Taking onion skin-derived food supplements of the flavonol quercetin can lower blood pressure for people with hypertension, according to a new study published in the British Journal of Nutrition.

Researchers from the University of Bonn in Germany conducted a double-blind study on 68 overweight or obese volunteers with pre-hypertension or stage 1 hypertension, giving them either a quercetin-rich onion skin extract supplement or a placebo for six weeks.

While the subjects who were not classed as having high blood pressure did not show significant changes, those with high blood pressure saw positive results.

“Supplementation with 162 mg/d quercetin from onion skin extract lowers [ambulatory blood pressure] in patients with hypertension, suggesting a cardioprotective effect of quercetin. The mechanisms responsible for the [blood pressure]-lowering effect remain unclear,” wrote the study’s authors.

Quercetin is an antioxidant commonly found in plants, including onions, kale, apple skins, berries, citrus fruits and tea.

“As demonstrated in cohort studies, dietary intake of flavonoids in general and of quercetin in particular is associated with a decreased risk for [cardiovascular disease]. Although the physiological mechanisms accounting for this benefit remain incompletely defined, animal studies and human intervention studies have identified many relevant effects — for example, supplementation of quercetin may reduce platelet aggregation and plasma concentrations of atherogenic oxidised LDL (oxLDL),” the authors wrote.

Participants with hypertension saw their systolic blood pressure fall an average of 3.6 mmHg, which the authors suggested “would be clinically meaningful when considered at the population level, particularly in view of the large population of people with pre-hypertension and stage I hypertension”.

The dose of 162 mg per day of quercetin-rich onion skin extract, in the form of three 54 mg capsules, was identified as being approximately 10 to 15 times the normal daily quercetin intake for people in the European Union. The study excluded people who consumed high levels of quercetin in their normal diets.

Mechanism of action unclear

While the researchers tried to identify how quercetin acted to reduce blood pressure as part of the study, none of the mechanisms investigated appeared to be responsible. The three main mechanisms covered in the study were quercetin’s effects on vascular endothelial function, angiotensin-converting enzyme (ACE) activity, and lipid and glucose metabolism, but none of them showed a link to the fall in blood pressure.

The authors also noted they could not rule out effects from other elements of the onion skin extract supplement: “We characterised the polyphenol spectrum of the extract, but we cannot exclude that other unknown components in the onion skin extract may have influenced our findings. Thus, strictly, the conclusion of our study is only true for quercetin-rich onion skin extract but not for pure quercetin.”

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