Abstract
The previous meta-analysis of clinical trials revealed a beneficial effect of vitamin E supplementation on serum C-reactive protein (CRP) concentrations; however, it is unknown whether this vitamin has the same influence on other inflammatory biomarkers. Also, several clinical trials have been published since the release of earlier meta-analysis. Therefore, we aimed to conduct a comprehensive meta-analysis to summarize current evidence on the effects of vitamin E supplementation on inflammatory biomarkers in adults. We searched the online databases using relevant keywords up to November 2019. Randomized clinical trials (RCTs) investigating the effect of vitamin E, compared with the placebo, on serum concentrations of inflammatory cytokines were included. Overall, we included 33 trials with a total sample size of 2102 individuals, aged from 20 to 70 years. Based on 36 effect sizes from 26 RCTs on serum concentrations of CRP, we found a significant reduction following supplementation with vitamin E (− 0.52, 95% CI − 0.80, − 0.23 mg/L, P < 0.001). Although the overall effect of vitamin E supplementation on serum concentrations of interleukin-6 (IL-6) was not significant, a significant reduction in this cytokine was seen in studies that used α-tocopherol and those trials that included patients with disorders related to insulin resistance. Moreover, we found a significant reducing effect of vitamin E supplementation on tumor necrosis factor-α (TNF-α) concentrations at high dosages of vitamin E; such that based on dose–response analysis, serum TNF-α concentrations were reduced significantly at the dosages of ≥ 700 mg/day vitamin E (Pnon-linearity = 0.001). Considering different chemical forms of vitamin E, α-tocopherol, unlike other forms, had a reducing effect on serum levels of CRP and IL-6. In conclusion, our findings revealed a beneficial effect of vitamin E supplementation, particularly in the form of α-tocopherol, on subclinical inflammation in adults. Future high-quality RCTs should be conducted to translate this anti-inflammatory effect of vitamin E to the clinical setting.
Highlights
Abbreviations CRP C-reactive protein IL-6 Interleukin 6 tumor necrosis factor-α (TNF-α) Tumor necrosis factor-α DM Diabetes mellitus cardiovascular diseases (CVDs) Cardiovascular disease NASH Nonalcoholic steatohepatitis NAFLD Non-alcoholic fatty liver disease HD Hemodialysis MetS Metabolic syndrome Randomized clinical trials (RCTs) Randomized clinical trial
33 eligible RCTs remained for inclusion in the current systematic review and meta-analysis[11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43]; out of them, 26 studies assessed serum concentrations of C RP11,13–20,22,25–31,33–35,37,38,40–43, 14 studies assessed serum concentrations of IL-617,20–23,26–31,33,37,38, and 12 studies evaluated serum concentrations of tumor necrosis factor-α (TNF-α) following vitamin E supplementation[12,17,20,21,24,29,30,31,32,36,38,39]
In the current meta-analysis, we found that vitamin E supplementation can exert a significant reducing effect, around 0.52 mg/L, on serum levels of CRP in adults
Summary
Abbreviations CRP C-reactive protein IL-6 Interleukin 6 TNF-α Tumor necrosis factor-α DM Diabetes mellitus CVD Cardiovascular disease NASH Nonalcoholic steatohepatitis NAFLD Non-alcoholic fatty liver disease HD Hemodialysis MetS Metabolic syndrome RCT Randomized clinical trial. Some clinical trials have shown a significant reducing effect of vitamin E supplementation on serum concentrations of C-reactive protein (CRP) and interleukin-6 (IL-6)[26,37,40], while others did not find any significant effect[17,19,27]. A meta-analysis in 2014 revealed a beneficial effect of vitamin E coated dialyzer on subclinical inflammation; authors in that meta-analysis included only clinical trials that performed on hemodialysis patients[45]. The current meta-analysis was conducted to summarize current evidence on the effects of vitamin E supplementation on inflammatory biomarkers in adults
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