The main role of vitamin D is to keep your bones healthy, primarily by helping you absorb intestinal calcium. In recent years, however, vitamin D deficiency has gained a lot of attention for being associated with a long list of diseases ranging from heart disease and cancer to diabetes mellitus, depression, dementia, colds, multiple sclerosis, and premature death (Click for Benefits of Vitamin D video).
As a result, many physicians, including myself, have been screening for vitamin D deficiency in people who are at risk—such as those who get limited sun exposure without sunscreen, have dark skin, are older, or obese—and have been recommending vitamin D supplementation.