INS and obesity due to melanocortin 4 receptor deficiency: Although several studies have shown a relationship between vitamin D deficiency and insulin resistance, one recent study has revealed that there was no relationship between vitamin D and insulin resistance when hyperinsulinemic-euglycemic clamp test, which is considered as a gold standard in peripheral insulin sensitivity, was performed the same study also stated that vitamin D deficiency did not directly affect development of insulin resistance but that the deficiency was associated with obesity(22).