The vitamin D endocrine system, calcium metabolism, and osteoporosis.

Although the nutritional aspects related to bone development and subsequent bone loss have been appreciated for many years, they are now being reemphasized in view of current information concerning the vitamin D endocrine system, the development of new assay procedures and more sensitive radiologic techniques to assess changes in bone mass, and the realization that clinical problems related to bone loss will increase as individuals live longer. The vitamin D endocrine system is complex, involving the skin, liver, and kidney for synthesis of the vitamin D metabolites and, primarily, the intestine and bone for biologic expression. Numerous factors and disorders affecting the skin, gastrointestinal tract, and kidney will adversely affect vitamin D metabolism. Vitamin D deficiency is common in elderly individuals, especially those who are chronically ill, house-bound, and poorly nourished. Subclinical vitamin D deficiency and osteomalacia may also be complicating problems in elderly patients with osteoporosis and hip fractures. At present the role of the vitamin D endocrine system in the pathogenesis and treatment of osteoporosis is unclear. There is little evidence that vitamin D or its metabolites are helpful in osteoporosis, except perhaps to heal osteomalacia which may be present. It is hoped that encouraging results will follow the use of more potent vitamin D metabolites, either alone or in combination with other agents. Calcium homeostasis is affected by numerous dietary factors (including protein, phosphorus, fiber, and lactose) and drugs (including alcohol, diuretics, and antacids), and calcium absorption in the intestine and the ability to adapt to low-calcium diets will decrease with advancing age. There are conflicting reports concerning the relation between low-calcium intake and osteoporosis, and about the role of calcium intake in the development and then maintenance of bone mass. There is little doubt that many older individuals ingest less calcium than is recommended, especially at a time when even more may be required to maintain bone mass. Several studies show that calcium supplementation producing a total calcium intake of 1,200-1,500 mg/day can slow the rate of bone loss. When the high doses of calcium are given along with vitamin D, periodic monitoring of blood and urine calcium is necessary to avoid hypercalcemia and hypercalciuria.