Pathogenesis of osteoporosis.

Five major risk factors for osteoporosis have been identified: age, initial bone density, the menopause, bioavailability of calcium, and sporadic factors. Age appears to be the major determinant of bone mass. During a lifetime, a woman will typically lose 50% of her trabecular bone and 35% of her cortical bone. Bone density is affected by the amount of bone developed during growth, as well as by the subsequent rate of loss. Strong evidence has shown that loss of ovarian function leads to an accelerated phase of bone loss. Bone turnover rates increase, but resorption occurs faster than formation. This acceleration slows with time, reaching the level of the underlying slower phase of bone loss approximately 10 years after the onset of menopause. The role of calcium intake in preventing osteoporosis remains a matter of debate. Other factors shown to affect the risk of osteoporosis include low weight, smoking, alcohol intake, and degree of physical activity. The fact that not all postmenopausal women develop osteoporosis suggests that other, as-yet-undetermined factors may play a role in this condition.