Effects of quercetin supplementation on glycemic control among patients with metabolic syndrome and related disorders: A systematic review and meta-analysis of randomized controlled trials

This systematic review and meta-analysis of randomized controlled trials was performed to determine the effect of quercetin supplementation on glycemic control among patients with metabolic syndrome and related disorders. Databases including PubMed, MEDLINE, EMBASE, Web of Science, and Cochrane Central Register of Controlled Trials were searched until August 30, 2018. Nine studies with 10 effect sizes out of 357 selected reports were identified eligible to be included in current meta-analysis. The pooled findings indicated that quercetin supplementation did not affect fasting plasma glucose (FPG), homeostasis model of assessment-estimated insulin resistance, and hemoglobin A1c levels. In subgroup analysis, quercetin supplementation significantly reduced FPG in studies with a duration of >= 8 weeks (weighted mean difference [WMD]: -0.94; 95% confidence interval [CI; -1.81, -0.07]) and used quercetin in dosages of >= 500 mg/day (WMD: -1.08; 95% CI [-2.08, -0.07]). In addition, subgroup analysis revealed a significant reduction in insulin concentrations following supplementation with quercetin in studies that enrolled individuals aged <45 years (WMD: -1.36; 95% CI [-1.76, -0.97]) and that used quercetin in dosages of >= 500 mg/day (WMD: -1.57; 95% CI [-1.98, -1.16]). In summary, subgroup analysis based on duration of >= 8 weeks and used quercetin in dosages of >= 500 mg/day significantly reduced FPG levels.