Quercetin is a naturally occurring plant compound (flavonoid) that has gained growing attention in scientific research due to its potential role in supporting overall health. Found in a wide variety of fruits, vegetables, herbs, and teas, quercetin is best known for its antioxidant and anti-inflammatory properties, which may contribute to improved immune function and protection against chronic disease.

Studies suggest that quercetin may offer several health benefits, including its possible ability to help reduce inflammation, support cardiovascular health by improving blood pressure regulation, and assist with blood sugar control. Research has also explored quercetin’s potential protective effects on the brain, its possible anti-cancer activity, and even its role in supporting exercise performance and recovery.

This powerful flavonoid is commonly found in foods such as onions, apples, berries, grapes, tomatoes, leafy greens, and tea. It is also available as a dietary supplement, sometimes combined with nutrients such as vitamin C, zinc, or bromelain for enhanced absorption and immune support.

In recent years, quercetin has attracted even more interest because of its potential antiviral properties. Some laboratory studies suggest that quercetin may interfere with viral replication and help regulate immune responses, making it an intriguing compound for immune health research. During the COVID-19 pandemic, quercetin was also studied for its possible supportive role in immune defence, although more large-scale human trials are still needed to confirm its effectiveness.

In this article, we’ll explore what quercetin is, the science-backed benefits associated with it, the foods that contain it, and what research suggests about its potential role in immune function and viral infections. We’ll also cover safe dosage guidelines and key precautions to consider before using quercetin supplements. Let’s discuss!

What is Quercetin?

Quercetin is a flavonoid that is found in many fruits and vegetables and is beneficial to our overall health and disease resistance due to its unique biological properties (Davis et al. 2009).  Flavonoid, which is generally described as a flavonol, is a water-soluble pigment that cannot be made by the human body and, as such, like water-soluble vitamins, must be consumed via our diet daily (Kumar et al. 2017). 

Research purports several benefits of quercetin, which include its ability to improve mental health and physical performance, reduce infection and help the body resist the development of diseases.  As such, it is reported to be anti-carcinogenic, anti-inflammatory, and anti-viral as well as acting as an antioxidant, thus helping the body to get rid of free radicals, which in and of themselves can lead to the development of chronic diseases. 

The quercetin molecule is widely distributed in plants and can be found in a variety of foods such as berries, apples, grapes, onions, green peppers, asparagus, tomatoes, and teas, especially green and black tea, as well as in some nuts and seeds.  It is also found in some medicinal plants such as the famed Ginkgo Biloba and St John’s-wor,t among others (Hakkinen et al. 1999; Wiczkowski et al. 2008; Williamson and Manach, 2005).  

As it pertains to onions, the purple or red variety is said to have a higher concentration of quercetin than the white or yellow kind.  Further, the highest concentration of this compound is found in the outermost rings of onions as well as in the section that is closest to the root (Smith et al. 2003).  Additionally, research contends that organically grown tomatoes had over seventy-nine (79%) percent more quercetin than those grown with pesticides or other chemicals (Mitchell et al. 2007).

While we are often encouraged to eat more fruits and vegetables, the dietary intake of quercetin will be different across countries and regions.  However, the more variety of fruits and vegetables with quercetin we consume, the more robust the immune system will become, which will ultimately aid the body in fighting diseases.  

This article will discuss five (5) impressive researched benefits of quercetin, from its antioxidant and anti-inflammatory properties to its anti-viral and cancer-fighting abilities.  To learn more about vitamins and minerals and their benefits to the body, you can read this article.

• Our ABC’s (Vitamins Overview)

Five (5) Impressive Health Benefits of Quercetin

• May possess anti-inflammatory and immune-boosting properties.
• Its possible effects on cancer.
• May be able to help with cardiovascular health.
• Possess anti-viral properties.
• May have a positive effect on Asthma.

Quercetin and its Anti-inflammatory and Immune-building Properties

Inflammation is described as the body’s typical response to injuries, infections or chemical irritations. As such, it is a critical body function as it provides signals to the immune system to either heal or repair damaged tissue or to protect itself from outside invaders (viruses and bacteria).  Studies have shown that quercetin displays anti-inflammatory and immune-strengthening capabilities in animal studies.  One such effect was observed in how quercetin was able to mitigate the inflammatory response stimulated by the controversial food additive, carrageenan (Morikawa et al. 2003), as well as that of a high-fat diet (Stewart et al. 2008).   

Therefore, it was found that a diet high in quercetin was able to reduce visceral adipose tissue, especially that found in belly fat (Rivera et al. 2008).  Additionally, quercetin was reported to be effective in decreasing the clinical indicators of arthritis when compared to the results of participants in a control group that were not treated with the flavonoid (Mamani-Matsuda et al. 2006).  Arthritis has largely been associated with inflammation in the body, which causes stiffness of the joints.

Additionally, it was found that quercetin, along with polysorbate 80(a synthetic compound), was very effective against inflammation in rats.  As such, the combination of both compounds prevented the formation of edema in the paws of the rats (Lin et al. 2012).   Notably, the absorption of quercetin was largely dependent on its degree of bioavailability.  The study found that quercetin glucosides found in onions or fresh shallots were better absorbed than those found in teas (Ader et al.2000). 

As such, research suggests that the simultaneous ingestion of vitamin C, resveratrol, folate and other flavonoids can drastically improve the bioavailability of quercetin in humans (Manach et al. 2005; Read, 1995).  It is best to consume a wide variety of fruits and vegetables to acquire the recommended allowance of quercetin, as well as to ensure that the body can appropriately absorb it and use it accordingly.

Quercetin and Cancer

A major focus of quercetin by researchers is its anti-carcinogenic property, which is due largely to its anti-oxidant activity.  One of its main uses and studies over the years is the development of cancer treatment to restrain the growth of cancer cells in humans (Kumar et al. 2017).  

Therefore, research contends that flavonoids can be highly effective as an anti-proliferation agent against certain types of cancers, particularly lymphoid, colorectal, ovarian and breast cancer (Aherne and O’Brien, 2002; Kumar et al., 2017).   As such, studies have shown that it is capable of slowing down the growth of cancer cells.  To this extent, it has been instrumental in the development of novel anti-cancer drugs over the years (Morand et al. 1998).

Quercetin and Cardiovascular Health

Cardiovascular disease has been denoted as a leading cause of death in developed countries (Kumar et al. 2017).  While the exact cause and mechanism of the occurrence of heart disease are still being studied, researchers contend that oxidative stress and inflammation play a huge role in its development (Kumar et al. 2017).  

Studies have shown that quercetin positively reduced several of the risk factors associated with heart diseases, such as blood pressure regulation and cytokine-induced C-reactive protein (CRP) expression.  As such, it has been used in some cases as an alternative to anti-inflammatory and antioxidant drugs, conventional methods used in the treatment of particular cardiovascular diseases (Kumar et al. 2017).

Quercetin and General Anti-Viral Properties

Quercetin is also known for its anti-viral properties, which have proven effective against viruses such as herpes, parainfluenza type 3, as well as cardio virus (Manach et al. 1998). The antiviral activity of quercetin is due largely to its ability to bind to viral coat protein as well as to DNA that has been damaged (Kumar et al. 2017). To this extent, research articulates that quercetin, along with ascorbic acid (vitamin C), enhances its antiviral activity and is thus effective in the prevention of viral or respiratory conditions.

Quercetin and Asthma

Asthma is a serious and chronic disease of the lungs, which causes swelling and narrowing of the airways, resulting in breathing challenges.  Research has shown that the anti-inflammatory effects of quercetin can reduce the inflammatory cells of the immune system as well as decrease the histamine levels, which then smooth the muscles of the airways (Rigolin et al. 2012).  

The researchers further reported that even a small concentration of quercetin was effective against asthma when compared to traditional asthma maintenance medications and steroid inhalers, which simply reduce the resistance to airflow (Rigolin et al. 2012).  To this extent, quercetin is widely considered a possible treatment both in the prevention and treatment of asthma in humans.

How Much Quercetin is Safe?

Studies have shown that therapeutic doses of quercetin can range from 250 mg to 500 mg three times daily (Kumar et al. 2017).   Additionally, research has shown that higher doses above 200 micrometres (200 μm) reduce cell viability (Pandey and Rizvi, 2009) while doses fewer than 200 micrometres were observed to increase cell viability.  Further, Werbach (1993) recommends dosages of 250 mg – 600 mg as well as 200 mg – 400 mg per day for persons suffering from allergic conditions as well as chronic hives.  

Nonetheless, research purports that low doses of quercetin were even beneficial in inhibiting the proliferation of breast cancer cells (Pietta, 2000).  Therefore, it would not be necessary to take high doses of this flavonoid unless under the supervision of a medical professional.

Additionally, the consumption of fruits and vegetables can contribute an average of 15 mg to 40 mg of quercetin per day (Kumar et al. 2017).  However, higher dietary intake can be achieved with higher consumption of fruits and vegetables.  Quercetin is also available via supplementation in the form of capsules, tablets, etc.

What are the Side-Effects of Quercetin?

While quercetin is considered relatively safe, reports have indicated a few side effects, such as headaches and stomach discomfort.  Research also shows that high doses of quercetin can harm the kidneys and, as such recommend periodic breaks during its supplementation (Kumar et al. 2017).  

In addition, animal studies have shown that consuming more than  Three Hundred Fourteen (314) mg and One Hundred Fifty-Seven (157) mg of quercetin per kilogram of body weight per day was found to increase the ratio of the weight of the liver and kidney to body weight ratios (Azuma et al, 2010). 

However, in human studies, it was found that consumption of doses of up to 1000 mg/per day for many months did not induce any adverse effects on blood parameters of both liver and kidney function or serum electrolytes.  Nonetheless, concern was raised as to the possible toxicity of high doses of quercetin with digoxin, thereby discouraging consumption of both substances together (Jae-Hoon, 2009). 

Illustrative Summary

Here is a summary of the health benefits of the flavonoid QUERCETIN

Let’s Sum Up!

Having a diet that is high in the consumption of fruits and vegetables is paramount due to the high levels of nutrients that these foods contain.  Fruits and vegetables are particularly high in antioxidants and anti-inflammatory properties, which help to strengthen the immune system and prevent the development of chronic diseases.

Quercetin is one of those compounds that is largely found in fruits and vegetables and has been touted by research as highly beneficial to human health.  Quercetin can be found in foods such as grapes, onions, asparagus, green peppers, berries, and apples, among others.

Some of the nutritional benefits of this powerful flavonoid include its high levels of antioxidants and anti-inflammatory properties, which not only help to strengthen the immune system but also help the body fight the occurrence of chronic diseases.  

Other health benefits include its ability to prevent the proliferation of cancer, particularly colorectal and breast cancer.  Research also shows that it has anti-viral properties.  As such, it can help the body to fight against viruses and respiratory conditions.  It is also said to be able to reduce some of the risk factors of cardiovascular disease, such as high blood pressure and oxidative stress. 

While we are aware of the importance of consuming our fruits and vegetables, we all become guilty at times of only doing so when we become ill or face particular life conditions.  However, let us vow today, if not already, to include more fruits and vegetables in our diets as well as nutrient supplementation where necessary.  This will help not only ward off infections and diseases but also allow us to maintain a robust immune system to withstand the attacks of diseases, small or great.   

Now that we have discussed the importance of quercetin; How will you add it (or more of it) to your diet?

You can read more on the benefits of vitamins and minerals in this article and this one as well.

• Our ABC’s (Vitamins Overview)

• Why Minerals?

• Is Ginger truly great for our health?

• Video: Let’s Talk Quercetin
  

Editor’s Note: Article last updated by a Research Consultant and Certified Nutrition Coach on February 12, 2026.