Vitamin D: metabolism and biological actions.

In the last few years, it has become clear that the vitamin D endocrine system is comprised of many more target cells and tissues than were imagined a decade ago; in addition to the intestine, kidney, and bone, the vitamin D endocrine system now includes the beta cells of the pancreas, breast tissue, placenta, the pituitary gland, cells of the reticuloendothelial system, and several other cells and tissues. The complexity of the metabolic pathway by which the active metabolite(s) of vitamin D are produced and further metabolized has emerged, as has the complexity of its regulation.