Does it work? Can quercetin act as an antioxidant?


BACKGROUND:Quercetin is found in a wide variety of fruits and vegetables, and increasingly in many types of food supplements. One independent database found more than 2,300 supplements that list quercetin as one of their ingredients. Quercetin is usually promoted as a natural antioxidant to overcome the negative effects of oxidants in the body.

It belongs to the class of plant compounds called flavonoids, common pigments in the rinds and barks of various plants. Foods and beverages that are rich in quercetin include black and green tea, red wine, red onions, apples, citrus fruits, broccoli, leafy green vegetables and berries.

Quercetin supplements have become widely promoted as beneficial for a number of health issues. The most common claims are that as an antioxidant, quercetin can prevent the damage that leads to many chronic illnesses, including heart disease, inflammatory conditions and cancer.


The vast majority of the studies conducted on quercetin were done with cell cultures and animals. Normal body metabolism leads to waste products which include reactive oxygen species and other compounds causing oxidation. These have some beneficial effects, but in excess lead to damage. Such damage can play a role in the development of different chronic illnesses.

To counteract these effects, antioxidants are produced by the body and consumed in the diet. Numerous types of antioxidants exist, each helping to counteract the effects of different oxidants. This is part of the reason why a variety of antioxidants from different foods is recommended in a balanced diet.

Quercetin has been tested in the lab and shown to have excellent antioxidant activity. It has over six times the antioxidant effect of vitamin C. It also has anti-inflammatory and other potentially beneficial effects.

However, very few studies have examined the effects of quercetin in humans. Two studies in healthy volunteers found that people given quercetin supplements (in one case, 1 gram/day) had elevated blood levels of quercetin, but levels of various disease markers did not change. This suggested that healthy people might not benefit from supplements. According to one recent review, only two studies have been conducted with unhealthy people. One examined the impact of quercetin (730mg/day) on blood pressure.

Those with very mildly elevated blood pressure showed no change, but those with stage 1 hypertension had reduced blood pressure. Another study gave quercetin to patients with a chronic inflammatory lung disease. Their blood antioxidant levels were increased, with those with the lowest initial levels benefiting the most. However, the study did not examine whether their lung symptoms were changed.


Some people experience headaches and tingling in the fingers and toes after taking quercetin orally. Higher doses have been given intravenously, which can lead to more serious side effects. No long-term studies have been conducted on the safety of quercetin supplementation. This is problematic because quercetin might be used for many years by people with chronic conditions.


One of the ironies of interest in “natural” remedies is how quickly it can lead to the use of “unnatural” formulations. The evidence clearly points to the general health benefits of consuming a variety of fruits and vegetables. Yet in seeking those benefits, many are opting for quercetin supplements which have not yet been demonstrated to have the same benefits.

At the moment, the potentially beneficial effects have been demonstrated only in laboratory studies. Even if these are confirmed in clinical studies, they may not provide the range of benefits available from the many different antioxidants found in fruits and vegetables.

Dónal OMathúna has a PhD in pharmacy, researching herbal remedies, and an MA in bioethics, and is a senior lecturer in the School of Nursing, Dublin City University.

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