The relative contributions of age and years since menopause to postmenopausal bone loss.

We have estimated the relative contributions of age and menopause to forearm mineral density in 485 normal postmenopausal women up to age 75 yr. In 87 pairs matched for years since menopause, in which 1 member was below 61 yr and the other was 61 yr or older, the mean bone density was significantly lower in the older than in the younger subjects despite their identical years since menopause (P less than 0.001). Further analysis suggested a model for bone loss after the menopause which comprises a menopausal component of exponential type and an age-related component which is linear and starts in the mid-50s. According to this model, a 70-yr-old woman has lost 11% of her bone due to menopause and 18% as a function of age. Thereafter, the age-related function is dominant. Early menopause is associated with a self-limiting loss of bone which does not progress further until aging exerts its effect. The main conclusion is that the significance of early menopause as a risk factor for osteoporosis has been overstated.