[Vitamin D metabolism, regulation, function]

In a review of the literature metabolism, regulation and function of vitamin D and its active metabolites is discussed. Vitamin D is absorbed from the gut and produced in the skin. 25-hydroxylation takes place in the liver, producing 25-hydroxy-vitamin-D. This metabolite is transferred and hydroxylated in the kidney. The most potent cholecalciferol is 1,25-dihydroxy-vitamin-D. This hydroxylation process is regulated by the need of phosphorus and calcium and by calcicotropic hormones, e. g. parathyroid-hormone. The active cholecalciferols regulate intestinal, renal and osseous handling of calcium and phosphorus by binding to a specific receptor protein. Thus, the control of serum calcium and -phosphate homeostasis by cholecalciferols guarantees adequate supply of these substances for mineralisation. Accordingly, perinatal period and puberty is associated with a marked increase in active vitamin D metabolites.