Vitamin D is a fat-soluble vitamin that is naturally present in very few foods. Vitamin D obtained from sun exposure, food and supplements. Vitamin D is essential for promoting calcium absorption in the gut and maintaining adequate serum calcium and phosphate concentrations to enable normal mineralization of bone and prevent hypocalcemic tetany, without sufficient vitamin D, bones can become thin, brittle, or misshapen, leading to rickets in children and osteomalacia in adults. Together with calcium, vitamin D also helps protect older adults from osteoporosis. It is available in tablet, capsule or liquid form.