Metabolism, function and clinical aspects of vitamin D.

Briefly reviewed herein are some of the contemporary findings on the metabolism of vitamin D, and the biochemical and physiological effects of this steroid in the animal. Certainly the most accepted major action of vitamin D is to enhance the intestinal absorption of calcium. Historically, there is also considerable evidence that the vitamin D is required for the resorption of calcium from bone, thereby aiding in maintaining normal serum calcium levels. Increasing evidence is becoming available that vitamin D does have a direct effect at the kidney level, and that the absorption and metabolism of the phosphate ion is also significantly affected by this steroid. As a consequence of vitamin D administration to the rachitic animal, some molecular changes in the intestine have been identified and these include the induction of the vitamin D dependent calcium binding protein, an increase in intestinal levels of alkaline phosphatase and calcium ATPase, and a stimulation of the adenylate cyclase system. A hallmark of recent efforts is a further understanding of the metabolism of vitamin D and the formation of its most active form, 1,25-dihydroxycholecalciferol. All of this knowledge will prove valuable in the rational treatment of certain abnormalities of calcium and bone metabolism for which examples are already available.