Quercetin: what is it, benefits, side effects, dosage

It’s no secret that a diet rich in colorful fruits and vegetables can help you ward off a number of undesirable health conditions such as heart disease, diabetes, and cancer. But did you ever wonder where these plants got their seemingly magical abilities?

One type of health-promoting compound found in plants is called bioflavonoids. These are found naturally in many fruits and vegetables and help your body ward off free radicals and other compounds that can cause disease. One of the best-studied of these is a flavonoid called quercetin. 

Here’s what you should know about quercetin, including what it is and its potential benefits and side effects.

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Quercetin, a plant pigment, is a type of biologically active compound called a flavonoid. Flavonoids are often responsible for many plants' bright colors and medicinal properties. Quercetin belongs to a subset of flavonoids called flavonols that the human body can’t produce (Yang, 2020; Anand David, 2016). 

This compound is present in a wide variety of plants that are commonly eaten and used as medicine. You can also take additional quercetin as a dietary supplement (Yang, 2020; Anand David, 2016).

Clinical trials have shown that quercetin has powerful antioxidant properties. Antioxidants are substances that can protect your cells from unstable molecules called free radicals that can cause disease and accelerate aging (Yang, 2020; Anand David, 2016).

Free radical damage has been implicated in illnesses such as heart disease, diabetes, and some types of cancer. Quercetin is the most effective free radical scavenger in the flavonoid family (Yang, 2020; Anand David, 2016).

While more study is needed, quercetin and other flavonoids have been shown to have several potential positive health benefits.

Antimicrobial

Studies have shown that quercetin has a strong ability to slow the growth of both bacteria and fungi. The flavonoid quercetin stops the growth of certain bacteria by (Yang, 2020):

• Changing or destroying the cell wall of bacteria

• Affecting the way bacteria can produce and express proteins and enzymes

• Preventing bacteria from being able to attach to other cells 

• Stopping bacteria from being able to make DNA and RNA

Quercetin’s effect on fungi is not as strong as its effects on bacteria, but when combined with the anti-fungal medication amphotericin B, its activity is greatly improved (Yang, 2020).

Anti-inflammatory

Quercetin has been reported to be a long-lasting anti-inflammatory substance. These strong anti-inflammatory effects have been shown to affect several different types of cells in both animals and humans (Li, 2016).

Anti-allergy

Allergic disorders have been increasing worldwide, causing an increased demand for compounds that help to treat and prevent them. Recently, much attention has been focused on flavonoids such as quercetin due to their antioxidant and anti-inflammatory properties (Mlcek, 2016). 

Quercetin has been shown to have potentially beneficial effects on the immune system. It can prevent the release of histamine and other allergy-producing substances in the body. It has also been shown to stabilize the cell membrane of mast cells, which means that it can help treat mast cell-related disorders, such as asthma, sinusitis, and rheumatoid arthritis (Mlcek, 2016). 

Heart protective

Since as early as 1993, human studies have shown a correlation between how much flavonoids you consume in your diet and your risk of cardiovascular disease. Many experimental and human studies have shown that quercetin has vasodilating and anti-clotting properties that can potentially reduce your risk of heart disease (Serban, 2016).

There is some evidence that quercetin can help lower blood pressure. One study found significant effects of quercetin in lowering blood pressure compared to placebo, but more studies are needed to confirm the effects and the optimal dose. It’s possible that one day, quercetin might be used alongside traditional anti-hypertensive medications for added benefits (Serban, 2016).

Cancer-fighting 

Quercetin has been shown in animal studies to help treat and prevent the spread of cancer cells. These include lung, prostate, liver, breast, colon, skin, ovarian, and cervical cancers. Its anti-cancer properties come from its ability to help regulate the pathways that cancer uses to spread. The body also uses it to help encourage cancer cells to self-destruct (apoptosis) (Xu, 2019).

In addition, quercetin is an antioxidant. It helps to reduce oxidative stress that can damage the DNA in your body, helping to block one process that could otherwise lead to the development of cancer cells (Xu, 2019).

Quercetin is widely found in many fruits, vegetables, seeds, nuts, flowers, bark, and leaves. Some of the foods with the highest concentrations of quercetin include (Yang, 2020; Li, 2016; Anand David, 2016):

• Apples

• Berries

• Citrus fruits

• Brassica vegetables

• Capers (very high concentration)

• Grapes and red wine

• Onions and shallots (high concentration)

• Green tea

• Tomatoes

• Honey

• Olive oil

• Buckwheat

• Ginkgo biloba

Together with kaempferol (another flavonol), quercetin is the most widely abundant dietary flavonoid found in plant foods, although it is generally present in only low concentrations except for a few foods (Mlcek, 2016; Anand David, 2016)

Quercetin is generally considered to be very safe. Studies looking at 1 g/day for three months have not found any significant toxicity. Some of the side effects that have been reported include (Colunga Biancatelli, 2020):

• Headache

• Tingling sensations

• Nausea

• Vomiting

• Sweating

• Flushing

• Trouble breathing (with high dose intravenous administration)

• Kidney problems (with high dose intravenous administration)

It is important to remember that quercetin can change how your body processes other medications, sometimes making them less effective. Make sure to check with your healthcare provider before taking quercetin with prescription (or non-prescription) medications.

You should also not take additional quercetin supplements (the quercetin in food is fine) if you are pregnant or breastfeeding. We don’t have any information about whether this is safe or not (Colunga Biancatelli, 2020).

Since quercetin is a supplement, there are no Food and Drug Administration (FDA)-approved dosages to take. Most supplement brands are typically dosed at 500–1000 mg per day. 

Quercetin has been found to have low bioavailability. This means that only a small percentage of what you take is absorbed into your system. Because of this, you will often find quercetin combined with other supplements that can help increase the amount that your body absorbs (Li, 2016).

Remember always to let your healthcare provider know if you’re taking any supplements such as quercetin.

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Every article on Health Guide goes through rigorous fact-checking by our team of medical reviewers. Our reviewers are trained medical professionals who ensure each article contains the most up-to-date information, and that medical details have been correctly interpreted by the writer.

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Current version

December 13, 2021

Written by

Ellyn Vohnoutka, BSN, RN

Fact checked by

Steve Silvestro, MD

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About the medical reviewer

Steve Silvestro, MD, Ro

Dr. Steve Silvestro is a board-certified pediatrician and Associate Director, Clinical Content & Education at Ro.