A recent study summarized evidence from randomized controlled trials (RCTs) in an effort to “assess the efficacy of vitamin D supplementation” on decreasing glycosylated haemoglobinA1c (HbA1c) and fasting blood glucose (FBG) levels. Included in the evaluation were 24 studies that assessed HbA1c levels, along with 18 studies that assessed fasting blood glucose. Utilizing a systematic method of statistical analysis to integrate data from a number of independent studies, it was observed that “vitamin D supplementation was associated with reduced HbA1c levels (standardized mean difference (SMD) -0.25 [-0.45 to -0.05])”. No association was observed in the meta-analysis between vitamin D and FBG. However, when subgroup analysis was performed, vitamin D supplementation was associated with both reduced HbA1c levels (SMD -0.39 [-0.67 to -0.10]) and reduced FBG (SMD -0.27 [-0.46 to -0.07]) in patients initially having deficiency levels of vitamin D. Vitamin D supplementation was also correlated to “significantly reduced HbA1c levels, which included type 2 diabetes patients.+
Chunhua Wu, Shanhu Qiu, Xiangyun Zhu, Ling Li. Vitamin D supplementation and glycemic control in type 2 diabetes patients: A systematic review and meta-analysis. Metabolism Clinical and Experimental. August 2017 73: 67–76.