Vitamin D designates a group of closely related compounds that possess antirachitic activity. It may be supplied through the diet or by irradiation of the body.
The primary function of vitamin D is to elevate plasma calcium and phosphorus to a level that will support normal mineralization of bone as well as other body functions. Recently, evidence also suggests a regulatory role of vitamin D (1,25-(OH)2D) in immune cell functions, the release of insulin in relation to glucose challenge, and reproduction in both males and females.
Sources of vitamin D are feedstuffs, irradiation, sebaceous material licked from skin or hair or directly absorbed products or irradiation formed on or in the skin. For dogs and cats (and presumably other carnivores), vitamin D must be obtained from dietary sources due to the inability of these species to synthesize and utilize vitamin D from precursors in the skin.