Absorption Metabolism and Excretion of Vitamin D

Vitamin D (vitamin D without a subscript represents either vitamin D2 or D3) is fat soluble and, therefore, once ingested vitamin D2 and vitamin D3 are incorporated into the chylomicron fraction and absorbed in the small intestine into the lymphatic system. Both dietary vitamin D2 and vitamin D3, and cutaneous vitamin D3 enter the circulation and are bound to a specific a1-globulin known as the vitamin D-binding protein. It is believed that this protein acts as a buffering system whereby it helps maintain circulating concentrations of 25(OH)D so that the free unbound form of 25(OH)D can enter into the renal tubular cells to be metabolized.

Neither vitamin D2 nor vitamin D3 possess any intrinsic biologic activity on calcium metabolism. They both require a hydroxylation on carbon 25 to form 25(OH)D (Figure 4). 25(OH)D is the major circulating form of vitamin D and at physiologic concentrations it has little biologic activity on calcium metabolism. It must undergo a hydroxylation

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