The prevalence of type 2 diabetes mellitus continues to climb in many parts of the globe in association with the rise in obesity. Although the latter is clearly a predominant factor in the pathogenesis of type 2 diabetes, other modifiable lifestyle factors such as exercise, alcohol consumption, smoking, and certain nutritional factors, such as vitamin D deficiency, are also believed to play a role. In contrast to the findings of observational studies, information pooled from vitamin D intervention trials lack conclusive evidence in support of vitamin D supplementation and changes in diabetes risk or measures of glucose intolerance, although an effect on insulin resistance may exist. Well-designed trials that focus on intermediate biomarkers of diabetes risk in response to increased vitamin D intake are still needed. It will be important to include in the design of these studies selection of insulin-resistant study subjects who have a low (< 50 nmol/L) initial serum vitamin D (25-hydroxyvitamin D) status and administration of sufficient vitamin D to adequately increase their vitamin D status to > 75 nmol/L serum 25-hydroxyvitamin D.