Vitamin D supplementation and glycemic control in type 2 diabetes patients: A systematic review and meta-analysis

Introduction. Low vitamin D status has been found to be associated with impaired glycemic control in patients who suffer from type 2 diabetes; however, whether vitamin D supplementation is associated with improved glycemic status remains controversial. The aim of this study was to summarize evidence from randomized controlled trials (RCTs) to assess the efficacy of vitamin D supplementation in reducing glycosylated haemoglobinAlc (HbA1c) and fasting blood glucose (FBG) levels. Materials/Methods. We searched PubMed, Web of Science and the Cochrane Library for reports published up to March 2017. We selected parallel RCTs investigating the effect of vitamin D or vitamin D analogues on HbA1c or FBG levels in type 2 diabetes patients. Cohen's.c1 was calculated to represent the standardized mean difference (SMD) for each study, and the SMDs with 95%confidence intervals (CIs) were pooled using a random effects model. Results. Twenty-four studies were included that evaluated HbA1c levels and 18 studies were included that evaluated FBG levels. Meta-analyses showed that vitamin D supplementation was associated with reduced HbA1c levels (standardized mean difference (SMD)-0.25 [-0.45 to-0.05]) but had no influence on FBG levels (SMD-0.14 [-0.31 to 0.03]). However, the subgroup analyses suggested that vitamin D supplementation was associated with reduced HbA1c levels (SMD-0.39 [-0.67 to-0.10]) and FBG (SMD-0.27 [-0.46 to-0.07]) among patients with 25-hydroxyvitamin D (25(OH) D) deficiency at baseline. Significantly reduced HbA1c levels were also observed in association with vitamin D supplementation in the subgroup including type 2 diabetes patients with a body mass index (BMI) <30 kg m(-2) (SMD-0.30 [-0.54 to-0.07]). Conclusions. Vitamin D supplementation could be effective at improving glycemic control in vitamin D deficient or non-obese type 2 diabetes patients. (C) 2017 Elsevier Inc. All rights reserved.