[Recent advances on vitamin D (author's transl)].

Our knowledge of vitamin D metabolism has undergone major advances within the last few years. The vitamin D3 produced by the skin or absorbed across the intestine is converted successively to 25-hydroxyvitamin D3 by the liver and to 1,25-hydroxyvitamin D3 (1,25-(OH)2D3) by the kidney. The latter metabolite is also the most active on three types of target tissue: gut, bone and kidney. The mechanism of action of 1,25-(OH2D3 has been investigated mostly in the intestinal tract, where it increased calcium absorption by stimulating the synthesis of such transfer proteins as the calcium binding protein (Ca BP). Although the level of 1,25 (OH)2D3 is regulated by vitamin D intake, it depends mainly on renal hydroxylation, the limiting step which controls 1,25-(OH)2D3 production. Both parathyroid hormone and calcium or phosphorus depletion stimulate renal 1-alpha-hydroxylase; 1,25-(OH)2D3 is also regulated directly or indirectly (through its action on the parathyroid glands) by a feedback system. In some diseases such as hepatic insufficiency and chronic renal failure, a profound impairment in endogenous synthesis of the biologically active metabolites of vitamin D results in sever calcium and skeletal disese. During the last decade, the therapeutic possibilities in vitamin D deficiency have been greatly improved by the synthesis of new derivatives of vitamin D and by a better knowledge of their bioavailability