Vitamin D is naturally present in a few foods and produced in the body when ultraviolet rays from sunlight come into contact with the skin. This fat-soluble vitamin is also available as a dietary supplement. The principal biological function of vitamin D is to maintain levels of calcium and phosphorus in the blood, aiding the absorption of calcium, and helping to form and maintain strong bones. Recent research findings have suggested that vitamin D also may provide protection from osteoporosis, high blood pressure, some cancers, and several autoimmune diseases, including diabetes. The authors provide this update.