Osteoporosis and the metabolites of vitamin D

Vitamin D is metabolized by sequential steps in the liver and kidney to its active form, a process that is strongly feedback-regulated. In old age, the activity of the enzyme, 25-hydroxyvitamin D 1 hydroxylase, which produces the vitamin D hormone, is diminished. The activities of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) extend beyond increasing intestinal absorption of calcium. The vitamin D hormone (or its analogs) is useful in the treatment of osteoporosis because it not only stimulates intestinal calcium absorption, but also is required for the stimulation of osteoblasts and many other cells in the body. Recent work demonstrates that it is possible to chemically synthesize analogs selective for specific actions of the vitamin D hormone, especially in inducing differentiation of promyelocytes and keratinocytes.