Effects of a quercetin-rich onion skin extract on 24 h ambulatory blood pressure and endothelial function in overweight-to-obese patients with (pre-)hypertension: a randomised double-blinded placebo-controlled cross-over trial

Randomised, double-blinded, placebo-controlled crossover trial in overweight-to-obese adults with pre-hypertension or stage I hypertension recruited from the community in Bonn, Germany. For the quercetin arm, 35 participants were randomised and 34 completed and were analysed; allocation was 1:1 versus placebo. Participants had BMI 25 to 35 kg/m^2 and largely had metabolic syndrome traits.

Participants received a quercetin-rich onion skin extract orally at 162 mg quercetin per day, given as three capsules daily with the principal meals for 6 weeks. Each quercetin capsule contained 54 mg quercetin derived from onion skin extract powder; the placebo capsules contained mannitol. The trial used a 6-week washout between crossover periods.

Quercetin did not significantly improve 24 h ambulatory blood pressure, office blood pressure, endothelial function, or inflammatory and oxidative markers in the total study population. In the hypertensive subgroup, quercetin lowered mean 24 h systolic BP by -3.6 (SD 8.2) mmHg from baseline (P = 0.022) and the treatment difference versus placebo was -3.9 (SD 11.1) mmHg (P = 0.049). Day-time systolic BP decreased by -4.6 (SD 9.0) mmHg (P = 0.014) and night-time systolic BP decreased by -6.6 (SD 9.9) mmHg (P = 0.007). No adverse events were reported during either treatment.

The overall sample was small, and the main blood pressure benefit appeared only in an exploratory hypertensive subgroup analysis. The crossover periods were short at 6 weeks each, and the mechanism for any blood pressure effect remained unclear because endothelial, inflammatory, oxidative, and ACE-related markers did not change. Generalizability is limited to overweight-to-obese adults with pre-hypertension or stage I hypertension.