The hypotheses of the influence of vitamin D on the risk of progression from prediabetes to Type 2 diabetes mellitus (T2DM) were based on in vivo and in vitro studies that demonstrated the capability of pancreatic Beta-cells to synthesize VDR and 1 alpha-hydroxylase, thus producing 1,25(OH)2D locally. This evidence concerns the gene VDR and type 2 diabetes mellitus.