The Effect of Vitamin D Supplementation on Insulin Resistance and Progression from Prediabetes to Diabetes Mellitus: A Systematic Review/Meta-Analysis

Abstract Background Emerging evidence suggests that altered vitamin D and calcium homeostasis play a role in type 2 diabetes mellitus development. Binding of vitamin D to its receptors stimulates the expression of insulin receptors and activates glucose transporters. Additionally, vitamin D regulates calcium concentration in β-cells, which enhances insulin secretion. Objective To evaluate the effect of vitamin D supplementation on insulin resistance and glycemic control in prediabetes. Materials and Methods We searched databases including CENTERAL, Medline (PubMed), Web of Science, EMBASE and Egyptian Knowledge Bank (EKB) for randomized controlled trials (RCTs) comparing vitamin D or it’s analogues with placebo, RCTs published in English language until December 2022 were included, participants were adults (≥18 years old) with laboratory-confirmed diagnosis of prediabetes by fasting blood glucose (FBG) and/or 2-hour plasma glucose after an oral glucose tolerance test (OGTT) and glycosylated hemoglobin (HbA1c). A total of 3616 studies were retrieved, and 3603 were excluded due to different populations, exposures, or languages. Studies with different outcomes, cross-sectional studies, cohort studies, case reports, case series, and commentaries, along with studies conducted on animals, were also excluded. Thirteen randomized controlled trials were included in this systematic review using Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) for reporting. Results Included thirteen RCTs showed improvement in FBG (Pooled standardized mean difference (SMD): −1.239, 95% confidence interval (CI): −1.858, −0.621, P value: <0.001), HbA1c (SMD: −0.736, 95% CI: −1.225, −0.246, P value: 0.003) and, OGTT (SMD: −0.665, 95% CI: −1.237, −0.094, P value: 0.023) respectively. Regarding heterogeneity, random effect model was considered as per I2 statistics for each FBG (96.52%, P value: <0.001), HbA1c (94.52%, P value: <0.001) and, OGTT (95.00%, P value:<0.001) respectively. Conclusion Vitamin D supplementation reduced the risk of new-onset type 2 diabetes mellitus. Additionally, it might boost the rate at which prediabetes returns to normal.