Relationship between body composition and bone mineral density in women with and without osteoporosis: relative contribution of lean and fat mass

To assess the relationship of total fat mass (TFM) and total lean mass (TLM) with bone mineral density (BMD) and bone mineral content (BMC), we studied 770 postmenopausal white women after total body measurements by dual-energy X-ray absorptiometry. Height-independent bone mineral density (HIBMD) was also tested. The effects of TFM and TLM on the dependent variables HIBMD, BMD, and BMC were assessed by the univariate general linear model (UGLM). Age, age at menopause, height, and bone area were entered in the models as controlling variables when appropriate. In the total population, TLM and TFM were associated with BMD, BMC, and HIBMD (P < 0.001). Taking the T-score cut-off as −2.5, women without (463) and with (307) osteoporosis were then tested separately. In nonosteoporotic women, TLM was significantly associated with BMD, BMC, and HIBMD (P < 0.001), while TFM was not. In osteoporotic women, both TLM and TFM were associated with BMD to the same extent (P < 0.05), but not with HIBMD. Women without osteoporosis were then tested according to whether their TFM/TLM fraction was less than or greater than 1. In those with TFM/TLM less than 1, both TLM (P < 0.001) and TFM (P < 0.01), tested separately, were associated with BMD and BMC, but not with HIBMD. When TLM and TFM were tested at the same time and assessed by the same UGLM, only TLM (P < 0.001) still affected these three bone parameters. In women with TFM/TLM greater than 1, testing the body components both separately and at the same time and using the UGLM showed that TFM affected both BMC and BMD (P < 0.05), while TLM did not. In conclusion, our data indicate that both TFM and TLM affect bone density, with different physiological/pathological conditions modulating this relationship.