The effect of vitamin D supplementation on glucose metabolism in type 2 diabetes mellitus: A systematic review and meta-analysis of intervention studies

Aims We aimed to assess whether vitamin D supplementation improves glucose metabolism in adults with type 2 diabetes. Methods PubMed and Cochrane database were searched up to July 1st 2016 for randomized controlled trials that assessed the relationship between vitamin D supplementation and glucose metabolism (change in hemoglobin A1C (HbA1C) and fasting blood glucose (FBG)) among adults with type 2 diabetes. Results Twenty nine trials (3324 participants) were included in the systematic review. Among 22 studies included in the meta-analysis, 19 reported HbA1C, 16 reported FBG outcomes and 15 were deemed poor quality. There was a modest reduction in HbA1C (− 0.32% [− 0.53 to − 0.10], I² = 91.9%) compared to placebo after vitamin D supplementation but no effect on FBG (− 2.33 mg/dl [− 6.62 to 1.95], I² = 59.2%). In studies achieving repletion of vitamin D deficiency (n = 7), there were greater mean reductions in HbA1C (− 0.45%, [− 1.09 to 0.20]) and FBG (− 7.64 mg/dl [− 16.25 to 0.97]) although not significant. Conclusions We found a modest reduction of HbA1C after vitamin D treatment in adults with type 2 diabetes albeit with substantial heterogeneity between studies and no difference in FBG. Larger studies are needed to further evaluate the glycemic effects of vitamin D treatment especially in patients with vitamin D deficiency.