Efficacy of vitamin D supplementation on glycaemic control in type 2 diabetes: An updated systematic review and meta‐analysis of randomized controlled trials

Abstract Aim To assess the effects of vitamin D interventions on glycaemic control in subjects with type 2 diabetes (T2D). Methods We searched PubMed, EMBASE, Web of Science and the Cochrane Library for relevant studies. Serum 25(OH)D, fasting blood glucose (FBG), HbA1c, fasting insulin and Homeostasis Model Assessment for Insulin Resistance (HOMA‐IR) were analysed. Results We identified 39 randomized controlled trials involving 2982 subjects. Results showed a significant decline in the vitamin D group, as shown by the FBG weighted mean difference (WMD; −0.49 [95% confidence interval {CI}: −0.69 to −0.28] mmol/L), HbA1c (WMD −0.30% [95% CI: −0.43 to −0.18]), HOMA‐IR (WMD −0.39 [95% CI −0.64 to −0.14]) and insulin (WMD −1.31 [95% CI: −2.06 to −0.56] μIU/mL). Subgroup analyses indicated that the effects of vitamin D supplementation on glycaemic control depend on the dosage and duration of supplementation, baseline 25(OH)D levels and the body mass index of patients with T2D. Conclusions Vitamin D supplementation can significantly reduce serum FBG, HbA1c, HOMA‐IR and fasting insulin levels in T2D patients; the effects were especially prominent when vitamin D was given in a short‐term, high dosage to patients with a vitamin D deficiency, who were overweight, or had an HbA1c of 8% or higher at baseline. Our study suggests that vitamin D supplements can be recommended as complementary treatment for T2D patients.