Vitamin D has been found to be involved in a variety of public health-significant diseases including bone diseases , diabetes , cardiovascular diseases , cancer , and metabolic syndrome . Due to its various extraosseous effects and the association between its deficiency and insulin resistance along with diabetes initiation, the serum vitamin D levels gain an extensive attention in the field of endocrinology . A large number of cross-sectional studies have shown a negative correlation between vitamin D status and T2DM prevalence [2, 6]. While several longitudinal studies in Europeans , African-Americans , South Asians , and in China  still demonstrated low levels of serum 25(OH)D which could predict the risk of type 2 diabetes, vitamin D supplementation also reduced the incidence of diabetes accordingly [7, 10]. Consistent with this, there was evidence to show that vitamin D deficiency is significantly higher in diabetic patients than in normal population in China . Recent studies find that vitamin D3 supplementation improved insulin secretion in diabetic patients  and show beneficial effects in diabetic patients with poor glycemic control . Maintaining adequate vitamin D level in the population will be an important strategy to reduce the incidence of diabetes. Many physicians have accepted that vitamin D can be a component of prescription for diabetic patients. However, before that, there is still an important question, in view of the potential hazard of excessive vitamin D3supplementation, what the level of vitamin D is sufficient for diabetic patients.