To what extent is bone mass determined by fat-free or fat mass?

One hundred sixty-four healthy black and white women aged 24-79 y were studied to determine to what extent bone mass is determined by fat-free mass (FFM). A multicomponent approach to body composition, with techniques that are not interdependent, was used. The measurements included dual x-ray absorptiometry (DXA), prompt gamma-neutron-activation analysis, inelastic neuron scattering, tritiated water dilution, and whole-body counting. Univariate correlations showed significant relationships of all the fat-free measures and most of the fat measures with bone mass measured by total body calcium (TBCa). Data from pre- and postmenopausal women were analyzed separately. The average FFM by itself explained 50-55% of the variability in TBCa whereas the average fat mass by itself explained only 5-18% of the variability. The contribution of fat mass was consistently greater in postmenopausal than in premenopausal women. When stepwise multiple regression with TBCa was performed to determine the influence of adding fat mass, height, and race to the relationship of FFM with TBCa, the variation explained by average FFM was 56% premenopausal, 50% postmenopausal; by height 3% premenopausal, 6% postmenopausal; by race 4% premenopausal, 8% postmenopausal; and average fat mass was not significant. Average values for fat mass and FFM were obtained by averaging all the methods used. In conclusion, in black and white healthy women, although bone mass may be partially influenced by fatness or race, the major determinant of bone mass is FFM. Fat mass may play a more important role in postmenopausal women.