The effect of vitamin D supplementation on oxidative stress parameters: A systematic review and meta-analysis of clinical trials.

Recent studies have shown that vitamin D supplementation beneficially decreases oxidative stress (OS) parameters; however, there is no consensus on this subject. In this systematic review and meta-analysis we aimed to investigate the effect of vitamin D supplementation on OS parameters. We searched electronic databases including Medline, Embase, Scopus, Web of Science and Cochrane Library up to July 2018 without any language restriction. Trials were reviewed using preferred reporting items for systematic reviews and meta-analyses (PRISMA) and Cochrane handbook. To compare the effects of vitamin D with placebo, standardized mean difference (SMD) with 95% confidence intervals (CI) were pooled using random effects model. Across thirteen clinical trials, we found that vitamin D supplementation increased serum levels of total antioxidant capacity (TAC) (SMD: 0.54 mmol/L, 95% CI: 0.29 to 0.79; I = 65.4%, P = 0.001) and glutathione (GSH) (SMD: 0.33; 95% CI: 0.11 to 0.54, P = 0.003; I = 61.2%, P = 0.001). Besides, malondialdehyde (MDA) concentration decreased significantly (SMD: -0.40 mmol/L, 95% CI: -0.60 to -0.21, P < 0.001) following vitamin D supplementation compared to placebo. However, its effects on nitric oxide (NO) was not significant (0.17, 95% CI: -0.10 to 0.45, I = 71.1%, P = 0.21). Finally, we showed that vitamin D supplementation can improve OS parameters; however, this finding may not be clinically significant. Further large prospective clinical trials are needed to confirm the effect of vitamin D supplement on other OS parameters.