Despite a large number of studies assessing relationships between putative risk factors and bone density, it is not known which factors influence the rate of axial bone loss in normal postmenopausal women. We have examined the relationships between the rate of bone loss (delta BMD) and variables related to
calcium metabolism, lifestyle, diet (
calcium,
sodium,
caffeine, and
protein), body composition, muscle strength,
sex hormones, and
spinal osteophytosis in 122 normal postmenopausal women participating in a 2-yr prospective randomized placebo-controlled trial of
calcium supplementation. Univariate correlation coefficients indicated that delta BMD at most sites was inversely related to baseline BMD and positively related to rate of change in
body weight (0.10 < r < 0.36) and fat mass (0.11 < r < 0.42) during the study. Lean mass and its rate of change showed no consistent relationship to delta BMD. There was no correlation between delta BMD and any of the lifestyle, muscle strength, dietary, or hormonal indices or with the severity of
spinal osteophytosis. Multiple regression analysis indicated that delta BMD in the total body was directly related to fat mass (P < 0.0001), the rate of change in fat mass (P < 0.0001), the renal tubular reabsorption of
calcium (P < 0.01), and
calcium treatment (P < 0.01) and inversely to the initial BMD (P < 0.0001; r2 = 0.42; P < 0.0001). Similar effects were seen throughout the skeleton, although the fraction of the variance accounted for was less in the subregions, consistent with the lower precision of measurement of regional bone density. It is concluded that baseline bone density, fat mass, and renal
calcium handling are important factors influencing bone loss in normal postmenopausal women.