Last updated by Dr Sarah Brewer on August 9th, 2018

Quercetin is the most abundant bioflavonoid in most people’s diet, and is found in a wide range of foods from capers and rocket/arugula leaves, to onions, berries, beans and buckwheat. Quercetin is also found in red wine and green tea and is an active ingredient of many herbal medicines such as Ginkgo biloba and St John’s Wort. At the end of this post is a list of foods that are naturally high in quercetin to help boost your intakes naturally.

Quercetin health benefits

Quercetin is a powerful antioxidant that reduces inflammation. Quercetin also activates a gene, SIRT1, which regulates cell metabolism and the rate at which fat is burned in the liver, in fat cells and in muscles.

Although quercetin is a less powerful activator of SIRT1 than resveratrol, quercetin appears to mimic some of the effects of SIRT1 using a different mechanism. Quercetin directly stimulates the growth and division of mitochondria, which are the tiny cell ‘batteries’ in which fat is burned to produce energy. This effect of quercetin increases weight loss and exercise tolerance, reduces insulin resistance and can improve glucose control and hormone balance in people with metabolic syndrome and polycystic ovaries.

Quercetin and weight loss

Quercetin has many beneficial effects on energy metabolism to suppress fat formation, boost fat burning and help speed weight loss. Quercetin switches off some genes needed for fat storage, and switches on other genes so that levels of SIRT1 protein (which are coded for by the SIRT1 gene) increase by around 60%.

Quercetin is also involved in suppressing appetite and reducing energy intake to support weight loss and the maintenance of a normal weight.

Quercetin has been shown to prevent diet-induced obesity in mice but surprisingly, no clinical trials have investigated the effects of quercetin supplements in supporting a weight loss diet in humans.

However, the good news is that all these beneficial effects can occur at quercetin intakes that occur with good dietary intakes and account for many of the benefits of the Mediterranean diet and the DASH (Dietary Approaches to Stop Hypertension) diet.

In fact, quercetin levels equivalent to those produced from good dietary intakes suppress the formation of triglycerides within fat cells by as much as 42%. Higher intakes are needed to suppress the laying down of fat in mature fat cells, however.

Quercetin and blood pressure

High quercetin intakes, including from supplements, can reduce blood pressure in people with hypertension. By activating SIRT1, quercetin helps blood vessels become more reactive, so they can dilate when needed to reduce blood pressure. Quercetin also has a unique effect on the kidneys to reduce sodium reabsorption and fluid retention in a similar way to some diuretic drugs.

One study found that quercetin supplements (150mg per day) reduced systolic blood pressure (the upper figure) by an average of 3.7mmHg in people with high blood pressure.

Another study found that higher doses of quercetin (730mg per day for 28 days) lowered blood pressure by 7/5 mmHg compared with placebo, but only in people with existing hypertension. It does not affect blood pressure control in those with a normal blood pressure.

Quercetin and heart disease

As a powerful antioxidant, quercetin protects circulating ‘bad’ non-HDL-cholesterol from the oxidation damage that triggers atherosclerosis.

A good intake of dietary flavonoids, including quercetin, helps to reduce hardening and furring up of the arteries (atherosclerosis) that can lead to heart attack or stroke.

Research suggests that much of the protection against atherosclerosis is due to quercetin activating SIRT1 in the cells lining blood vessel walls to reduce the inflammation that would otherwise trigger atherosclerosis (hardening and furring up of the arteries).

Quercetin also appears to reduce the formation of foam cells (cholesterol-stuffed macrophages) that cause the accumulation of cholesterol plaques.

Quercetin and diabetes

Quercetin is under investigation as a potential diabetes treatment, as it interacts with cells in the small intestine to regulate glucose uptake, with the pancreas to improve insulin release, with muscle and fat cells to reduce insulin resistance, and within the liver to regulate the production of new glucose.

Quercetin also increases the number and activity of cell mitochondria so that more glucose is burned to produce heat and energy rather than stored as fat.

Quercetin and polycystic ovary syndrome (PCOS)

Because of its effects in reducing insulin resistance, quercetin is helpful for women with PCOS. A study involving 84 women with PCOS found that taking 1g quercetin (two 500 mg capsules) daily for 12 weeks was more effective than placebo in improving insulin resistance and the hormonal profile with improvements in testosterone and luteinising hormone levels.

Quercetin and allergies

Quercetin can suppress allergic reactions in a number of ways, including reducing histamine release, the formation of IgE antibodies, and restoring normal immune responses away from abnormal allergic reactions towards those that fight infection.

Within the skin, quercetin can reduce symptoms of atopic dermatitis, contact dermatitis and eczema.

Research suggests that quercetin is more effective than the drug, cromolyn, in stabilising mast cells that release histamine and trigger allergic symptoms. Many people find quercetin helpful in reducing allergy symptoms such as hayfever, mold allergy and histamine intolerance.

Quercetin and immunity

Quercetin has an anti-viral action that inhibits the replication of common cold viruses, and reduces hypersensitivity of airway cells which may help to reduce exacerbations in people with asthma and chronic obstructive pulmonary disease (COPD).

Quercetin is a popular supplement among athletes to support immunity during periods of intense exercise and heavy training. When 40 elite cyclists took either 1g quercetin or placebo a day for 3 weeks to cover intensive cycle training for a race, only 1 of the 20 taking quercetin developed a cold in the two weeks after the race, compared with 9 out of the 20 taking placebo.

Quercetin and cancer

Quercetin and other flavonoids contribute to the health benefits of eating more fruit and vegetables for cancer prevention. Research suggests that people who have a frequent intake of quercetin-rich foods have a lower risk of developing lung cancer than those who eat them infrequently.

In the laboratory, quercetin can block the growth of cancer cells derived from breast, colon, prostate, ovarian, womb, liver and lung cancers and leukaemia cells. The way in which quercetin protects against cancer is still unclear, but it appears to suppress the proliferation of abnormal cells by inducing the natural way in which abnormal cells are identified and destroyed (apoptosis).

Quercetin supplements

Quercetin supplements are widely available and often contain quercetin derived from red vine leaves which are a particularly rich source. Quercetin that is described as ‘activated’ means it is combined with other ingredients such as other bioflavonoids, vitamin C or bromelain to increase its absorption and effectiveness.

Supplements providing quercetin dihydrate have the best bioavailability followed by quercetin glycosides, quercetin aglycone and quercetin rutinoside.

Quercetin has been safely used in amounts up to 1 gram daily (500 mg twice a day) in studies that lasted for 12 weeks. It is not known if longer-term use or larger amounts are safe, however.

Quercetin foods

Fruit and vegetables are the main dietary sources of quercetin. Average dietary intakes of all bioflavonoids, including quercetin, is relatively low, at around 13mg per day, but are easily increased by eating more quercetin rich foods.

The most common dietary sources of quercetin are onions, apples, tea and dark berries. An average red onion provides over 30mg quercetin, for example, while a typical red delicious apple, eaten with the skin, provides at least 5mg quercetin.

Heating, especially boiling in water, significantly reduces the amount of quercetin available for absorption. Aim to eat quercetin-rich fruit and vegetables either raw or only lightly cooked.

Quercetin rich foods list

Sources: Various including Phenol-Explorer 

Quercetin side effects

As a natural component of many fruit and vegetables, quercetin is considered safe and is well tolerated at usual dietary intakes.

In those taking high doses there have been a few reports of headache, gastrointestinal effects and bruising although it is not known if quercetin was responsible.

Very high doses may have toxic effects on the kidneys.

Image credits: daria_yakovleva/pixabay; nattika/shutterstock; phakpoom_mahawat/shutterstock;