CIRCULATING INSULIN LEVELS ARE RELATED TO BONE-DENSITY IN NORMAL POSTMENOPAUSAL WOMEN

We recently established that the dependence of bone mineral density (B MD) on body weight in women is mainly attributable to a close relation ship between total body fat mass and BMD. The present study assesses w hether this latter relationship might be contributed to by the hormone s insulin or amylin, both of which may influence fat mass and calcium metabolism. Fifty-three normal postmenopausal women underwent a 75-g1 glucose tolerance test with measurement of plasma insulin and amylin c oncentrations every 30 min for 2 h. Body composition and BMD/height (t o provide a quantity with the dimensions of volumetric density that is independent of body size) were measured by dual-energy X-ray absorpti ometry, and volumetric density of the third lumbar vertebral body was calculated. Circulating insulin concentrations correlated with BMD/hei ght and volumetric density of the third lumbar vertebral body (r = 0.2 8-0.52). They also were related to body weight (r = 0.34-0.56) and fat mass (r = 0.38-0.56) but were not independently related to lean mass on multiple regression. There were no consistent relationships between amylin levels and these variables. Multiple-regression analyses with fat mass and insulin levels as independent variables indicated that BM D/height of total body and femoral trochanter were primarily related t o fat mass, whereas, in femoral neck, the significant relationship was with insulin. Volumetric density of the third lumbar vertebral body w as related to insulin levels alone on this analysis. It is concluded t hat circulating insulin levels are consistently related to bone densit y throughout the skeleton and that this may be mediated by direct anab olic effects of insulin on osteoblasts or its inhibition of synthesis of sex hormone-binding globulin. However, the present data do not esta blish that insulin underlies the fat mass-bone density relationship at all skeletal sites.