There is animal and human observational evidence to suggest that vitamin D and calcium are important in modifying t2DM risk but there are critical gaps in our knowledge about the clinical magnitude of the association with t2DM and potential mechanisms in humans. We are conducting a randomized trial to quantify the effect of vitamin D and calcium supplementation on beta-cell function, insulin sensitivity, glucose tolerance and systemic inflammation and other cardiometabolic outcomes in ambulatory adults at high risk for t2DM. We anticipate that the research proposed in this application is significant because it will provide the basis for defining feasible nutritional interventions that promotes prevention of t2DM. Based on the results of the proposed studies and future work in this area, vitamin D and calcium supplementation can assume an important role in the treatment of t2DM and in the prevention of the disease in the 41 million Americans who are at risk of developing t2DM.