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One of the most available antioxidants, quercetin is a naturally occurring flavonoid found in many foods. Some people take it as a supplement because of its immune-boosting benefits. Its ability to regulate the body’s defense response makes it a good choice for reducing inflammation, treating allergy symptoms and preventing or treating cold and flu viruses.

What Is Quercetin?

Quercetin is a pigment that gives fruits, vegetables and grains their color. You’ll find quercetin in food like:

  • Cherries
  • Berries
  • Grapes
  • Red apples
  • Onions
  • Tomatoes
  • Broccoli
  • Kale

Quercetin is considered a flavonoid. Along with color, these chemicals are also responsible for antioxidant effects. This means quercetin benefits include an immune system boost that can help related conditions, such as autoimmune diseases. It can also help allergies, reduce inflammation and treat cold symptoms.

Although you probably get some in your diet, quercetin isn’t well-absorbed by the body. This is why some take it as a supplement in powder or capsule form. Combining quercetin with vitamin C or bromelain (digestive enzyme) can also maximize its effects.

Quercetin side effects are mild (headache or stomach ache) but can happen if you consume more than 1,000 mg per day. It’s considered generally safe to take, but always consult a dietician or physician before adding a new supplement to your routine.

Quercetin Immune System Benefits

Quercetin and immune system function are closely linked, allowing the flavonoid to fight off harmful free radicals, lowering your risk of diseases and inflammation.

Acts as an Antioxidant

The most well-known quercetin health benefits are its antioxidant effects. Antioxidants scour the body for free radicals, which are unstable atoms that roam freely.

Free radicals are necessary, but unhealthy activities like drinking or smoking can create too many. An overload of free radicals puts the body into a state of oxidative stress, where cells and tissues undergo damage. Oxidative stress is linked to chronic diseases like:

  • Alzheimer's
  • Cancer
  • Kidney diseases
  • Heart disease
  • Diabetes

Antioxidants put a stop to an accumulation of free radicals by restoring the body to a healthy balance. Quercetin specifically does this by removing reactive oxygen species (ROS), which resists oxidative damage, such as to the respiratory tract.

Although quercetin’s antioxidant powers are now well-researched, the flavonoid has been widely used in Traditional Chinese Medicine and botanical medicine for those same benefits. Studies have shown that its antioxidant effects allow it to protect against some cancer forms, tumors and lung and cardiovascular diseases.

Perhaps most impressive, quercetin bioflavonoids can hunt out two potentially dangerous and highly reactive radicals—peroxynitrite and hydroxyl. According to a review of research, quercetin’s unique ability to fight these two harmful chemicals may be what gives the flavonoid its protective properties. 

Provides Anti-Allergy Support

Some people also take quercetin for allergies. Flavonoids are known to have anti-allergic effects, but in particular, quercetin could be a front-runner. Quercetin’s effect on the immune systems, plus its antioxidant effects, make it a good supplement to take if you’re experiencing allergic reactions.

Your immune system creates histamines in response to threats. Once a threat is recognized, histamines trigger your body to expel them, such as through coughing or sneezing. For example, if you have a dust allergy, your immune system sees it as a threat and overreacts. That triggers an inflammatory response, including histamine, which causes you to sneeze or itch.

Quercetin can stop histamine release, minimizing allergic symptoms. There are a few other reasons it helps regulate immune responses:

  • Decreases pro-inflammatory cytokines—immune system proteins that cause inflammation
  • Suppresses a protein called interleukin IL-4, which is a hallmark of allergic inflammation
  • Stabilizes the reaction of various immune systems cells

In one study on mice, quercetin supplements suppressed severe allergic reactions to peanuts

Combined, this research suggests that quercetin may be beneficial for seasonal and food allergies, as well as asthma.

Reduces Inflammation

When our bodies are exposed to something harmful, the immune system produces an inflammatory response. Inflammation protects tissues from threats so that immune cells can remove them and begin healing.

Another way we develop inflammation is through oxidative stress, whereby the body produces inflammation in response to an overload of free radicals. Since quercetin’s antioxidant power combats free radicals, it can also help alleviate chronic inflammation caused by oxidative stress. Quercetin also regulates inflammation by inhibiting two inflammatory enzymes—cyclooxygenase and lipoxygenase—as well as other inflammation markers. 

In a 2008 study, researchers found that eating certain foods with flavonoids can lower the levels of c-reactive protein (CRP)—a marker linked to inflammatory diseases like lupus and rheumatoid arthritis, as well as heart disease. A 2019 review of research also found that flavonoids like quercetin have anti-inflammatory effects that are particularly beneficial for cardiovascular disease.

Quercetin’s natural ability to regulate both acute and chronic inflammation makes it an important nutrient for disease prevention and maintaining overall health.

Quercetin Uses

Since quercetin regulates the immune system, it’s often used to treat or manage conditions tied to inflammation, infections or allergies. The most common dose is 500 mg per day, but some people take as much as 1,000 mg.


Taking quercetin for eczema and allergies may lessen symptoms and act like a natural antihistamine.

Lab studies show that quercetin stops immune cells from releasing histamines. So, whether you’re eating quercetin-rich foods or taking a supplement, you may notice a reduction of allergic symptoms.

Although you should always seek medical care for severe reactions, you can try taking quercetin if you’ve accidentally eaten something that causes irritation. If you have seasonal allergies, it’s best to take a supplement regularly for at least six to eight weeks.

Cold and Flu

Promising research shows that taking a quercetin supplement may lessen your chance of cold and flu infection.

A 2016 study showed that quercetin prevented flu infection from a variety of viral strains. Another research review found that flavonoids, like quercetin, reduced the incidence of upper respiratory tract infections in healthy participants. 

In addition to preventing infections, quercetin might also help treat flu symptoms. In 2011, researchers found that quercetin prevented severe complications related to the H1N1 flu. While researchers don’t yet know what mechanisms in quercetin help reduce flu symptoms, they did conclude that the flavonoid’s combination of antiviral and antioxidant properties might be an effective treatment choice in severe cases.

Autoimmune Diseases

Autoimmune diseases are due to an overreaction of the immune system where it attacks healthy cells and creates inflammation. Because quercetin appears to have immuno-modulating and anti-inflammatory benefits, taking it as a supplement may help to manage certain autoimmune disorders.

A recent lab study compared the inflammation-suppressing properties of three flavonoids and found that quercetin showed the most promising results. Not only did quercetin demonstrate strong anti-inflammatory effects, it also helped to modulate the immune response of certain white blood cells.

In a 2016 study, authors looked at the effects of the flavonoid on women with rheumatoid arthritis. After taking 500 mg a day for eight weeks, the patients experienced significantly reduced morning stiffness, pain and after-activity pain. The number of patients with active disease also significantly decreased. 

The anti-inflammatory and immune-stabilizing effects of quercetin make it a promising supplement to help manage an overactive immune system.

Quercetin in Summary

Quercetin is one of the most abundant flavonoids, found in many fruits and vegetables. It can also be taken as a supplement for additional support.

Research shows that quercetin can fight free radicals, temper inflammation and stabilize immune responses. From managing allergies and autoimmune diseases to preventing cold and flu, quercetin is a beneficial supplement to include in your regular diet. 



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