The
polyphenol quercetin may prevent CVD due to its
antihypertensive and
vasorelaxant properties. We investigated the effects of
quercetin after regular intake on blood pressure (BP) in
overweight-to-obese patients with
pre-hypertension and stage I
hypertension. In addition, the potential mechanisms responsible for the hypothesised effect of
quercetin on BP were explored. Subjects (n 70) were randomised to receive 162 mg/d
quercetin from onion skin extract
powder or placebo in a double-blinded, placebo-controlled cross-over trial with 6-week treatment periods separated by a 6-week washout period. Before and after the intervention, ambulatory blood pressure (ABP) and office BP were measured; urine and blood samples were collected; and endothelial function was measured by EndoPAT technology. In the total group,
quercetin did not significantly affect 24 h ABP parameters and office BP. In the subgroup of hypertensives,
quercetin decreased 24 h systolic BP by -3·6 mmHg (P=0·022) when compared with placebo (mean treatment difference, -3·9 mmHg; P=0·049). In addition,
quercetin significantly decreased day-time and night-time systolic BP in hypertensives, but without a significant effect in inter-group comparison. In the total group and also in the subgroup of hypertensives, vasoactive
biomarkers including
endothelin-1, soluble endothelial-derived adhesion molecules,
asymmetric dimethylarginine,
angiotensin-converting enzyme activity, endothelial function, parameters of oxidation,
inflammation,
lipid and
glucose metabolism were not affected by
quercetin. In conclusion, supplementation with 162 mg/d
quercetin from onion skin extract lowers ABP in patients with
hypertension, suggesting a cardioprotective effect of
quercetin. The mechanisms responsible for the BP-lowering effect remain unclear.