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.