HEALTH AND MEDICAL PRODUCTS:Vitamin D.

Vitamin D

While researching a solution to the bone disease called rickets in 1922, E. Mellanby discovered vitamin D. In the United States, the enrichment of milk with this vitamin was, and continues to be, extremely effective against rickets, which is the faulty mineralization of bones and teeth in growing young children, producing soft bones and conditions known as “bowlegs” and “knockknees.” Normally, a vitamin is defined as a substance that is essential for the maintenance of life-sustaining metabolic body processes yet is not synthesized by the body on its own. The single ex- ception to this rule is vitamin D, which can be synthesized in the skin, but only when exposed to direct sunlight (ultraviolet light). Vitamin D refers to a group of chemically distinct steroids that exhibit qualitatively the biological activity of calciol [also called cholecalciferol; vitamin D3; (3�,5Z,7E)-9,10-secocholesta-5,7,10(19)-trien-3-ol; C27H44O]. Stable when exposed to heat, light, acids, alkalis, and oxidation, vitamin D is a fat-soluble compound concentrated in the liver, skin, and kidneys that is essential for calcium and phosphorus metabolism in animals. Vitamin D is therefore important for normal mineralization of bone and cartilage, neuromuscular functioning, tooth formation, and blood clotting. The biologically active form of the hormone is 1a,25-dihydroxycholecalcif- erol, also called calcitriol. Active calcitriol is derived from ergosterol (pro- duced in plants) and from 7-dehydrocholesterol (produced in the skin). After ultraviolet irradiation, ergosterol is converted to ergocalciferol (vitamin D2) in plants, and 7-dehydrocholesterol is converted to cholecalciferol (vitamin D3) in the skin. The same enzymatic pathways in the body then process vitamins D2 and D3 to D2-calcitriol and D3-calcitriol, respectively. Cholecalciferol (or ergocalciferol) is then absorbed from the intestine and transported to the liver for further chemical modification. For example, in the liver, cholecalciferol is hydroxylated to form 25- hydroxy-D3, which is the major blood-circulating form of vitamin D in the body. Subsequent conversion of 25-hydroxy-D3 to calcitriol (its bio- logically active form) occurs in the kidneys and bone tissues. Calcitriol functions in concert with both parathyroid hormone and the hormone calcitonin to regulate blood serum calcium and phosphorus levels. Major dietary sources of vitamin D include fish liver oils, egg yolk, butter, and fortified milk.