Quercetin is a powerful plant compound found in a variety of foods ranging from leafy greens to fruits and teas. It is classified as a flavonoid, which is a broad sub-class of polyphenols (plant chemicals) that are known to deliver various health benefits from decades of cell, animal, and human research.

Quercetin is also thought to have adverse effects when combined with dozens of different prescription medications, including medicines for hypertension, diabetes, blood thinners, antipsychotics, and more (see Potential Health Risks and Side Effects below).

Quercetin is believed to have 3 key mechanisms of action. These include functioning as an antioxidant, anti-inflammatory, and anti-viral compound.

Overview of potential Quercetin health benefits:

• Cellular Protection from Oxidative Stress
• Reduced Inflammation
• Reduced Symptoms of Metabolic Syndrome
• Obesity Prevention
• Type 2 Diabetes Prevention
• Improved Brain Health and Memory
• Improved Mood
• Reduced Risk of Cancer
• Allergy Relief

Overview of potential risks and side effects:

• Risk of Exacerbated Kidney Damage
• Risk of Adverse Reactions with Prescription Medications

Reducing oxidative stress

Like many flavonoids, quercetin demonstrates great efficacy as an antioxidant. This appears to be one of the primary mechanisms of action driving the plant compound’s ability to promote cellular health and longevity.

Antioxidants perform several functions, but the one most often linked to longevity is their ability to reduce oxidative stress. Quercetin’s unique chemical structure makes it a potent antioxidant capable of doing just that.

In a 2019 article published by the journal Molecules, researchers illustrated quercetin’s ability to neutralize free radicals as they were produced by mitochondria, the powerhouses that create energy within each cell.

They explain that quercetin also appears to promote the body’s natural antioxidant system by interacting with NFRB, AMPK, and MAPK signalling pathways within the cell.

Quercetin has also been shown in both animal and cell studies to promote synthesis of glutathione, a vital antioxidant that is produced in the body as a line of defense against free radical damage.

This vital antioxidant is endogenous (meaning it is produced internally) and plays a critical role in tissue detoxification, reducing oxidative stress, and regulating immune system and healthy cell development.

However, one in vitro cell study indicates quercetin can actually degrade into a free radical called semiquinone. This study also indicates cardiovascular exercise may worsen degradation and cause tissue and DNA damage.

To date, researchers disagree on whether supplementing with high doses of quercetin delivers a net positive or negative result.

Anyone interested in supplementing with quercetin should consult a physician, stay up to date on the latest research, and read more in the Quercetin Dosages and Supplementation section below.

Reducing inflammation and related disease

Many studies correlate inflammation with aging and age-related disease. In fact, a 2014 article published by The Journals of Gerontology, Series A: Biological Sciences and Medical Sciences coined the term ‘Inflamm-aging’ to underscore the importance of this phenomenon and identify the core biological systems involved.

Longevity research has identified several of these systems, and quercetin appears to have a beneficial impact on at least three of them: NF-κB, MAKP, and immune system cytokines.

NF-κB and MAKB are two vital cellular signaling pathways. The first is known to induce inflammation in response to environmental stress, while the second is a protein network that facilitates communication within cells. Quercetin seems to suppress each, thereby reducing the onset and spread of inflammation.

Research shows that pro-inflammatory cytokines can make disease and infection worse, while anti-inflammatory cytokines can attenuate symptoms.

Remarkably, quercetin has been shown to decrease pro-inflammatory cytokines while simultaneously increasing the anti-inflammatory cytokines associated with healing.

Functioning as an antiviral agent

Various studies indicate quercetin has potential anti-viral properties. In a collative paper published by the peer-reviewed journal Frontiers in Immunology, researchers attribute quercetin’s anti-viral potential to its ability to suppress and/or prevent the following replication enzymes and activities within viral cells:

• Polymerases
• Proteases
• Reverse transcriptase
• DNA gyrase
• Binding viral capsid proteins

Each of the aforementioned processes play a role in viral replication and spread. For example, polymerase is an enzyme that is required to copy and assemble both DNA and RNA.

This chain reaction is called Polymerase Chain Reaction, or PCR—an acronym widely used to describe the type of COVID-19 test that looks for signs of this reaction to identify the virus.

Potentially functioning as a senolytic

It is worth noting there is conflicting research on quercetin’s potential as a senolytic. Such compounds are believed to promote longevity by regulating healthy cellular aging and senescence.

To date, the research is mixed. In 2018, a cell-based study indicated quercetin had zero impact on cellular senescence.

In 2020, researchers found the direct opposite to be true. Many more studies offer inconclusive evidence, indicating the truth might be somewhere in the middle. Moving forward, it will be exciting to see if quercetin continues to show promise as a senolytic compound in human trials.

According to information published by the UC Cooperative Extension Center for Health and Nutrition Research at University of California Davis, the most potent food sources for quercetin include the following common fruits, vegetables, and drinks.

Note: Quercetin content is expressed as total milligrams per 100 grams of food/drink:

• Onions: 21.42mg
• Kale: 7.71mg
• Chives: 4.77mg
• Pears: 4.51mg
• Apples: 4.27mg
• Cherry Tomatoes: 2.76mg
• Cranberries: 2.76mg
• Tea: 2.74mg*
• White Currant: 2.68mg
• Sweet Cherries: 2.64mg
• Broccoli: 2.51mg
• Apricots: 2.08mg
• Red Grapes: 1.38mg
• Fava Beans: 0.55mg
• Leeks: 0.10mg

*Specific variety of tea not mentioned in the publication referenced above. However, green and black teas are among the most studied varieties due to high concentrations of polyphenols.

According to research published by the United States Department of Agriculture, brewed green and black teas contain about 2.69mg and 2.19mg of quercetin per 100 grams, respectively.

In the same publication, the USDA also mentions that decaffeination and fermentation are not known to affect flavonoid levels.

Disclaimer: Always consult your health care professional prior to making changes in your diet or exercise regimen, including whether to use supplements like quercetin.

Prevention and Alleviation of Viral Infection

As noted above, quercetin shows several key antiviral properties, like polymerase and protease inhibition. The same antiviral properties appear to make quercetin a promising tool in the fight against other kinds of viral infections.

In a 2020 cell-based study, quercetin appeared to inhibit the spread of human herpesvirus. Another cell-based study indicates quercetin may also prevent the spread of hRSV, or human respiratory syncytial virus.

This finding is particularly encouraging, as authors note hRSV is one of the most common respiratory infections driving hospitalizations among young children and the elderly.

Cellular Protection from Oxidative Stress

Just as fire produces smoke, cellular activity produces free radicals. These harmful molecules are inherently unstable and create a state of oxidative stress within the cells.

Damage to lipids, proteins, and DNA are just a few consequences of prolonged oxidative stress. Fortunately, nature has a built-in mechanism for bringing things back into balance: Antioxidants.

Antioxidants have the remarkable ability to neutralize free radicals before they cause harm. Some are produced by the body, while others can be consumed in the form of vitamins and other phytonutrients, like quercetin.

And when it comes to antioxidant potential, quercetin is exceptional. In 2011, research published in the book series Advances in Experimental Medicine and Biology showed the flavonoid to have more than three times the Total Antioxidant Capacity (TAC) as curcumin.

In other studies, quercetin showed the ability to increase antioxidant enzymes in red blood cells, prevent DNA damage, and promote DNA repair.

In addition to the activities listed above, quercetin has been shown to promote the body’s natural production of other endogenous antioxidants to further aid in reducing oxidative stress, bolstering immunity, and regulating healthy cellular reproduction and longevity.

Obesity Prevention

In a 2015 study involving both cell and animal models, quercetin again showed the ability to suppress both creation and accumulation of fatty tissue by down-regulating the MAPK signaling pathway.

As a result, mice that received quercetin experienced an astounding 40% decrease in body weight. Commenting on the study, researchers noted “quercetin may represent a potential therapeutic agent for other metabolic disorders” and suggest further investigation in humans.

In another cell-based study, onion-derived quercetin was found to convert white fatty tissue into brown fatty tissue.

Modern longevity research has been increasingly interested in the role of brown fatty tissue in preventing obesity and related diseases. Writing for MayoClinic.org, Donald Hensrud, MD, explains that brown fat is activated when humans feel cold.It has more mitochondria than normal fatty tissue, and it has the unique ability to burn the body’s excess fat stores when activated.

In addition to keeping warm, this process also increases total fat utilization, which may prevent excess accumulation that leads to obesity.

Researchers have also noted that brown fat may “increase thermogenesis and help achieve long-lasting weight loss and an improved metabolic profile”.

As noted above, meta-analysis of metabolic syndrome research highlights the potential of quercetin to stabilize blood sugar and reduce the incidence of insulin resistance. In turn, these potential benefits may ward off the onset of type 2 diabetes. But how?

A 2017 cell-based study sheds light on this mechanism, explaining that quercetin inhibits glucose utilization in the intestine and increases glucose utilization in peripheral tissues.

Though the results are promising, the authors of the study warn that human trials are needed to further explore quercetin’s anti-diabetic potential in humans.

And while human trials are needed, animal studies have already shown promise. A systematic review of quercetin literature published by the journal Biomedicine and Pharmacology provides substantial evidence from rodent studies indicating quercetin’s ability to improve symptoms of rodents with diabetes.

Improved Mood

Quercetin may also help to improve mood. In general, plant-derived flavonoids are thought to have an impact on mood and cognitive function through two main mechanisms: Altering brain kinase activation and neuronal communication, and acting as an adenosine A1 receptor agonist.

For context, consider that caffeine is perhaps the most common adenosine agonist in the Western world. Just as a warm cup of coffee or tea produces a subtle lift in mood, so too does quercetin—at least in animal studies.

One study expanded on this finding, demonstrating quercetin’s ability to alleviate symptoms of depression in rodents.

Additionally, quercetin appears to provide an anti-anxiety effect in at least one separate rodent study. Commenting on the results, researchers attribute the effects to quercetin’s ability to reduce inflammation and modulate brain derived neurotrophic factor (BDNF), thus alleviating anxiety-like behaviors.

Reduced Risk of Cancer

Quercetin may also help to reduce the risk of cancer by halting its spread. In a study published by the journal Nature, quercetin was found to enhance certain drugs that inhibit the proliferation of cancer cells.

Although this does not mean quercetin alone can stop the progression of cancer, it does indicate the compound’s potential to play a role in augmenting existing cancer treatments.

In 2011, a cell-based study of quercetin demonstrated a more direct means of preventing the spread of cancer. Studying breast cancer cells, researchers found quercetin to “significantly suppress” COX-2 mRNA and protein expression, thereby preventing cellular growth and proliferation.

Quercetin also shows promise as a potent antioxidant and natural anti-inflammatory, as illustrated in the How Does Quercetin Work section above.

Through these mechanisms of action, the plant compound may indirectly reduce the risk of cancer by promoting cellular longevity, reducing oxidative stress, and regulating the immune system.

Allergy Relief

Quercetin also shows an ability to reduce allergic response. In a 2016 article published by the peer-reviewed journal Molecules, authors note quercetin demonstrated efficacy as a histamine inhibitor.

In animal studies, quercetin has been shown to exacerbate kidney damage. This is particularly dangerous for individuals who currently take prescription medications that are known to be damaging to the kidneys.

Quercetin exhibits several adverse reactions with common prescription drugs, including:

• Quinolone (antibiotics): Synergies may amplify effects
• Cyclosporine (Neoral, Sandimmune): May reduce bioavailability and lessen effects
• Any medication that is broken down by the liver, including Cytochromes P450 2C9, P450 2D6 (CYP2D6), and P450 3A4 (CYP3A4): May affect how the liver processes these drugs and cause corresponding changes in effects and side effects
• P-Glycoprotein substrates and other medications mediated by pumps in cells: May reduce delivery of medication into cells and inhibit effects
• Warfarin (Coumadin): May interfere with warfarin therapy, resulting in increased risk of bruising or bleeding
• Losartan (Cozaar) and other antihypertensive drugs, including thiazide diuretics, calcium channel blockers, ACE inhibitors, angiotensin II receptor antagonists (ARBs), and beta blockers: May cause dangerous decreases in blood plasma concentration
• Midazolam (Versed): May cause adverse interactions
• Pravastatin (Pravachol): May increase the effects of this statin
• Diclofenac (Voltaren): May have adverse effects on metabolism and utilization of this common arthritis medication
• Quetiapine (Seroquel): When taken together, quercetin may increase the effects of this prescription medication
• Diabetes medications: Taking both quercetin and diabetes medications may cause dangerous drops in blood sugar. This includes standard insulin medications for type 1 diabetes, as well as medications for type 2 diabetes like alpha-glucosidase inhibitors, biguanides, dopamine agonists, DPP-4 inhibitors, GLP-1 receptor agonists, meglitinides, SGLT-2 inhibitors, sulfonylureas, and thiazolidinediones.