In a randomized, double-blind, placebo-controlled crossover trial from the University of Bonn, hypertensive adults who took 162 mg of quercetin daily from onion skin extract for six weeks saw their 24-hour systolic blood pressure fall by 3.6 mmHg compared with baseline. The paper, published in 2015 in the British Journal of Nutrition, found no effect in participants with normal pressure. Only those already in the hypertensive range benefited.
Quercetin is a flavonoid, a plant compound with antioxidant and anti-inflammatory properties. Onions are one of the richest food sources of it, concentrated especially in the papery outer skin.
What the bone research actually shows
The bone finding comes from a separate study by Matheson and colleagues, who analyzed data from the U.S. National Health and Nutrition Examination Survey (NHANES) 2003-2004. Perimenopausal and postmenopausal non-Hispanic white women aged 50 and older who ate onions once a day or more had bone mineral density about 5% higher than women who ate them once a month or less, according to a peer-reviewed review that cites the trial.
The same paper estimated that daily onion eaters could see their hip fracture risk fall by more than 20% compared with women who never ate onions, as summarized by NutritionFacts.org. Because the analysis is observational, it can show an association but not prove that onions caused the difference.
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A 12-week onion powder trial
A Japanese research team tested 9 grams of quercetin-rich onion powder daily against a quercetin-free placebo in 54 adults for 12 weeks. The study in Nutrients did not find a significant drop in visceral fat, the deep abdominal fat linked to heart disease, across the whole group. In a subgroup with lower HDL cholesterol, visceral fat area was significantly lower in the onion group, and liver enzyme ALT also fell. Four participants reported mild gastrointestinal symptoms, and no serious safety concerns were reported.
The heart mechanism, briefly
Quercetin can protect low-density lipoprotein (LDL) cholesterol from oxidation, an early step in the buildup of arterial plaque. Reviews of the flavonoid describe it as a strong antioxidant that scavenges free radicals and can bind metals, effects that in cell and animal work translate into slower LDL oxidation. Human evidence for hard cardiovascular endpoints such as heart attack and stroke is thinner and still developing.
Who should be careful
Onions are high in fructans, a fermentable carbohydrate in the FODMAP group. For people with irritable bowel syndrome or gastroesophageal reflux, both raw and cooked onions can trigger bloating, gas or heartburn by relaxing the lower esophageal sphincter. Anyone with new or worsening reflux, or persistent bloating, should raise it with their doctor before changing the diet.
Raw, cooked, or extract
Cooking method changes how much quercetin survives. Japanese laboratory work found that boiling onions caused about a 30% loss of quercetin glycosides, with the compound leaching into the cooking water. Frying and microwaving preserved more of it, according to research on flavonoid content after cooking.
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None of the trials used the amount of onion in a typical dinner. The blood pressure and Nutrients studies used concentrated skin extract or powder, which packs far more quercetin than the vegetable itself. The bone finding, by contrast, tracked real-world daily onion eaters over ordinary meals.
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This article is made and published by Jesper Bengtson, who may have used AI in the preparation.