Overweight children have poor bone strength relative to body weight, placing them at greater risk for forearm fractures

Introduction: Obesity is thought to be a protective factor for bones in adults but not in children based on the evidence of the greater incidence of forearm fractures in obese children. Our objective was to investigate the effect of adiposity on bone strength in relation to the mechanical challenge placed onto the forearm bones in case of a fall.

Methods: Cross sectional areas (CSA) were obtained at the mid- and distal radius by peripheral quantitative computed tomography in 486 children (241 boys), mean age 8.3 years (range 6.9–9.7), participating in the LOOK Project. The following parameters were measured: bone mass and bone CSA (both sites), and muscle and fat CSA (mid-forearm only). Bone strength indices combining bone size and total volumetric density were calculated at each site.

Results/Discussion: Overweight children (BMI > percentile equivalent to 25 kg/m² in adults) have higher bone parameters than normal-weight peers (Z-scores +0.6 to +0.9SD, p < 0.0001). These differences disappear after adjustment for muscle CSA. Adiposity (fat CSA/muscle CSA) was negatively correlated with bone mass, size and strength at the distal radius only (r = −0.1, p < 0.05). After adjustment for body weight (estimate of the load during a fall), the negative correlations were stronger and observed at both the mid- and distal radius (r = −0.37 to −0.55, p < 0.0001).

Conclusion. Overweight children have stronger bones due to greater muscle size. However, children with high fat mass relative to muscle mass (increased adiposity) have poorer bone strength, independent of weight, which may contribute to the increased risk of fracture in obese children.