vitamin Dchemical compound

any of a group of fat-soluble vitamins important in calcium metabolism in animals. It is formed by ultraviolet radiation (sunlight) of sterols present in the skin.

The most important of these sterols are 7-dehydrocholesterol, formed by metabolic processes in animals, and ergosterol, present in vegetable oils. The action of sunlight converts these two compounds, respectively, to cholecalciferol (vitamin D₃) and ergocalciferol (also called calciferol or vitamin D₂). Vitamins D₂ and D₃ are equivalent in human metabolism, but in birds vitamin D₂ is much less effective than D₃, which therefore is preferred in the formulation of poultry-feed supplements.

After the vitamins are formed from the sterols, further chemical reactions occur first in the liver and then in the kidneys, changing them into hormones that participate in absorption of calcium into the bloodstream and formation of bone. It is not usually essential that vitamin D be taken in food because of synthesis from exposure of skin to sunlight. However, winter sunshine in northern regions and sunlight that has passed through certain kinds of glass, clouds, or the contaminated air of cities may lack sufficient amounts of ultraviolet rays to bring about adequate production of the vitamin. Under these conditions, supplementary vitamin D must be ingested. In the United States and Canada it is customarily added to milk. Vitamin D is one of the most potent vitamins. Adequate intake of vitamin D in the daily diet for children and most adults up to age 50 is considered to be 200 IU (International Units; for vitamin D, 200 IU is the equivalent of 5 μg). For adults age 51 to 70, adequate intake is 400 IU (10 μg), and for adults over 70 it is 600 IU (15 μg). However, the dramatic rise in the number of children and adults with vitamin D deficiency in the late 20th and early 21st centuries, especially in countries in northern latitudes, has prompted reevaluation of vitamin D intake guidelines.

Lack of vitamin D causes rickets in children and osteomalacia in adults. These diseases are characterized by poorly calcified, softened bones. Unlike the water-soluble vitamins, a surplus of vitamin D in the body is not eliminated in the urine but remains in the body, sometimes reaching toxic levels, a condition called hypervitaminosis D. An individual experiencing vitamin D poisoning may complain of weakness, fatigue, loss of appetite, nausea, and vomiting. In infants and children there may be growth failure. Because vitamin D is involved in the intestinal absorption and mobilization of calcium, this mineral may reach abnormally high concentration in the blood (hypercalcemia) and the widespread deposition of calcium phosphate throughout the body, particularly in the kidneys. Toxic manifestations have been observed in adults receiving 50,000 to 100,000 IU (1,250 to 2,500 μg) of vitamin D daily and in infants on relatively low daily intakes of 2,000 to 4,000 IU (50 to 100 μg). Excessive exposure to sunlight does not lead to vitamin D toxicity. Treatment includes discontinuing the use of the vitamin.

Vitamin D may play a role in protecting against cancer, most notably against colorectal cancer. Both vitamin D and a component of bile called lithocholic acid (LCA)—a substance implicated in colorectal cancer that is produced during the breakdown of fats in the digestive tract—bind to the same cellular receptor. Binding of either substance to the receptor results in increased production of an enzyme that facilitates the metabolism and detoxification of LCA. Thus, the presence of vitamin D in sufficient levels results in increased production and activity of the enzyme, essentially priming it for efficient detoxification of LCA.