Relation of bone mineral density and content to mineral content and density of the fat-free mass

Differences in the mineral fraction of the fat-free mass (M-FFM) and in the density of the FFM (D-FFM) are often inferred from measures of bone minera l content (BMC) or bone mineral density (BMD). We studied the relation of B MC and BMD to the M-FFM and D-FFM in a heterogeneous sample of 216 young me n (n = 115) and women (n = 101), which included whites (n = 155) and blacks (n = 61) and collegiate athletes ( n = 132) and nonathletes (n = 584). Who le body BMC and BMD were determined by dual-energy X-ray absorptiometry (DX A; Hologic QDR-1000W, enhanced whole body analysis software, version 5.71). FFM was estimated using a four-component model from measures of body densi ty by hydrostatic weighing, body water by deuterium dilution, and bone mine ral by DXA. There was no significant relation of BMD to M-FFM (r = 0.01) or D-FFM (r = -0.06) or of BMC to M-FFM (r = -0.11) and a significant, weak n egative relation of BMC to D-FFM (r = -0.14, P = 0.04) in all subjects. Sig nificant low to moderate relationships of BMD or BMC to M-FFM or D-FFM were found within some gender-race-athletic status subgroups or when the effect s of gender, race, and athletic status were held constant using multiple re gression, but BMD and BMC explained only 10-17% of the variance in M-FFM an d 0-2% of the variance in D-FFM in addition to that explained by the demogr aphic variables. We conclude that there is not a significant positive relat ion of BMD and BMC to M-FFM or D-FFM in young adults and that BMC and BMD s hould not be used to infer differences in M-FFM or D-FFM.