Posted By: Elynor Moss on March 08, 2019 |
Vitamin D has 2 main forms:
D2 ( ergocalciferol)
D3 ( cholecalciferol): The naturally occurring form and the form used for low-dose supplementation
Vitamin D3 is synthesized in skin by exposure to direct sunlight (ultraviolet B radiation) and obtained in the diet chiefly in fish liver oils and salt water fish (see Table: Sources, Functions, and Effects of Vitamins). In some developed countries, milk and other foods are fortified with vitamin D. Human **** milk is low in vitamin D, containing an average of only 10% of the amount in fortified cow’s milk.
Vitamin D levels may decrease with age because skin synthesis declines. Sunscreen use and dark skin pigmentation also reduce skin synthesis of vitamin D.
Vitamin D is a prohormone with several active metabolites that act as hormones. Vitamin D is metabolized by the liver to 25(OH)D, which is then converted by the kidneys to 1,25(OH)2D (1,25-dihydroxycholecalciferol, calcitriol, or active vitamin D hormone). 25(OH)D, the major circulating form, has some metabolic activity, but 1,25(OH)2D is the most metabolically active. The conversion to 1,25(OH)2D is regulated by its own concentration, parathyroid hormone (PTH), and serum concentrations of calcium and phosphate.
Vitamin D affects many organ systems (see Table: Actions of Vitamin D and Its Metabolites), but mainly it increases calcium and phosphate absorption from the intestine and promotes normal bone formation and mineralization.
Vitamin D and related analogs may be used to treat psoriasis, hypoparathyroidism, and renal osteodystrophy. Vitamin D's usefulness in preventing leukemia and ****, prostate, and colon cancers has not been proved, nor has its efficacy in treating various other nonskeletal disorders (1) or preventing falls (2,3,4) in the elderly. Whether vitamin D is useful in preventing fractures in the frail or healthy elderly is under study (5).
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