The relative contributions of menopause and aging to postmenopausal vertebral osteopenia.

Lumbar spine bone mineral density (BMD) was measured by dual energy x-ray absorptiometry in 2190 normal postmenopausal women (PMW). BMD was loosely correlated (r = 0.15) with height, but stronger positive relations were found with body mass index (BMI; r = 0.25) and body weight (r = 0.31). BMD decreased with age (r = 0.25), but the relation with years since menopause (YSM) was the most potent (r = 0.36). BMD shows a rapid and highly significant (P < 0.0001) decrease in the first 5 YSM, when no relation between BMD and age is present. As the menopausal bone loss is not linear, the BMD values were regressed on the logarithmic transformation of YSM, and the equation was y = -0.183 logYSM + 1.149 (r = 0.44; P < 0.0001). When lumbar BMD was simultaneously regressed on different variables, the correlation with BMI and logYSM remained highly significant, whereas the correlation with age was not significant. The age-related component accounts for a linear 0.4% decrease/yr, starting at the age of 55 yr. In 139 pairs of PMW up to and over 60 yr of age (58 +/- 1.9 and 62.5 +/- 1.5 yr, respectively), matched for YSM (10.3 +/- 2.3), no significant differences in height, weight, or BMI were found. The BMD was significantly (P < 0.001) lower in the younger (0.945 +/- 0.13 g/cm2) than in older PMW (1.006 +/- 0.18 g/cm2) despite their identical YSM. In conclusion, the menopausal (hormonal) component of bone loss and a younger at the menopause represent the major factors in determining involutional vertebral osteopenia in women.