INS and type 2 diabetes mellitus: A 2017 systematic review of 29 trials investigating vitamin D supplementation in adults with T2D reported no impact on fasting blood glucose and only a moderate reduction in glycosylated hemoglobin.51 Furthermore, a systematic review and meta-analysis that examined vitamin D3 supplementation found no significant impact on insulin resistance, insulin secretion, glycosylated hemoglobin, or the prevention of T2D.52