The effects of vitamin D supplementation on indices of glycemic control in Iranian diabetics: A systematic review and meta-analysis

This systematic review and meta-analysis aimed to assess the effects of vitamin D supplements on indices of glycemic control [homeostatic model assessment-insulin resistance (HOMA-IR), hemoglobin A1C (HbA1C), fasting blood glucose (FBG), and quantitative insulin-sensitivity check index (QUICKI) and lipid profile in diabetic patients. Eight databases were searched, for randomized controlled trials (RCTs) or cross-sectional and cohort studies that have been published up to December 2017. We used the comprehensive meta-analysis (CMA) software for all statistical analysis and used the I2 index for assessing heterogeneity. A p value of <0.05 was considered as statistically significant. We found 621 articles, and after the exclusion of ineligible publications, 82 studies remained to be assessed of which 37 were used for meta-analysis. Vitamin D supplementation was associated with a significant improvement in FBG (p = 0.001 and 95% CI: −0.526 to −0.136) and HbA1C (p = 0.003 and 95% CI: 1.719 to −0.361) in individuals with type 2 diabetes mellitus (T2DM); while in women with gestational diabetes mellitus (GDM) the reduction in FBG (p = 0.071 and 95% CI: −0.873 to −0.035) and HbA1C (p = 0.199 and 95% CI: 3.270 to 0.681) failed to reach statistical significance. Treatment with vitamin D supplements was associated with an improvement in HOMA-IR in pregnant diabetic women (p = 0.028 and 95% CI: 0.924 to −0.053) and for individuals with diabetes mellitus (p = 0.005 and 95% CI: 1.772 to −0.319). The pooled result of the cross-sectional meta-analysis indicated that serum vitamin D concentrations were significantly lower in diabetic patients than in healthy controls (p = 0.018 and 95% CI: 0.587 to −0.054). This meta-analysis suggests that vitamin D supplementation improves indices of glycemic control (FBG, HOMA-IR, and HbA1C) in patients with diabetes mellitus. Hence, vitamin D supplements may be of potential therapeutic value in diabetic patients, as an adjuvant therapy along with other treatments.