Timing of menopause, reproductive years, and bone mineral density - A cross-sectional study of postmenopausal Japanese women

Age at menopause has been found to be associated positively with bone miner al density, and age at menarche has been found to be associated negatively with bone mineral density. However, there have been few studies on the rela tions of timing of menopause and length of the reproductive period with bon e mineral density. The purpose of this study was to examine the relations o f timing of menopause and reproductive years (calculated as age at menopaus e minus age at menarche) with mineral density of the second metacarpal bone in postmenopausal Japanese women. The study population consisted of 1,035 naturally menopausal women aged 40-70 years who were screened in 1996-1997. Using computed x-ray densitometry, the authors measured bone mineral densi ty by analyzing radiographic films of the right second metacarpal bone. Usi ng the women with early menopause (age <49 years) as the reference group an d adjusting for age, subjects with late menopause were at decreased risk fo r low bone mineral density (odds ratio (OR) = 0.69, 95% confidence interval (CI) 0.49-0.97). After adjustment for additional covariates (grip strength , physical activity, body mass index, smoking, and calcium intake), the ass ociation was unchanged (OR = 0.70, 95% CI 0.50-0.99). Postmenopausal women with more reproductive years (greater than or equal to 40 years) were at de creased risk for low bone mineral density compared with those with fewer re productive years, after adjustment for age (OR = 0.73, 95% CI 0.40-1.30) an d potentially confounding factors (OR = 0.76, 95% CI 0.41-1.37); the p-valu e for trend was not statistically significant. In multiple linear regressio n analysis, early menopause and fewer reproductive years were independent p redictors of low bone mineral density. In this study, postmenopausal Japane se women who had a late menopause and more reproductive years were at decre ased risk for low bone mineral density, and may therefore be less prone to osteoporosis.