Overweight children have a greater proportion of fat mass relative to muscle mass in the upper limbs than in the lower limbs : implications for bone strength at the distal forearm

<b>Background:</b> The influence of adiposity on upper-limb bone strength has rarely been studied in children, despite the high incidence of forearm fractures in this population. <br><br><b>Objective: </b>The objective was to compare the influence of muscle and fat tissues on bone strength between the upper and lower limbs in prepubertal children. <br><b><br>Design:</b> Bone mineral content, total bone cross-sectional area, cortical bone area (CoA), cortical thickness (CoTh) at the radius and tibia (4% and 66%, respectively), trabecular density (TrD), bone strength index (4% sites), cortical density (CoD), stress-strain index, and muscle and fat areas (66% sites) were measured by using peripheral quantitative computed tomography in 427 children (206 boys) aged 7–10 y. <br><br><b>Results:</b> Overweight children (<i>n =</i> 93) had greater values for bone variables (0.3–1.3 SD; <i>P <</i> 0.0001) than did their normal-weight peers, except for CoD 66% and CoTh 4%. The between-group differences were 21–87% greater at the tibia than at the radius. After adjustment for muscle cross-sectional area, TrD 4%, bone mineral content, CoA, and CoTh 66% at the tibia remained greater in overweight children, whereas at the distal radius total bone cross-sectional area and CoTh were smaller in overweight children (<i>P <</i> 0.05). Overweight children had a greater fat-muscle ratio than did normal-weight children, particularly in the forearm (92 ± 28% compared with 57 ± 17%). Fat-muscle ratio correlated negatively with all bone variables, except for TrD and CoD, after adjustment for body weight (<i>r =</i> −0.17 to −0.54; <i>P < </i>0.0001). <br><b><br>Conclusions: </b>Overweight children had stronger bones than did their normal-weight peers, largely because of greater muscle size. However, the overweight children had a high proportion of fat relative to muscle in the forearm, which is associated with reduced bone strength.<br>