Role of the vitamin D-endocrine system in the pathophysiology of postmenopausal osteoporosis.

Impaired calcium absorption and impaired adaptation to a low calcium diet are common features of aging in women and these processes are even more severely impaired in patients with osteoporotic fractures. The calcium absorption defects are associated with several abnormalities of the vitamin D-endocrine system including secondary hyperparathyroidism, intestinal resistance to 1,25-dihydroxyvitamin D (1,25(OH)(2)D) action, decreased 1,25(OH)(2)D production due to impaired 25(OH)D 1alpha-hydroxylase activity, and, in some elderly persons, nutritional deficiency of vitamin D. However, in postmenopausal women, most of these abnormalities are normalized by administration of physiologic replacement dosages of estrogen and, thus, appear to be secondary consequences of estrogen deficiency. Nonetheless, a minority of them, especially nutritional vitamin D deficiency and impaired 25(OH)D 1alpha-hydroxylase activity late in life, appear to be primary and are independent of estrogen deficiency.