Background: Dietary flavonoids may improve endothelial function and ultimately lead to beneficial cardiovascular effects. Objective: The objective was to assess whether pure dietary flavonoids can modulate nitric oxide and endothelin-1 production and thereby improve endothelial function. Design: A randomized, placebo-controlled, crossover trial in 12 healthy men was conducted to compare the acute effects of the oral administration of 200 mg quercetin, (−)-epicatechin, or epigallocatechin gallate on nitric oxide, endothelin-1, and oxidative stress after nitric oxide production was assessed via the measurement of plasma S-nitrosothiols and plasma and urinary nitrite and nitrate concentrations. The effects on oxidative stress were assessed by measuring plasma and urinary F₂-isoprostanes. Plasma and urinary concentrations of quercetin, (−)-epicatechin, and epigallocatechin gallate were measured to establish the absorption of these flavonoids. Results: Relative to water (control), quercetin and (−)-epicatechin resulted in a significant increase in plasma S-nitrosothiols, plasma nitrite, and urinary nitrate concentrations (P < 0.05), but not in plasma nitrate or urinary nitrite. Epigallocatechin gallate did not alter any of the measures of nitric oxide production. Quercetin and (−)-epicatechin resulted in a significant reduction in plasma endothelin-1 concentration (P < 0.05), but only quercetin significantly decreased the urinary endothelin-1 concentration. None of the 3 treatments significantly changed plasma or urinary F₂-isoprostane concentrations. Significant increases in the circulating concentrations of the 3 flavonoids were observed (P < 0.05) after the corresponding treatment. Conclusions: Dietary flavonoids, such as quercetin and (−)-epicatechin, can augment nitric oxide status and reduce endothelin-1 concentrations and may thereby improve endothelial function.

Objective— Animal and clinical studies have suggested that polyphenols in fruits, red wine, and tea may delay the development of atherosclerosis through their antioxidant and anti-inflammatory properties. We investigated whether individual dietary polyphenols representing different polyphenolic classes, namely quercetin (flavonol), (−)-epicatechin (flavan-3-ol), theaflavin (dimeric catechin), sesamin (lignan), or chlorogenic acid (phenolic acid), reduce atherosclerotic lesion formation in the apolipoprotein E (ApoE) ^−/− gene–knockout mouse. Methods and Results— Quercetin and theaflavin (64-mg/kg body mass daily) significantly attenuated atherosclerotic lesion size in the aortic sinus and thoracic aorta ( P <0.05 versus ApoE ^−/− control mice). Quercetin significantly reduced aortic F ₂ -isoprostane, vascular superoxide, vascular leukotriene B ₄ , and plasma-sP-selectin concentrations; and augmented vascular endothelial NO synthase activity, heme oxygenase-1 protein, and urinary nitrate excretion ( P <0.05 versus control ApoE ^−/− mice). Theaflavin showed similar, although less extensive, significant effects. Although (−)-epicatechin significantly reduced F ₂ -isoprostane, superoxide, and endothelin-1 production ( P <0.05 versus control ApoE ^−/− mice), it had no significant effect on lesion size. Sesamin and chlorogenic acid treatments exerted no significant effects. Quercetin, but not (−)-epicatechin, significantly increased the expression of heme oxygenase-1 protein in lesions versus ApoE ^−/− controls. Conclusion— Specific dietary polyphenols, in particular quercetin and theaflavin, may attenuate atherosclerosis in ApoE ^−/− gene–knockout mice by alleviating inflammation, improving NO bioavailability, and inducing heme oxygenase-1. These data suggest that the cardiovascular protection associated with diets rich in fruits, vegetables, and some beverages may in part be the result of flavonoids, such as quercetin.