Gender-related differences in the relationship between densitometric values of whole-body bone mineral content and lean body mass in humans between 2 and 87 years of age.

The mineral, lean, and fat contents of the human body may be not only allometrically but also functionally associated. This report evaluates the influence of muscle mass on bone mass and its age-related changes by investigating these and other variables in both genders in the different stages of reproductive life. We have analyzed the dual-energy X-ray absorptiometry (DEXA)-determined whole-body mineral content (TBMC), lean body mass (LBM), and fat body mass data (FBM) of 778 children and adolescents of both genders, aged 2-20 years [previously reported in Bone 16(Suppl.): 393S-399S; 1995], and of 672 age-matched men and women, aged 20-87 years. Bone mass (as assessed by TBMC) was found to be closely and linearly associated with muscle mass (as reflected by LBM) throughout life. This relationship was similar in slope and intercept in prepubertal boys and girls. However, while keeping the same slope of that relationship (50-54 g increase in TBMC per kilogram LBM): (1) both men and women stored more mineral per unit of LBM within the reproductive period than before puberty (13%-29% and 33%-58%, respectively); (2) women stored more mineral than age-matched men with comparable LBM (17%-29%) until menopause; and (3) postmenopausal women had lower values of bone mineral than premenopausal women, similar to those of men with comparable LBM. Men showed no age effect on the TBMC/LBM relationship after puberty. Multiple regression analyses showed that not only the LBM, but also the FBM and body height (but not body weight), influenced the TBMC, in that decreasing order of determining power. However, neither the FBM nor body height could explain the pre/postpubertal and the gender-related differences in the TBMC/LBM relationship. Accordingly: (1) calculated TBMC/LBM and FBM-adjusted TBMC/LBM ratios were lower in girls and boys from 2-4 years of age until puberty; (2) thereafter, females rapidly reached significantly higher ratios than age-matched men until menopause; and (3) then, ratios for women and age-matched men tended to equalize. A biomechanical explanation of those differences is suggested. Sex hormones or related factors could affect the threshold of the feedback system that controls bone remodeling to adapt bone structure to the strains derived from customary mechanical usage in each region of the skeleton (bone "mechanostat"). Questions concerning whether the mineral accumulation in women during the reproductive period is related or not to an eventual role in pregnancy or lactation, or whether the new bone is stored in mechanically optimal or less optimal regions of the skeleton, are open to discussion.