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Vitamin D (VD for short) is a fat-soluble vitamin. It is a cyclopentane polyhydrophenanthrene compound. It is a group of vitamins that are structurally related to sterols and can prevent rickets in function. The most important ones are vitamin D3 and D2. The former is formed by ultraviolet radiation of 7-dehydrocholesterol under human skin. The latter is formed by ultraviolet radiation of ergosterol contained in plants or yeast. The main function of vitamin D is to promote the absorption of calcium and phosphorus by small intestinal mucosal cells. The absorption of calcium ions in the intestine requires a calcium-binding protein. 1,25-dihydroxyvitamin D3 can induce the synthesis of this protein and promote the absorption of Ca2+. It can also promote the renewal of calcium salts and the formation of new bone, as well as the absorption of phosphorus and renal tubular cells. The reabsorption of calcium and phosphorus can increase blood calcium and phosphorus concentration, which is conducive to new bone formation and calcification.
In addition, vitamin D also promotes the growth and differentiation of skin cells and regulates immune function. Adults who are often exposed to sunlight will not cause deficiency diseases. Infants, pregnant women, nursing mothers and the elderly who do not often go outdoors should increase the daily supply of vitamin D to 10μg (equivalent to 400 international units). Children who lack vitamin D can suffer from rickets, and adults suffer from osteomalacia.