Vitamin D: Historical Overview.

A history of vitamin D has been provided, dating from the earliest description of rickets, the disease resulting from vitamin D deficiency, to a current understanding of vitamin D metabolism and the mechanism of action of its hormonal form in regulating gene expression in target organs. Vitamin D is produced in skin by impact of 280-310 nm light on 7-dehydrocholesterol. The vitamin D is then converted in the liver to a circulating form, 25-hydroxyvitamin D that is converted largely, if not exclusively, in the kidney to the final hormone, 1α,25-dihydroxyvitamin D. This hormone functions through a nuclear receptor that regulates expression of key genes in target organs. Among its many resulting functions are increased intestinal calcium and phosphate absorption, bone calcium mobilization, and renal reabsorption of calcium. The resultant increase in serum calcium and phosphate supports bone mineralization, curing rickets, and osteomalacia. There are many other functions of vitamin D that remain to be described that contribute to its health supporting role.