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Vitamin D3 (cholecalciferol)

The vitamin D or calciferols arise from their provitamins through a cleavage, catalysed by sunlight, of the B-ring in the sterane rings. With adequate exposure to sunlight, dietary supplements are not normally required. However, not all individuals are exposed to the adequate levels of sunlight, especially in the winter. Vitamin D3 is a derivative of provitamin D3 (7-dehydrocholesterol), the immediate biological precursor of cholesterol. Their most important representatives are vitamin D3 (cholecalciferol) and vitamin D2 (ergocalciferol), which differ slightly only in the side chains, but which are similarly metabolised and have identical biological effects. Whereas provitamin D2 must be taken in with the food, the provitamin D3 can be formed in the human organism. Vitamin D3 is well known to play a vital role in the metabolism of calcium, by promoting intestinal absorption of calcium and phosphorus, maintaining adequate serum levels of calcium and phosphorus and stimulating mobilisation of calcium from the bone fluid compartment in the presence of parathyroid hormone. Vitamin D3 is a critical element in the metabolism of calcium, promoting intestinal absorption of calcium and phosphorus, maintaining adequate serum levels of calcium and phosphorus, and stimulating flux of calcium into and out of bone. Vitamin D3 analogues have been recognized as having important biological activities. It is well established that these biological functions of vitamin D require metabolism to hydroxylated metabolites which are the biologically active agents. Vitamin D3 is metabolized in human to 25-hydroxyvitamin D3 and then to 1.alpha.,25-dihydroxyvitamin D3, and it is the latter compound, in particular, that is responsible for the stimulation of calcium transport in intestine and the liberation of calcium from bone mineral. Vitamin D3 deficiency, or disturbances in the metabolism of vitamin D3 cause such diseases as rickets, renal osteodystrophy and related bone diseases, as well as, generally, hypo- and hyper-calcemic states. Vitamin D3 and its metabolites are therefore crucial in maintaining normal development of bone structure by regulating blood calcium levels.

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