Diabetes is a serious condition affecting about 1 in 10 people in the United States. Lifestyle changes, such as healthy eating, exercise and weight loss, can decrease the chances of developing diabetes. However, many people still develop diabetes despite trying to change their lifestyle. The goal of this study is to see whether taking vitamin D can lower the risk of getting diabetes in people with a high risk for diabetes. As a food ingredient or dietary supplement, vitamin D is generally recognized as safe. However, we do not know if vitamin D is effective in preventing diabetes. Taking vitamin D to prevent diabetes is considered experimental.