- What is Quercetin? Food Sources + Dosage & Side Effects
- What is Quercetin?
- The Master Flavonoid
- Antioxidant Potential & Research
- How it Works
- Typical Dosage
- For Dogs
- What Increases its Absorption?
- EGCG Potential Synergy
- Quercetin Side Effects & Safety
- Brain cells
- Homocysteine Levels
- Thyroid Function
- Children and Pregnant Women
- Drug Interactions
- COMT Expression
- Further Reading
- Quercetin Uses, Benefits & Dosage — Drugs.com Herbal Database
- Adverse Reactions
- Uses and Pharmacology
- What Is Quercetin? — Quercetin Supplement Health Benefits & Risks
- What is quercetin?
- Potential quercetin benefits
- Heart disease
- Cancer risk
- How can you add quercetin to your diet?
- So do you really need to take a quercetin supplement?
- ⚠️ Quercetin side effects ⚠️
What is Quercetin? Food Sources + Dosage & Side Effects
Quercetin, an antioxidant flavonoid, is found in many plant-based foods from apples to nuts to capers. What are the best sources, how does it work, and are there any side effects to its use? Read on to find out.
What is Quercetin?
Most people have heard of flavonoids, plant-based antioxidant pigments that are being touted for many alleged health benefits. Flavonoids give plants their color and belong to the class of polyphenols. Polyphenols became a hot topic recently when some studies suggested their benefits in preventing heart disease, cancer, and other chronic diseases [1, 2].
The Master Flavonoid
Many vegetables, fruits, nuts, honey, and medicinal herbs are rich in quercetin. Raw capers have the highest amount of quercetin, while apples are the most common food source. Quercetin makes about 75% of all flavonoids consumed through diet .
Quercetin is also relatively better researched than most other flavonoids. Thus, quercetin has earned the nickname “master flavonoid” .
However, this doesn’t mean its potential health benefits are clear. In fact, most of the alleged health benefits of quercetin have not been verified by proper clinical trials. Quercetin has not been shown to treat or prevent cancer or other diseases.
Although quercetin supplements are widely available, they have not been approved by the FDA for medical use. Supplements generally lack solid clinical research. Regulations set manufacturing standards for them but don’t guarantee that they’re safe or effective. Speak with your doctor before supplementing.
The FDA has issued warning letters to several manufacturers advertising unauthorized health claims. Their quercetin product labels and websites listed claims such as treating diseases, for which quercetin has never been approved. Making such claims places these products in the “unapproved drugs” category by the FDA.
On the other hand, quercetin has cultural and historical significance. Ever since antiquity, people put great value on quercetin-rich foods. For example, people consumed pomegranate as an elixir for good health and longevity.
Antioxidant Potential & Research
Quercetin is an antioxidant flavonoid. Limited studies suggest it may scavenge free radicals and reduce tissue and DNA damage.
It seems to boost antioxidant defense, which might be helpful for health conditions linked to oxidative stress.
According to some theories, most chronic health problems in the modern world have been linked with excessive oxidative stress and free radicals [4, 5, 2].
A PubMed search returns almost 17k studies about Quercetin. However, clinical trials are rare, small, and of questionable quality. About 200 clinical trials involving quercetin have been carried out so far. So is there anything special about it and how weak is the evidence?
How it Works
Limited studies suggest quercetin may [6+, 2, 7]:
- Neutralize free radicals and ROS
- Reduce oxidative damage to fats
- Boost levels of glutathione
- Increase the blood’s overall antioxidant power
- Reduce inflammation by blocking inflammatory substances and pathways (including COX-2 and CRP)
- Be active against some bacteria and viruses
- Reduce the expression of inflammatory genes (such as those that make TNF-alpha)
- Block the release of histamine, which affects allergies and mast cell activation
The dosage in clinical trials varied between ~100-1,000 mg/day. The most common dose was 500-1,000 mg/day .
The main problem with Quercetin is its poor bioavailability. Quercetin bioavailability in typical oral supplements is ~2%.
It’s important to remember that Quercetin is available in many forms: free quercetin (the aglycone) or quercetin bound to various sugar molecules. Rutin from apples, for example, is sugar-bound quercetin. Not all of these types of quercetin have the same bioavailability. For example, Quercetin from onion powder is better absorbed than quercetin from apple peel powder .
Once quercetin is ingested through food, the sugar bound forms are degraded and free quercetin is released. Free quercetin is metabolized very quickly in the small intestine, the kidneys, the large intestine, and the liver, giving rise to numerous metabolites that are probably not active .
Once quercetin is in the gut, its bioavailability also depends on how well it’s modified to be made more soluble [6+].
Quercetin is sometimes added to commercially-available dog food. Similar to humans, dogs metabolize quercetin very quickly. Its bioavailability in dogs is also low. Un for humans, dogs absorb the rutin form of Quercetin found in apples better than humans .
Some people use Quercetin to reduce allergies in dogs. In one study, dogs fed antioxidant- and Quercetin-rich diets had better metabolism and less free radicals .
The human dosage could be adapted to dogs if using Quercetin supplements, although the bioavailability remains uncertain.
the dog size, the dosage may need to be reduced. For example, very small dogs would need only 1/10 of a typical human dose (if the dog is 1/10 the size and weight of an average person).
Talk with your vet before giving quercetin to your dog.
What Increases its Absorption?
The following may increase quercetin absorption and bioavailability:
- Taking it with fats or oils. The oils stimulate bile production, which can make quercetin soluble in the gut and easier to absorb 
- Liposomal or nano-quercetin [12, 13, 14]
- Adding it to foods, such as cereal bars (possibly) 
- Taking quercetin from onion powder instead of from apple peel powder 
- Quercetin 3-glucose as opposed to the free quercetin (in rats) 
- Alcoholic tinctures, estimated to be ~40% bioavailable [6+]
- Combining it with bromelain, which increases both its bioavailability and anti-inflammatory effects 
EGCG Potential Synergy
The combination of quercetin and resveratrol, a polyphenol from grapes may have added health benefits. In rats, only the combination of both reduced fat deposits, while each resveratrol or quercetin alone did not have any effects .
Flavonoids may act in synergy to increase antioxidant defense. Quercetin increases the bioavailability of EGCG and other antioxidant flavonoids [6+].
Quercetin Side Effects & Safety
Quercetin is generally considered to be safe. However, proper safety trials are lacking .
The side effects mentioned below were observed in animal or cellular studies. More clinical studies would need to determine the side effects of quercetin in different formulations and doses.
Quercetin was toxic to rat brain cells. Higher concentrations caused more brain cells to die .
It’s uncertain how cellular effects and doses could translate to humans.
In human liver cancer cells, quercetin significantly increased homocysteine levels .
The same effect has not been observed in clinical trials.
High doses of quercetin and other flavonoids acted as thyroid disruptors in animal studies. People with thyroid problems should use caution .
Children and Pregnant Women
Quercetin is ly safe if taken through a diet of healthy quercetin-rich foods in small amounts during pregnancy and childhood.
Children and pregnant women should avoid quercetin supplements due to a lack of safety data.
Quercetin reduced fertility in female mice in one study [22, 23].
It’s unknown how quercetin interacts with drugs in humans.
In cells, quercetin blocks the following CYP enzymes [6+]:
It also blocks a drug transporter in the gut (pgp) that helps eliminate many drugs from the body.
It’s possible that Quercetin can affect the levels of commonly used drugs that are eliminated through these pathways, although no clinical studies have confirmed this.
Rutin may also reduce the effects of warfarin, so caution is advised for people on this anticoagulant.
COMT, the worrier or warrior gene, helps with methylation and also breaks down important neurotransmitters .
People with SNPs that predispose them to low COMT levels might want to avoid Quercetin, since it has a catechol structure and can block COMT gene expression .
Quercetin is found in a large number of foods. How the food was grown and transported will impact the concentrations of Quercetin. It’s possible that organic food is higher in Quercetin. In one study, organically-grown onions were higher in Quercetin and other flavonoids .
Food sources of quercetin include :
- Vegetables such as capers (highest concentration), onions, eggplant, celery, asparagus
- fruits, especially berries, but also apples and oranges
- Black and green tea
- 8 Quercetin Supplement Benefits for Allergies & More
Quercetin Uses, Benefits & Dosage — Drugs.com Herbal Database
Medically reviewed by Drugs.com. Last updated on Jun 19, 2018.
Scientific Name(s): 2-(3,4-Dihydroxyphenyl)-3,5,7-trihydroxy-4H-1-benzopyran-4-one, 3, 3′, 4′, 5, 7-pentahydroxy-2-phenylchromen-4-one, 3,3′,4′,5,7-pentahydroxyflavone
Common Name(s): Pentahydroxyflavone, Quercetin, Quercetine, Vitamin P
Interest in quercetin as an antioxidant is ongoing. Chemoprotective and antihypertensive effects show promise, but clinical studies are limited. Preliminary data indicate potential benefit for improving adaptive and social functioning in children with autism.
Although specific evidence to support dosing recommendations is limited, most clinical studies use quercetin 500 to 1,000 mg per day in divided doses.
Contraindications have not been identified. Avoid coadministration with the cardiac glycoside digoxin.
Generally recognized as safe (GRAS) when consumed as food. Avoid dosages above those found in foods because information regarding safety and efficacy in pregnancy and lactation is lacking for such doses. Fetal growth retardation was observed in a study in rats exposed to quercetin by oral gavage.
A case report of a clinically relevant warfarin interaction resulting in supratherapeutic international normalized ratio (INR) values has been documented.
No clinically important adverse effects were reported in clinical studies.
Oral supplemental doses of up to 1,000 mg per day for as long as 12 weeks showed no evidence of toxicity. However, data on long-term safety at high doses are lacking, and concerns regarding carcinogenicity remain unresolved. Nephrotoxicity has been reported with high doses of intravenous (IV) quercetin.
The flavonol quercetin is found as glycosides in many vegetables and fruits, as well as in seeds, nuts, flowers, bark, and leaves.
Rich sources of quercetin include apples, asparagus, berries, Brassica vegetables (eg, broccoli), capers, grapes, onions, shallots, tea, and tomatoes. Quercetin is also found in large amounts in ginkgo, St. John’s wort, and elder.
The outermost layers of onions and dried, not fresh, shallots are rich in quercetin.1, 2, 3, 4, 5 See also Onion monograph.
Initially, quercetin was considered to be a vitamin and given the name «vitamin P.» It was identified in the 1930s, but was slow to gain recognition because it did not seem to be an essential micronutrient. As epidemiological studies in the 1990s pointed to the benefits of flavonoids in cardiovascular health, more researchers started investigating quercetin in depth.6
Quercetin (3, 3′, 4′, 5, 7-pentahydroxy-2-phenylchromen-4-one) is a flavonol, sharing the common hydroxylated 3-ringed structure with attached hydroxyl groups of other flavonoids.
2 Quercetin is bright yellow and soluble in alcohol and lipids, but poorly soluble in hot water and insoluble in water.
The aglycone form is more lipophilic than quercetin glycosides such as rutinoside from tea, and is absorbed more readily than glycoside forms, which must be hydrolyzed to release quercetin. Analytical techniques for the identification of quercetin have been described.1, 5, 7, 8, 9
Uses and Pharmacology
Quercetin exhibits potent antioxidant activity in experimental models and in animal studies.1, 7, 10, 11, 12
Patients with sarcoidosis showed increased plasma antioxidant capacity and reduced markers of oxidative stress and inflammation with quercetin supplementation.7, 13 However, in a clinical study of quercetin's effect on hypertension, none was found on markers of oxidative stress,14 and no antioxidant effects were observed in healthy volunteers despite increased plasma quercetin levels.7
Data from a small, prospective, open-label trial (n = 40; 87.
5% boys) in children with autism spectrum disorder showed significant improvement in adaptive functioning and overall behavior after 26-week administration of a supplement containing luteolin from chamomile (100 mg), quercetin (70 mg), and the quercetin glycoside rutin (30 mg); 1 capsule per 10 kg of weight was given daily with food.
Changes in raw and age-equivalent scores were significant for all domains except communication raw scores and were greater than those expected by maturation per se. No major adverse effects were documented; however, 6 children from the original 50 enrolled withdrew due to increased irritability caused by the formulation.50
In vitro and animal studies have attempted to elucidate possible mechanisms of action for quercetin in cancer.
Aside from its potent scavenging of reactive oxygen and nitrogen species and metal chelation, such mechanisms include antiproliferative, growth-suppressing, and antiangiogenesis activity; inhibition of telomerase and induction of senescence and cell death; and activation of immune and autophagic activity.
Studies in rodents have included cancers of the colon, small intestine, tongue, skin, lung, and mammary gland. Other in vivo studies have used melanoma and prostate cancer cells. Quercetin also demonstrates estrogenic activity and may exert a direct effect on androgen receptors in prostate cancer cells.1, 8, 15, 16, 17, 18, 19, 20, 21
Clinical trials are lacking to support findings from epidemiological and animal studies. Phase 1 (safety) clinical studies have shown positive results; however, the number of participants was too small to draw conclusions. Other studies have used mixed preparations of quercetin with curcumin or are of limited robustness.1, 22
Animal models and studies using isolated cardiovascular tissues suggest a role for quercetin in cardiovascular disease.1, 15 However, not all effects observed in animal studies have translated to similar effects in humans.
23, 24 Antioxidant effects, including reduced low-density lipoprotein oxidation, decreased experimental reperfusion injury, improved endothelial function, decreased inflammation, and anti-aggregating and antihypertensive effects of quercetin have been demonstrated mostly in rodents.
1, 15, 24 Improved cholesterol profiles and decreased insulin resistance have also been demonstrated,6, 15 while a role for quercetin metabolites has also been suggested.25
Epidemiological studies on the role of flavonoids in decreasing risk factors of cardiovascular morbidity support a place for quercetin.
1, 15 Studies in healthy volunteers and among patients with prehypertension (systolic blood pressure [BP] 120 to 139 mm Hg and diastolic BP 80 to 89 mm Hg) have found no effect of quercetin supplementation on blood pressure.
14, 26 In 23 patients with stage 1 hypertension (systolic BP 140 to 159 mm Hg, diastolic BP 90 to 99 mm Hg), quercetin 730 mg per day over 28 days reduced systolic pressure by 7 mm Hg (±2) and diastolic pressure by 5 mm Hg (±2). Mean arterial pressure was also reduced. Measures of oxidative stress, however, were not affected.
14 In a trial of overweight patients, systolic BP was reduced by 2.6 mm Hg (P< 0.01) in all participants and by 3.7 mm Hg (P< 0.01) in participants with hypertension who consumed quercetin 150 mg per day over 6 weeks.
27 Another clinical trial suggested that the efficacy of quercetin is dependent on the apolipoprotein (Apo) genotype, with subjects presenting with subtype ApoE 3 demonstrating decreased systolic pressure, while those with subtype ApoE 4 did not.28 Inhibition of platelet aggregation and postulated reductions in the risk of thrombosis have been shown in healthy volunteers given quercetin alone and as onion soup.29, 30 Limited studies evaluating quercetin’s effect on lipids have produced equivocal results.28, 31
Endothelial dysfunction and inflammation biomarkers were evaluated after 35 prehypertensive adults ingested pure epicatechin 100 mg/day and quercetin-3-glucoside 160 mg/day for 4 weeks in a placebo-controlled, double-blind, crossover study. Of the 5 endothelial dysfunction biomarkers measured, soluble endothelial selectin was significantly reduced (P = 0.
03) by epicatechin and quercetin supplementation. No other biomarkers were significantly affected by epicatechin; however, pure quercetin supplementation also reduced inflammatory markers (ie, interleukin [IL]-1-beta [P = 0.009], z score for inflammation [P = 0.02]).
As a reference, it’s important to note that 4 cups of black tea contain about 19 mg of quercetin.53
The effects of quercetin supplementation on C-reactive protein (CRP) (a strong predictor of cardiovascular disease) were assessed in a systematic review and subsequent meta-analysis of 7 double-blind, randomized, placebo-controlled trials (N=549), including 4 crossovers.
The studies had sample sizes of 40 to 93 adult participants (1 study enrolled only males, 2 studies enrolled only females) and administered quercetin at dosages of 150 to 500 mg/day for 6 to 10 weeks. Baseline CRP levels ranged from 1.28 to 5.7 mg/L (mean, 3.
44 mg/L) in patients with body mass indices of 21.4 to 31.1 kg/m2 and who presented with a variety of conditions (eg, metabolic syndrome, pre-hypertension, mild hypertension, rheumatoid arthritis, type 2 diabetes, various apolipoprotein genotypes).
Overall, data showed a statistically significant reduction in CRP (weight mean difference, −0.33 mg/L; P
What Is Quercetin? — Quercetin Supplement Health Benefits & Risks
There’s a reason plant-based diets get so much healthy hype. Loading up on fruits and vegetables has been linked with a lower risk of chronic disease and plants themselves are packed with tons of vitamins, minerals, and nutrients that promote overall wellness.
Often, too, those plant-based pigments—say, the beta-carotene in orange veggies or anthocyanins in purple fruits—get singled out for their powerful roles in fighting inflammation, stress, and diseases diabetes, heart disease, and even cancer.
One in particular: a flavonoid called quercetin found in various fruits and vegetables, which has been touted for its ability to boost cardiovascular health and possibly even reduce cancer risk.
So it’s no surprise, then, that some people are popping it in supplement form. In fact, one quercetin product on Amazon has nearly 400 reviews with a 4.
3-star rating, mainly due to the positive impact reviewers have experienced with allergy relief.
But what exactly is quercetin? And do you really need a pill to reap the benefits of the so-called plant-based powerhouse? Below, we dig into the research and consult with registered dietitians to find out.
What is quercetin?
“Quercetin is a flavonoid known for its antioxidant and anti-inflammatory health benefits,” explains Yasi Ansari, RDN, CSSD, a registered dietitian in the Los Angeles and San Francisco areas.
Basically, it’s a health-promoting compound found in a variety of fruits and vegetables, apples, onions, citrus fruits, leafy greens kale, as well as black and green teas. It comes in supplement form on its own and is included in both ginkgo biloba and St. John’s Wort, two popular herbal supplements.
Potential quercetin benefits
In large, quercetin’s health benefits boil down to its ability to fight inflammation and its antioxidant properties, meaning it can neutralize damaging free radicals (a type of molecule in the body), which is a common precursor to several chronic diseases. That’s where many of the proposed perks of supplementation come in.
Research—which, by the way, has mostly been done in animals and in labs—has suggested quercetin has both brain-protective and antidepressant effects and may reduce inflammation and oxidative stress, a condition in which free radicals cause damage to cells.
Some studies show that quercetin may protect against the buildup of plaque in the arteries, decreasing “bad” LDL levels, thus reducing risk of heart disease and high blood pressure, explains Ansari.
Quercetin has also shown some promise in relieving allergies as it stabilizes mast cells, which release histamine, says Robin Foroutan, RDN, an integrative medicine dietitian and spokesperson for the Academy of Nutrition and Dietetics. “Histamines are to blame for symptoms seasonal allergies, so high doses of quercetin can be really helpful to reduce seasonal allergy and hay fever symptoms.”
Some in vitro data (meaning the research was done outside of a living organism, a culture dish) has even indicated that quercetin may help to decrease the formation of liver cancer cells and quantities of rectal tumors, explains Ansari.
Unfortunately, though, there are few large-scale clinical studies in people and the ones that exist have shown varying effects of quercetin, according to the Memorial Sloan Kettering Cancer Center.
Plus, it may even interfere with certain chemotherapy drugs because of its antioxidant properties.
“Although there is some level of decreased cancer incidence, there is no evidence that it is the activity of quercetin alone versus the combination of more than one antioxidant providing the health benefits,” says Ansari.
How can you add quercetin to your diet?
Your body has a tough time absorbing quercetin properly, so naturally a supplement—which usually contains concentrated amounts of the stuff—would make sense if you’re trying to up your intake.
However, some research actually finds the quercetin in whole foods is more bioavailable to the body than capsule varieties. Certain studies suggest the quercetin from onions is more bioavailable than that from apples, for example, and that eating the antioxidant alongside heart-healthy fats can improve absorption, too.
Of course, not all studies point to this and ultimately, more research in humans is needed to understand absorption from both supplementation and food. Ansari, however, favors a food first approach.
“I think there is some evidence to suggest health benefits of quercetin supplementation for those suffering from high blood pressure and overall increased risk of chronic disease and inflammation,” says Ansari. “But it’s too early to tell as additional research is still needed on dosage, type, and what to pair supplements with that would help increase absorption.”
So do you really need to take a quercetin supplement?
As of now, there are no specific public health recommendations for quercetin intake, such as dietary reference intakes (DRIs) or daily value amounts (DV), from the Food and Drug Administration for the antioxidant, says Ansari.
If you are curious to try them anyway, research generally suggests that a dosage under 1,000 mg a day seems safe, she notes. Just note that it can effect everyone differently, so be aware of the potential downsides a quercetin supplement may have on your body, especially when take for long periods of time at high doses.
⚠️ Quercetin side effects ⚠️
- Negative interaction with certain medications, such blood thinners ( aspirin and warfarin), chemotherapy drugs, antibiotics, corticosteroids, and other drugs
- Potential kidney damage
- Upset stomach
- Heartburn or acid reflux
If you decide to take a supplement, always consult with your doctor or discuss your decision with a registered dietitian, especially if you are pregnant or taking prescription medications.
- Robin Foroutan, MS, RDN, HHC, health coach at The Morrison Center and national spokesperson for the Academy of Nutrition and Dietetics
- Yasi Ansari, MS, RDN, CSSD, registered dietitian in Southern California working with patients with a variety of health conditions, such as diabetes, cancer, and heart disease