Vitamin D, absorbed in the small intestine with the assistance of bile salts, is necessary for the intestines to absorb calcium. Vitamin D plays a major role in regulating the metabolism of calcium and phosphorus.
This kind of vitamin has two forms : D2, called ergocalciferol; and D3, called cholecalciferol. D2 is a synthetic fortified form of Vitamin D. D3 is the natural form of Vitamin D that is produced in the skin by ultraviolet sunlight.
Once absorbed, it is converted into calcifediol in the liver and then converted to an activated form of calcifediol in the kidneys. The active form is a hormone that combines with parathyroid hormone (PTH) and calcitonin to regulate calcium and phosphorus metabolism in the body and stimulate the reabsorption of calcium and phosphorus by bone. When serum levels of calcium are low, more Vitamin D is used to create the active form of calcifediol.
Low serum levels of calcium cause a decrease in the creation of the active form of calcifediol. Excess Vitamin D is then excreted in bile and a small amount is excreted in urine.