What is Quercetin and what are its health benefits?

Quercetin is an antioxidant and belongs to a group of plant pigments called flavonoids. It is found naturally in many fruits, vegetables, flowers, bark, and leaves but is not made in the human body. Studies show quercetin may help to protect against many health conditions associated with oxidative stress, including heart disease, rheumatoid arthritis and cancer.

Antioxidants neutralize free radicals which can damage our cells, alter DNA and cause cell death. Free radical molecules are often a by-product of pollution, cigarette smoke, sunlight, alcohol consumption, and other environmental toxins. Like other antioxidants, quercetin may help to protect against oxidative stress and inflammation. According to Rice and colleagues, quercetin is relatively more potent compared to other well-known antioxidants like vitamin C or E.

Most people get quercetin from their diet, but quercetin supplements are also available at retail stores without a prescription. When taken as a dietary supplement, experts are not sure if quercetin has the same antioxidant properties against disease as seen in the laboratory or from the plant source.

Researchers have suggested that quercetin, when used as a dietary supplement, may be beneficial against inflammation that may contribute to diseases such as cardiovascular disease, cancer, allergies, diabetes, stomach ulcers, high blood pressure, and to help boost the immune system. Adequate, randomized, controlled trials are lacking for most of these uses.

Other names for quercetin are: pentahydroxyflavone, Quercetine, and Vitamin P.

10 health benefits of quercetin

Conditions where quercetin may possibly have a positive health effect include:

• Allergies
• Asthma
• Heart disease
• High cholesterol
• High blood pressure
• Rheumatoid arthritis
• Gastritis / Stomach Ulcers
• Cancer
• Prostatitis
• Alzheimer’s disease

For most uses, randomized, controlled studies in humans are still needed to validate findings.

Allergies / asthma

In lab studies, quercetin has been shown to prevent cells from releasing histamine, which could have a beneficial effect in conditions such as seasonal and year-round allergies (runny nose, itchy eyes, hives and swelling).

Studies have shown quercetin inhibits the release of histamine from mast cells; however, here is no evidence yet that it works in humans as an antihistamine.

Heart disease, high cholesterol, high blood pressure

It has been shown that the risk for cardiovascular disease is lessened in those who follow a plant-based diet like the Mediterranean Diet, which is abundant in fruit, vegetables, olive oil, and moderate consumption of red wine.

When studying heart disease, most human studies have looked at the effect of flavonoids like quercetin from food and not in dietary supplements. Some studies have suggested flavonoids like quercetin may reduce the build-up of plaque in arteries that can lead to heart disease, heart attack or stroke.

Quercetin may also protect against damage from LDL (“bad”) cholesterol, lower blood pressure and inhibit platelet aggregation.

• In a 6-week study, quercetin given at 150 mg/day was reported to reduce systolic blood pressure and LDL levels in overweight subjects at high risk of heart disease.
• More studies are needed to determine if the man-made supplement can have the same effect.

Rheumatoid arthritis (RA)

In animal studies, quercetin has been shown to inhibit both acute and chronic inflammation and exhibited significant anti-arthritic action.

• Some reports have noted that in a vegan diet (with high levels of quercetin in uncooked berries, fruits, vegetables, nuts, roots, seeds and sprouts), fewer arthritis symptoms were seen.
• However, there is no direct evidence that the effects were due to antioxidants or that quercetin supplements can treat RA.

Stomach ulcers and gastritis

Quercetin has been shown to inhibit gastric acid secretion, oxidation of gastric cells, and Helicobacter pylori, lending a gastroprotective effect.

• Suzuki and colleagues studied the antioxidant and antiulcer effects of quercetin in ethanol-induced gastric acid injury in animal models and found it had favorable anti-ulcer activity.

Cancer

Scientists have long considered a diet high in fruits and vegetables as having a protective effect against cancer. Researchers have noted that quercetin has potential anticancer properties which may include antioxidant properties and slowing of tumor growth.

Laboratory and animal studies suggest quercetin and other flavonoids may have anticancer properties in breast, colon, prostate, ovarian, endometrial and lung cancer.

• In one small study, Cruz-Correa and colleagues studied the effects of curcumin (found in turmeric) and quercetin supplements in patients with familial adenomatous polyposis (precancerous colorectal polyps).
• Patients received curcumin 480 mg and quercetin 20 mg orally 3 times a day for 6 months. At the end of the study period, a 60% reduction in the number and a 50% reduction in the size of ileal and rectal polyps was seen. Minimal side effects were reported.

Prostate gland inflammation (prostatitis)

A small, placebo-controlled study reported preliminary evidence that quercetin might reduce symptoms of prostate gland inflammation. More research is needed.

Alzheimer’s disease

Oxidative stress and free radical development can contribute to certain neurological diseases like Alzheimer’s disease.

• In a diet-based, 24-week, randomized, double-blind, placebo-controlled study from 2021, healthy Japanese individuals 60 to 79 years old consumed quercetin-rich onion (vs. a group consuming quercetin-poor onion).
• Results showed that 24-week continuous intake of quercetin-rich onion reduced age-related cognitive decline.
• However, more research is needed to determine if quercetin-rich onions can have a significant role in the management of Alzheimer’s disease.

Does quercetin improve sports performance?

Several studies have evaluated the use of quercetin supplements on sports performance, as noted by Up to Date. Study results have been mixed.

• One study did find a small improvement in exercise output in untrained individuals. Other studies have found no benefit.
• Some studies have suggested improvements during resistance training, lowering of muscle soreness, reduction of inflammation and improved endurance.

Where does Quercetin come from?

Quercetin (3,3',4',5,7-pentahydroxyflavone) is one of the most important bioactive flavonoids. It is found in multiple fruits, vegetables, plants and herbs. Food sources with a high content of quercetin include onions and citrus are fruits.

Quercetin may also be found in Ginkgo biloba, milk thistle, American elder, St. John’s Wort, and mulberry. These plants grow in various regions around the world, ranging from China, India, Europe, and North and South America.

One brand of Quercetin supplement (Pure Encapsulations) contains quercetin derived from the Fava d’anta tree (Dimorphandra mollis) seed pods, a tree often found in Brazil. Quercetin in oral supplements may be derived from different sources.

The name quercetin is derived from the Latin word “Quercetum” which means Oak Forest. It is yellow in color and insoluble in water but soluble in alcohol. It is noted to be one of the most common bioflavonoids used for metabolic and inflammatory disorders worldwide.

Related Questions

• What are antioxidants and should you take supplements?
• How long does a rheumatoid arthritis flare-up last?
• What are the 4 stages of rheumatoid arthritis?

Which foods contain quercetin?

Fruits and vegetables are the primary sources of dietary quercetin, particularly citrus fruits and onions. It is found in varying amounts in:

• shallots (dried)
• apples
• asparagus
• dill
• onions
• dark berries (cherries, blueberries, blackberries, bilberries)
• nuts and seeds
• tea
• citrus fruits
• cabbage, cauliflower
• kale
• parsley
• red wine
• sage
• tomatoes
• capers
• cranberry
• olive oil
• green beans
• grapes
• coffee beans

How does quercetin work?

As a flavonoid, quercetin may prevent injury caused by free radicals through its scavenging of reactive oxygen species (ROS) and antioxidant properties. Other proposed mechanisms for flavonoids or quercetin include: metal chelating, reduction of alpha-tocopheryl radicals, inhibition of oxidases, lowered stress caused by nitric oxide, and reduction of C-reactive protein levels.

In addition, quercetin has been shown to inhibit inflammatory enzymes cyclooxygenase (COX) and lipo-oxygenase, which may help to decrease prostaglandins and leukotrienes, compounds that may lead to inflammation.

The true health benefits of quercetin supplements are not yet fully known, as adequate studies have not been completed in humans and more well-controlled, randomized studies are needed.

How safe is quercetin?

Side effects

The most common side effects with quercetin are headache and upset stomach, but it appears to be well-tolerated in most people. There is not enough evidence to recommend quercetin for use in children.

Quercetin is generally recognized as safe (GRAS) when consumed as food. If you are pregnant, breastfeeding or have kidney disease, you should avoid quercetin supplements or speak to your healthcare provider first before using it. If you are being treated for cancer, talk to your oncologist before taking quercetin.

Dosing

Oral supplemental doses up to 1,000 mg per day for up to 12 weeks showed no evidence of toxicity. However, data on long-term safety at high doses are lacking, and concerns regarding cancer-causing potential remain unresolved.

High doses of quercetin (over 1,000 milligrams per day) have been reported to cause kidney damage. Animal studies suggest a potential risk of tumor promotion in estrogen-dependent tumors. Also, there is a concern that a byproduct of quercetin may lead to a loss of protein function.

High doses given intravenously to cancer patients have led to vomiting, high blood pressure, kidney toxicity, and reduction in serum potassium.

Although specific evidence to support dosing recommendations is limited, most clinical studies use quercetin 500 mg to 1,000 mg per day in divided doses. Few studies have evaluated side effects in humans using a dose of 1,000 milligrams per day, which is significantly higher than typical dietary intake.

Drug interactions

Quercetin can be associated with drug interactions, including blood thinners like warfarin, aspirin or clopidogrel (Plavix), and some antibiotics. Avoid the coadministration of quercetin with the cardiac glycoside digoxin.

Tell your doctor and pharmacist about all of the prescription, over-the-counter and dietary supplements you use so that a drug interaction review can be performed.

Where to buy quercetin?

Quercetin supplements can usually be found in many pharmacies, retail grocery stores or nutrition shops, such as Walgreens, Walmart, Target or GNC stores. It comes in tablet or capsule forms in various strengths (commonly 250 mg or 500 mg). These doses may be much higher than those consumed in the diet.

Quercetin supplements may be found on the shelves combined with other ingredients such as vitamin C, vitamin D, zinc, bromelain (pineapple enzyme) or probiotics, so be sure to check labels for added products.

Beware of purchasing supplements online and from unverified online pharmacies, as these supplements may contain or toxic ingredients.

As with all dietary supplements, quercetin has not been evaluated by the FDA for safety or effectiveness. Dietary supplements do not need to be approved by the FDA before they are marketed in the U.S. This supplement is not intended to diagnose, treat, cure, or prevent any disease.

Dietary supplement products containing the USP Verified Mark on the package have met standards for product quality and consistency. Ask your pharmacist if you need help finding a USP Verified product. USP Verified status is not available for all dietary supplements.

Related: Drugs.com User Reviews for Quercetin for Herbal Supplementation

Bottom Line

• Quercetin is a common flavonoid found in a wide range of fruits and vegetables. It is an antioxidant and a component of a healthy diet. Consuming abundant daily fruits and vegetables with quercetin can help you to naturally maintain this bioflavonoid in your diet. It is also available as an over-the-counter oral dietary supplement in tablet or capsule form.
• Due to its antioxidant and free radical scavenger properties, quercetin has been studied in a variety of conditions such as heart disease, arthritis and some cancers, but further research is needed to determine its full benefits and side effects.
• As with any dietary supplement, consult with your healthcare provider before using this product, especially if you are pregnant, breastfeeding, undergoing surgery, giving to a child, have chronic medical conditions or take other medications.