Correlations between skeletal muscle mass and bone mass in children 6-18 years: Influences of sex, ethnicity, and pubertal status

A constant sex-specific relationship between skeletal muscle mass and bone mass was observed in healthy adults based on TBK/TBCa, using TBK (total bod y potassium) by K-40 counting and TBCa (total body calcium) by in-vivo neut ron activation analysis (Ellis and Cohn, 1975). We revisited this topic in children by studying correlations between TBK and TBCa, and by comparing TB K/TBCa between sexes, pubertal groups (prepubertal and pubertal) and ethnic groups in 141 white, 101 black, and 62 Asian healthy children, aged 6 - 18 years, living in New York City. TBK was measured by 40K counting, and TBCa by dual energy x-ray absorptiometry. TBK and TBCa were significantly corre lated from 6 to 18 years (r > 0.93), but the correlation equations varied b y gender and ethnicity. Boys had significantly more TBK and greater TBK/TBC a than girls at a given age and weight, reflecting greater skeletal muscle mass in boys from 6 years, the age at which the study started. TBK/TBCa in blacks was significantly smaller than whites and Asians in both sexes in pr epuberty and puberty, and pubertal black girls had the smallest mean TBK/TB Ca. No significant differences were found between whites and Asians. TBK/TB Ca decreased as body weight increased in prepubertal girls, and decreased a s body weight and age increased in pubertal girls, but did not change with body weight or age in boys of any subgroup. The inverse relationship betwee n TBK/TBCa and age in pubertal girls suggests greater increase in TBCa comp ared to TBK than in other groups, while the constant TBK/TBCa in boys refle cts proportional increases in TBK and TBCa. Thus TBK/TBCa can be used as an index of relative growth in skeletal muscle mass and bone mass in white, b lack, and Asian children according to sex, age and pubertal status.