Pure dietary flavonoids quercetin and (-)-epicatechin augment nitric oxide products and reduce endothelin-1 acutely in healthy men

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 F-2-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 F-2-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.