Intestinal calcium absorption: a vitamin D-hormone-mediated adaptive response.

The process of intestinal calcium absorption represents the mechanism for dietary calcium to enter into the physiological processes that contribute both to the skeletal growth of the organism and to the maintenance of calcium homeostasis (both intracellular and extracellular). Because there is a large variation worldwide in the availability of dietary calcium (300-1500 g/d for man) and because there is a changing physiological need throughout life (growth, puberty, pregnancy, lactation, and menopause) for absorption of 50-500 mg/d of dietary calcium, it is essential that the process of intestinal calcium absorption be adaptable and responsive to both the dietary and physiological circumstances. This article reviews the evidence that this adaptation process is largely orchestrated by the vitamin D endocrine system. In this model the steroid hormone 1,25-dihydroxycholecalciferol has been shown to stimulate intestinal calcium absorption by both genomic (receptor mediated) and nongenomic (transcaltachia mediated) mechanisms.