INS and type 2 diabetes mellitus: Various biological factors support the association between VDD and T2DM: (1) studies have found out that the VDR is prominently expressed in both pancreatic beta-cells that secrete insulin and in peripheral target tissues that respond to insulin, such as skeletal muscle and adipose tissue [62, 63]; (2) VDR gene polymorphisms in humans have been associated with variation in insulin secretion and sensitivity [64]; (3) animal studies show that mice with VDR mutations have impaired insulin secretion and poorer glucose tolerance than those with normal VDR [65].