Known as the sunshine vitamin, one of the key roles of vitamin D is maintaining serum calcium and phosphorous balance. The body makes vitamin D by converting vitamin D2 to D3, or cholecalciferol, when exposed to sunlight. Vitamin D3 is also the form which the body derives from dietary cholesterol. While vitamin D is available in both forms as supplements, studies have found vitamin D3 is the preferable form, as it has been found to maintain active vitamin D levels for a longer period of time.