J Am Vet Med Assoc, 1982. 181(11): p.1434-1436.
Vitamin D and parathyroid hormone play important roles in maintaining normal circulating concentrations of calcium and phosphorus. Vitamin D is obtained from dietary sources and produced endogenously from precursors in the skin. Ultraviolet irradiation of ergosterol, a sterol found in certain plants, produces vitamin D, (ergocalciferol). In the skin, ultraviolet light converts 7-dehydrocholesterol, a byproduct in the synthesis of cholesterol, to vitamin D(cholecalciferol).