Vitamin D plays a key role in regulating the stores of calcium and phosphorus in the body by controlling the absorption of these minerals from the intestine. When experiencing a deficiency of vitamin D young horses will absorb too little calcium and phosphorus to enable building of well mineralized, healthy bones, even when the ration contains adequate amounts of the minerals. In severe vitamin D deficiency, rickets will develop. In addition to its effects on mineral metabolism and bone, we now know that vitamin D has significance for the normal functioning of the immune system and a variety of cell functions. Along with vitamin A and E, Vitamin D is one of the group of fat-soluble vitamins and because it is fat-soluble, vitamin D reserves can be stored in body fat for several months.