[Bone and Nutrition. Vitamin D independent calcium absorption]

Vitamin D endocrine system is required for normal calcium and bone homeostasis. Trans-epithelial calcium absorption is initiated with calcium entry into the intestinal epithelial cells from luminal fluid through calcium permeable channels, and those expressions are strongly supported by vitamin D action. On the other hands, dietary treatment, mineral supplementation or restriction, successfully improves intestinal calcium absorption in global vitamin D receptor knock-out (VDR KO) mice, though vitamin D dependent active transport pathway is lacking. Dietary rescue of intestinal calcium absorption provided a positive calcium balance in this mouse model, and suggested that the major role of vitamin D function on calcium homeostasis was considered to be intestinal active absorption. To elucidate the entire process of intestinal calcium absorption, vitamin D independent calcium transport system was characterized into either trans-cellular or para-cellular process.