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

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Abstract
Differences in the mineral fraction of the fat-free mass (MFFM) and in the density of the FFM (DFFM) are often inferred from measures of bone mineral content (BMC) or bone mineral density (BMD). We studied the relation of BMC and BMD to the MFFM and DFFM in a heterogeneous sample of 216 young men ( n = 115) and women ( n = 101), which included whites ( n = 155) and blacks ( n = 61) and collegiate athletes ( n = 132) and nonathletes ( n = 84). Whole body BMC and BMD were determined by dual-energy X-ray absorptiometry (DXA; Hologic QDR-1000W, enhanced whole body analysis software, version 5.71). FFM was estimated using a four-component model from measures of body density by hydrostatic weighing, body water by deuterium dilution, and bone mineral by DXA. There was no significant relation of BMD to MFFM( r = 0.01) or DFFM ( r = −0.06) or of BMC to MFFM ( r = −0.11) and a significant, weak negative relation of BMC to DFFM( r = −0.14, P = 0.04) in all subjects. Significant low to moderate relationships of BMD or BMC to MFFM or DFFM were found within some gender-race-athletic status subgroups or when the effects of gender, race, and athletic status were held constant using multiple regression, but BMD and BMC explained only 10–17% of the variance in MFFM and 0–2% of the variance in DFFM in addition to that explained by the demographic variables. We conclude that there is not a significant positive relation of BMD and BMC to MFFM or DFFM in young adults and that BMC and BMD should not be used to infer differences in MFFM or DFFM.