Any of a group of fat-soluble alcohol s important in calcium metabolism in animals to form strong bones and teeth and prevent rickets and osteoporosis .
It is formed by ultraviolet radiation (sunlight) of sterols (see steroid ) present in the skin. The most important of these sterols are 7-dehydrocholesterol, formed by metabolic processes in animals, and ergosterol, present in vegetable oils. The action of sunlight on the skin converts these two compounds, respectively, to cholecalciferol (vitamin D 3 ) and ergocalciferol (vitamin D 2 ). Vitamin D is added to margarine, milk, and cereals for the benefit of those who may not get enough sunlight in winter. As little as 5 micrograms each day appear adequate for children. Because the body cannot excrete it, prolonged high intake can cause a toxic reaction including fatigue, nausea, and abnormal calcium accumulation.