3,553 words, 19 minutes read time
Updated on November 29, 2023. Fact checked by Robert Underwood, Research Lead.
Quercetin is among the most studied of all flavonoids in modern longevity science. Decades of research support this powerful plant compound’s ability to reduce inflammation, prevent infection, and promote healthy aging. Learn more about quercetin, an extraordinary tool in the fight against aging and age-related disease.
What is Quercetin?
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 one of the most studied flavonoids in the field of longevity research. In fact, a quick search for this well-known plant compound on the National Library of Medicine website returns over 2,300 results. Of those results, 30 are clinical trials.
Many foods are rich in this potent anti-aging compound, including berries, citrus fruits, leafy greens, and even red wine.
For individuals interested in supplementing their diet with high doses of quercetin, a variety of products are available. And as a supplement, the U.S. Food and Drug Administration has labeled quercetin a “Generally Recognized as Safe” (GRAS) ingredient.
Despite being a plant compound and having a GRAS label from the FDA, several studies indicate quercetin may be harmful to certain individuals.
Those who suffer with kidney issues, and especially those who take medications known to cause kidney damage, are particularly at risk.
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 Benefits and Risk
Disclaimer: Always consult your health care professional prior to making changes in your diet or exercise regimen, including whether to use supplements like quercetin. Pregnant women, women who are nursing, and children especially must always consult a physician prior to taking supplements or medications.
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
- Allergy Relief
Overview of potential risks and side effects:
- Risk of Exacerbated Kidney Damage
- Risk of Adverse Reactions with Prescription Medications
How Does Quercetin Work?
Polyphenols represent a large and diverse class of plant compounds that have played a key role in human evolution for thousands of years. Within that class lies a potent sub-class of phytochemicals called flavonoids.
After decades of research, a growing body of evidence indicates flavonoids have an anti-oxidative, anti-inflammatory, and anti-carcinogenic effect at the cellular level. Studies indicate at least 4 potential mechanisms of action for this anti-aging compound, including:
- Reducing oxidative stress
- Reducing inflammation and related diseases
- Functioning as an antiviral agent
- Potentially functioning as a senolytic
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.
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.
Inflammation is also mediated by the immune system through the use of unique proteins called cytokines. Some cytokines are pro-inflammatory, while others are anti-inflammatory.
Research shows that pro-inflammatory cytokines can make disease and infection worse, while anti-inflammatory cytokines can attenuate symptoms.
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
What Foods Naturally Contain Quercetin?
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.
Quercetin Dosages and Supplementation
Disclaimer: Always consult your health care professional prior to making changes in your diet or exercise regimen, including whether to use supplements like quercetin.
Dosage
Typical dosage ranges from 500mg to 1,000mg per day. Individual quercetin supplements may vary.
Frequency
Absorption
Quercetin is characterized by relatively poor oral absorption. However, pairing quercetin with a food-based lecithin has been shown to increase absorption by a factor of 20. This is a common grouping in quercetin supplements, as is delivering quercetin in a liposomal (fat-based) matrix to improve absorption.
Potential Health Benefits of Quercetin
Quercetin is one of the most-studied flavonoids on the planet. Still, a majority of research conducted to date has utilized cell-based and animal models. For that reason, one should approach quercetin with caution.
Many researchers are skeptical that health benefits observed in cell cultures can ever be replicated in humans.
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.
Reduced Inflammation
In a cell-based study, quercetin was shown to reduce inflammation by inhibiting the pathways and chemical messengers that are known to cause an inflammatory response. Four key findings from this study include:
- Quercetin appears to block the activation of NF-κB, a critical signaling network that modulates the human inflammation response. NF-κB is a pro-inflammatory pathway that is activated in times of stress.
- Quercetin seems to down-regulate this pathway, helping to alleviate stress response and reduce corresponding inflammation.
- Quercetin also appears to block the MAKP pathway. MAKP can be thought of as an on/off switch that allows one cell to communicate with its neighbors.
- By keeping this pathway switched “off”, quercetin appears to inhibit the spread of inflammatory signals from cell-to-cell.
- Quercetin also appears to suppress certain inflammatory cytokines, specifically IL-1β and IL-6.
- The immune system uses these chemical messengers to induce inflammation; by suppressing them, quercetin is thought to act as an anti-inflammatory.
- Finally, quercetin appears to promote an anti-inflammatory cytokine called IL-10. Anti-inflammatory cytokines function opposite the cytokines described above, acting to reduce swelling and promote healing.
Reduced Symptoms of Metabolic Syndrome
Metabolic syndrome is characterized by high blood pressure, blood sugar, cholesterol, and body fat.
Metabolic syndrome is typically progressive as symptoms worsen over time. Together, these conditions increase the risk of cardiovascular disease, stroke, and type 2 diabetes.
A 2021 meta-analysis of metabolic syndrome research articles shows evidence that quercetin improves critical elements of healthy metabolism. According to the analysis, quercetin may reduce blood glucose levels, blood pressure, insulin resistance, and rates of obesity.
In a separate cell-based study, quercetin was also shown to be an effective inhibitor of LDL (“bad”) cholesterol oxidation.
Obesity Prevention
According to the Journal of the American Medical Association, obesity kills about 280,000 Americans every year.
In addition to curbing calories, plant-based diets provide a wide array of anti-obesity phytonutrients. Quercetin has shown promise as one such compound.
In 2008, researchers discovered quercetin had the ability to mediate both AMPK and MAPK networks—two key signaling pathways that influence cellular activity and inflammation.
In doing so, quercetin demonstrated the ability to slow adipogenesis, the process by which fat cells develop and accumulate in the body.
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”.
Type 2 Diabetes Prevention
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 Brain Health and Memory
Quercetin has also shown promise as a neuro-protectant in various cell-based and animal studies. In a 2018 review of such studies, the following pathways and receptors were identified as possible mechanisms of action:
Most recently, an article published in the November 2021 edition of Scientific Report named quercetin as a possible therapeutic in the fight against Alzheimer’s Disease.
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.
Allergy Relief
Quercetin also shows an ability to reduce allergic response. In a 2016 article published by the peer-reviewed journal Molecules, authors note that quercetin can stop the activation of mast cells, a type of immune cell, and can slow and even stop the release of histamine, a chemical in the immune system that triggers an allergic response. Quercetin also suppresses inflammatory molecules (interleukins IL-4 and IL-13) by stopping certain key components in the body’s allergic response system. Through these mechanisms, quercetin can help the immune system calm down and not overreact to allergy triggers.
Potential Health Risks and Side Effects
Quercetin is a plant compound, and it is commonly ingested in small amounts as part of a healthy plant-based diet. As indicated above, the compound is also “Generally Regarded As Safe” by the US Food and Drug Administration.
However, taking high doses of quercetin over long periods of time may have certain risks and side effects. More research is needed to determine what those side effects are.
Individuals must always consult a physician prior to taking a new supplement, and that is especially important when combining new supplements with existing medications.
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.
New Quercetin Research 2023
New research published in the journal Sec. Cancer Immunity and Immunotherapy in 2023 has reviewed the potential anti-cancer effects of quercetin on cancers of the lung, blood, and prostate.
The major review found that quercetin – a primary bioflavonoid in the human diet – has notable anti-inflammatory and chemo-preventive effects and can hinder cancer progression by down-regulating a variety of proteins and feedback systems. These proteins and feedback systems include mutant p53 proteins, mutant p53 proteins, G1 phase arrest, and tyrosine kinase inhibition. They also include downregulation of cell survival, proliferative and anti-apoptotic proteins, G1 phase arrest, and downregulation of cell survival, proliferative and anti-apoptotic proteins.
Another meta-analysis on the potential of quercetin for treating COVID-19 patients found that the flavonoid decreased the risk of patients being admitted to intensive care and lowered the incidence of hospitalisation, but did not lower the risk of all cause mortality and no recovery rates. Researchers noted that quercetin may benefit patients with COVID-19, particularly when administered as a phytosome formulation to improve its bio-availability, although further research is needed to confirm the findings.
Conclusion: Quercetin is a Potentially Powerful Longevity Tool
Flavonoids like quercetin are vital tools in the fight against aging and age-related disease. Decades of cell, animal, and human studies support quercetin’s efficacy as a potent antioxidant, anti-inflammatory, and anti-viral compound.
Remarkably, this unique phytochemical shows exceptional promise as a treatment for reducing symptoms and preventing infection.
Although some studies indicate quercetin to be toxic in high doses for prolonged periods of time, the supplement is “Generally Regarded as Safe” by the US Food and Drug Administration. Outlook is generally positive, and it will be exciting to see what future clinical trials teach us about this powerful longevity compound.
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Glossary of Terms
Anti-apoptotic proteins: Proteins that inhibit programmed cell death and allow for the growth and proliferation of cancerous cells.
Bio-availability: The percentage of a compound or substance that is able to reach the systemic circulation after ingestion.
Bioflavonoids: Also known as flavonoids, bioflavonoids are polyphenolic secondary metabolites found in a range of plant sources.
Chemopreventive: Relating to any compound or substance that can hinder the development and progression of cancers.
Diastolic blood pressure: An indication of the level of blood pressure in the arteries while the heart is resting in between heartbeats.
Free radicals: Unstable atoms which are capable of causing cell damage and accelerating the onset of ageing and disease.
G1 phase arrest: Growth 1 phase is the first of four cell cycle phases present in cell division. In the G1 phase, cells produce proteins and mRNA to prepare for mitosis. G1 phase arrest refers to the prevention of this phase of cellular division.
Mutant P53 proteins: These proteins prevent autophagic mechanisms in cells, effectively protecting cancer cells and enabling them to proliferate.
Systolic blood pressure: A measurement of the level of pressure present in the arteries while the heart is beating.
Tyrosine kinase: A cellular enzyme that acts as a ‘switch’ to turn cellular functions on and off.
