Numerous studies have reported that VD insufficiency or deficiency increases the risk of MetS [127] and that VD supplementation improves outcomes in patients, including improving insulin sensitivity and β-cell function, decreasing fasting glucose level, insulin, and hemoglobin A1c (HbA1c), increasing HDL level and reducing LDL and TG, and reducing the incidence of type 2 diabetes [128]. This evidence concerns the gene INS and type 2 diabetes mellitus.