean cloth with clean china, etc., can stimulate, whereas when the same food is set out in a careless and dirty manner with dirty china and dirty linen, not only docs it not evoke a flow, but actually inhibits the secretion.

Emotional disturbances like fear, anger, etc., act as marked inhibitors of the digestive juices. Hence the necessity of peace, contentment, and happiness at mealtimes. The ancient writer enunciated a sound physiological truth when he wrote: “ Better is a dinner of herbs where love is than a stalled ox and hatred therewith.”

In the present state of our knowledge of the science of nutrition, it is not possible to lay down any hard and fast lines for the composition of the ideal dietaries of different peoples, or, generally speaking, even for different individuals. But broad general principles, which must underlie all dietaries that are universally applicable, may, it is believed, be stated as follows:

x. The diet must be sufficient in quantity—i.e., the amount ingested must cover the energy requirements.

2.    The diet must be sufficient in quality—i.e., it must satisfy fully die biological needs.

3.    The diet must be as appetising as possible.

4.    The diet must, so far as possible, conform to racial dietary habits.

General principles may be stated, the results of past experience may be cited, and the ideals in the light of the general principles and past experience may be divined, but to maintain, or even imagine, that there is one absolutely correct dietary for all men is folly. Much better, and in the end safer, to adopt the dictum of the writer of the verse below than to follow that will-o’-the-wisp, the perfect diet.

“ Eat all kind nature doth bestow,

It will amalgamate below,

If the mind says it shall be so.

But, if you once begin to doubt,

The gastric juice will find it out:

Calm courage conquers Sauerkraut.”

BIBLIOGRAPHY

Principles of Human Physiology. Starling. (Churchill, 1926.)

Science of Nutrition. Lusk. (Saunders, 1928.)

Chemistry of Food and Nutrition. Sherman, (lvlr t millan, 1918.)

Basal Metabolism in Health and Disease. Du Be-(Lea and Febiger, 1927.)

The Newer Knowledge of Nutrition. McCollum. (Macmillan, 1922.)

Metabolism and Practical Medicine. Vol. I. V. Noorden. (Heinemann, 1907.)

Endocrinology and Metabolism. Vol. III., edited by Barker. (Appleton, 1922.)

Analyses and Energy Values of Foods. Plimmer. (H.M. Stationery Office, 1921.)

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No.

1.    HISTORY OF ENGLAND

2.    WORLD OF GREECE AND

ROME

3.    EASTERN ART AND

LITERATURE

4.    ROMAN BRITAIN

S- ORIGINS OF CIVILIZATION 6 ORIGINS OF AGRICULTURE 1. NUTRITION & DIETETICS

8.    HISTORY OF EUROPE,

476-1925

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GERMAN LITERATURE RUSSIAN LITERATURE MYTHS OF GREECE AND ROME 67. ARCHITECTURE 87. THE ENGLISH NOVEL 102. MODERN SCIENTIFIC IDEAS

102. AGE OF THE EARTH

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118.    INTRO. TO BOTANY

140.    SIR ISAAC NEWTON

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143. STRUCTURE OF MATTER ■ 45- THE WEATHER 151. RELIGIONS OF THE WORLD

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SYSTEM 277. TRADE 279. MONEY

227. THEORY OF MUSIC 230. ENGLISH FURNITURE 232. THE ENGLISH DRAMA 252. NELSON

252. OLIVER CROMWELL

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