BackgroundQuercetin, the most abundant dietary flavonol, has antioxidant effects in cardiovascular disease, but the evidence regarding its effects on blood pressure (BP) has not been conclusive. We assessed the impact of quercetin on BP through a systematic review and meta‐analysis of available randomized controlled trials.Methods and ResultsWe searched PUBMED, Cochrane Library, Scopus, and EMBASE up to January 31, 2015 to identify placebo‐controlled randomized controlled trials investigating the effect of quercetin on BP. Meta‐analysis was performed using either a fixed‐effects or random‐effect model according to I2 statistic. Effect size was expressed as weighted mean difference (WMD) and 95% CI. Overall, the impact of quercetin on BP was reported in 7 trials comprising 9 treatment arms (587 patients). The results of the meta‐analysis showed significant reductions both in systolic BP (WMD: −3.04 mm Hg, 95% CI: −5.75, −0.33, P=0.028) and diastolic BP (WMD: −2.63 mm Hg, 95% CI: −3.26, −2.01, P<0.001) following supplementation with quercetin. When the studies were categorized according to the quercetin dose, there was a significant systolic BP and diastolic BP‐reducing effect in randomized controlled trials with doses ≥500 mg/day (WMD: −4.45 mm Hg, 95% CI: −7.70, −1.21, P=0.007 and −2.98 mm Hg, 95% CI: −3.64, −2.31, P<0.001, respectively), and lack of a significant effect for doses <500 mg/day (WMD: −1.59 mm Hg, 95% CI: −4.44, 1.25, P=0.273 and −0.24 mm Hg, 95% CI: −2.00, 1.52, P=0.788, respectively), but indirect comparison tests failed to significant differences between doses.ConclusionsThe results of the meta‐analysis showed a statistically significant effect of quercetin supplementation in the reduction of BP, possibly limited to, or greater with dosages of >500 mg/day. Further studies are necessary to investigate the clinical relevance of these results and the possibility of quercetin application as an add‐on to antihypertensive therapy.

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Currently, experimental and clinical evidences showed that polyphenols-rich natural products, like nutraceuticals and food supplements, may offer unique treatment modalities in type 2 diabetes mellitus (DM), due to their biological properties. Natural products modulate the carbohydrate metabolism by various mechanisms, such as restoring beta-cells integrity and physiology, enhancing insulin releasing activity, and the glucose using. Sea buckthorn berries, red grapes, bilberries, chokeberries and popular drinks like cocoa, coffee and green tea are all rich in polyphenols and may decrease the insulin response, offer in g a natural alternative of treatment in diabetes. Therefore, researches are now focused on potential efficacies of different types of polyphenols, including flavonoids, phenolic acids, lignans, anthocyans and stilbenes. Animal and human studies showed that polyphenols modulate carbohydrate and lipid metabolism, decrease glycemia and insulin resistance, increase lipid metabolism and optimize oxidative stress and inflammatory processes. It is important to understand the proper dose and duration of supplementation with polyphenols-rich extracts in order to guide effective therapeutic interventions in diabetic patients.

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This meta-analysis of RCTs showed a significant effect of H. sabdariffa in lowering both SBP and DBP. Further well designed trials are necessary to validate these results.