There are two forms of vitamin D that are important in humans: vitamin D2, which is made by plants, and vitamin D3, which is made by human skin when exposed to sunlight. Foods may be fortified with vitamin D2 or D3. Vitamin D is important because it maintains normal blood levels of calcium and phosphorus. It also helps the body absorb calcium, which forms and maintains strong bones. 15 minutes a day in the sun will get you the vitamin D levels that are recommended.