Vitamin D3 (cholecalciferol) is formed in the skin of animals from irradiation of 7-dehydrocholesterol by sunlight. Two vitamin D3 metabolites (products of vitamin D3 metabolism) act on the intestines, bones, and kidneys to regulate calcium homeostasis (balance) and promote synthesis of calcium-binding proteins.
There are no documented cases of vitamin D deficiency in horses unless they are kept inside away from all sunlight. Rickets has occurred in foals fed vitamin D-deficient diets and kept in the dark. Vitamin D toxicity, on the other hand, involves calcification of soft tissues, blood vessels, heart, and/or kidneys due to bone resorption abnormalities. Supplementation of vitamin D, if needed, is recommended at 10% of the amount of supplemental vitamin A.« Back to Glossary Index