Quercetin is the “master flavonoid,” a strong antioxidant and one of the best natural antihistamines–one that can stabilize mast cells and reduce inflammation. People have known about its health benefits since ancient times. Read to understand what the latest science says about quercetin, and what you can do to overcome its poor bioavailability.

What is Quercetin?

Most people have heard of flavonoids, plant-based antioxidant pigments that are being touted for many amazing health benefits. Flavonoids give plants their color and belong to the class of polyphenols. Polyphenols became a hot topic recently for their benefits in preventing heart disease, cancer, and other chronic diseases [1, 2].

The Master Flavonoid

Well, quercetin can be viewed as the “master flavonoid”. It’s the most well-researched and most abundant flavonoid of all [3].

Many vegetables, fruits, nuts, honey, and medicinal herbs are rich in it. Raw capers have the highest amount of quercetin, while apples are the most common food source. In fact, quercetin makes about 75% of all flavonoids consumed through diet [3].

Ever since antiquity, people put great value on the quercetin, consuming pomegranate juice rich in it as an elixir for good health and longevity.

A Strong Antioxidant

Quercetin is a strong antioxidant flavonoid that can scavenge free radicals and reduce tissue and DNA damage. Quercetin boosts antioxidant defense against a wide range of health conditions linked to oxidative stress. In fact, most chronic health problems in the modern world are caused by increased oxidative stress and free radicals [4, 5, 2].

Nowadays, Quercetin supplements are widely available and affordable. Quercetin has many benefits for those wanting to heal or optimize their health.

A PubMed search returns almost 17k studies about Quercetin. And although clinical trials are less common, over 200 have been carried out so far. So what’s so special about it?

How it Works

As the strongest flavonoid antioxidant, Quercetin acts to [6+, 2]:

  • Neutralize free radicals and ROS, protecting tissues and organs in the whole body
  • Reduce oxidative damage to fats, which reduces cholesterol and protects against artery clogging, heart disease, and neurodegenerative diseases
  • Boost levels of glutathione, the master antioxidant
  • Increase the blood’s overall antioxidant power
  • Reduce inflammation by blocking important inflammatory substances and pathways (including COX-2 and CRP)
  • Fight bacteria and viruses
  • Block the release of histamine, reducing allergies and mast cell activation

Snapshot

PROs

  • Anti-inflammatory
  • Natural Antihistamine
  • Increases antioxidant protection against a wide range of health conditions

CONs

  • Bioavailability is not good for most supplements
  • May alter thyroid function
  • Inhibits COMT

Benefits of Quercetin

1) Protects the Brain

Quercetin is a powerful antioxidant that may protect brain cells from oxidative stress [7].

It shields the brain from toxic substances, according to studies in mice. It increased an antioxidant enzyme, superoxide dismutase (SOD) and decreased markers of oxidative stress. It also improved cognition, learning, and memory [8].

In mice, it also had an epigenetic effect: increasing the expression of genes that boost the function of existing brain cells and regenerate old or damaged ones[8].

Flavonoids, including quercetin, block inflammatory molecules linked to serious brain disorders like Alzheimer’s disease, meningitis, AIDS dementia, and stroke [9].

Additionally, quercetin reversed cognitive deficits in aged mice with Alzheimer’s disease. It also prevented disease worsening but had the strongest beneficial effect in the early-middle stages of the disease [10].

What’s more, antipsychotics used to treat schizophrenia can cause movement disorders as a side effect, which are similar to Parkinson’s. Quercetin reduced these side effects in mice given antipsychotics [7].

2) Might Heal Leaky Gut

Chronic stress can over-activate mast cells in the gut, which leads to increased inflammation and permeability of the gut lining (leaky gut). Quercetin has dual action: it helps rebuild a healthy gut barrier and prevents mast cells from flooding the gut with histamine [11, 12].

In rat studies, it blocked histamine release and stabilized gut mast cells. This may help both prevent and heal leaky gut [13].

Quercetin reversed leaky gut in mice and helped rebuild the gut barrier. Once the gut lining was restored, inflammation decreased as well [14].

This antioxidant tightened the junctions between gut cells in a cellular study. It increased the activity of genes that make the gut lining and reduce gut inflammation. This may help strengthen the gut barrier and prevent unwanted food particles from entering the bloodstream in the long run [15].

3) Reduces Inflammation

Quercetin decreases inflammatory cytokines that can damage tissues in humans [16, 6+].

There’s scientific ground for using quercetin to reduce prostate inflammation and enlargement. In an initial small study, it reduced symptoms of prostate inflammation in men by 75% [17].

In the next study of 30 men, quercetin (1000 mg/day) reduced prostate inflammation and pain in 67% of them after 1 month. Then 17 men also received bromelain and papain as an add-on to enhance quercetin absorption, along with palmetto and cranberry. This combination improved symptoms in over 80% of the men [18].

Quercetin can reduce the expression of inflammatory genes. It turned off pro-inflammatory genes, such as those that make TNF-alpha, in human immune cells. As a result, TNF-alpha levels dropped, preventing an inflammatory response [19].

In cells, quercetin decreases the release of MCP-1, IL-6, and IL-8, which all trigger and worsen inflammation [20].

Quercetin blocks the production of prostaglandins, key mediators of the inflammatory response. It also blocks a very important inflammatory enzyme, COX-2, similar to most NSAID drugs like Advil [21, 22+].

4) May Be Anti-Aging

Researchers suggest that quercetin may work as a natural anti-aging supplement.

Senescence is a process during which cells lose the ability to regenerate with aging. These become “senescent” cells, they lose their function, produce inflammatory substances, and damage other cells. Senescence is thought to underlie aging [23].

Quercetin is considered a “senolytic”, a natural compound that may stop the aging process. In studies on human skin cells, it eliminated harmful, aging senescent cells [6+].

In mice, Quercetin increased exploratory behavior, spatial learning, and memory. Quercetin can enhance brain function, limit oxidative stress and increase glutathione levels [24].

Quercetin extended the lifespan and increase stress resistance in worms. Another study confirmed that it increases the ability of worms to handle stress while increasing their average lifespan by 15% [25, 26].

However, it is unclear how these cellular and worm studies translate to humans. A recent study found that Quercetin does not extend the lifespan of animals. More research is needed [27].

5) May Prevent Cancer

Scientists have long considered Quercetin and other flavonoids from in fruits and vegetables important in cancer prevention. People who eat more fruits and vegetables tend to have a lower risk of certain types of cancer [28].

Frequent intake of Quercetin-rich foods leads to a lower risk of developing lung cancer, according to an observational study of almost 10,000 people. The association was stronger in those under 50 years of age [29].

In fact, smokers who consume lots of antioxidant-rich phenols have enhanced protection against bladder cancer [30].

Quercetin lowers TNF-a and, which lowers the cancer risk; TNF-a encourages the growth and spreading of most tumor cells. In fact, mice with TNF-a deficiencies are resistant to skin cancer. Quercetin led to a 5-fold increase in the lifespan of mice with leukemia [31, 32].

Other cellular studies confirmed the anti-cancer properties of TNF-a-lowering flavonoids. Quercetin specifically [33]:

  • Stops quickly-dividing cancer cells in the early phases of the cell cycle. This freezes tumor growth and initiates the mitochondria to kill the cancerous cell (via apoptosis) [34, 32]
  • Triggers cell death in human colon cancer cells (by blocking the NF-κB pathway) [34].
  • Slows the growth and Induces the death of leukemia cells (by inhibiting Cox-2 expression and initiating apoptosis) [35, 28].
  • Reduces the spreading of tumors in the brain and liver (cell studies) [36, 37].
  • Enhances the efficacy of conventional chemotherapeutic drugs, like doxorubicin (colon cancer cell studies) [38].
  • Kills leukemia and breast cancer cells without any harmful effects on normal, healthy cells [32].

Scientists think Quercetin may be useful for rare cancers resistant to conventional drugs. However, much more research is needed before we can claim it has any anti-cancer effects in people [38].

6) Reduces the Histamine Response

Quercetin has a profound effect on histamine release. It acts to:

  • Reduce histamine and other inflammatory substances like leukotrienes, and prostaglandins [2].
  • Epigenetically reduce inflammation in cells, which may help control the histamine response long term. Quercetin turns off the histamine receptor gene (H1R), making the body less sensitive to histamine [39].

Many people with histamine issues claim that quercetin helped them, especially those who are TH2-dominant.

7) Fights Bacteria

Quercetin kills various types of bacteria, based on cellular studies. It reduced Staphylococcus infections, including antibiotic-resistant S. aureus (MRSA). Plus, Quercetin increased the susceptibility of MRSA to other antibacterial compounds [40, 40, 41].

Quercetin was active against the H. Pylori infections in test tubes [41].

Quercetin may also boost oral health. It blocked the growth of a wide range of oral bacteria in cellular studies. Quercetin may be useful for dental cavities, gum infections, and mouth infections [42].

The antibacterial activity of Quercetin is partially due to its ability to block an important enzyme bacteria need to survive (DNA gyrase) [43].

With increased antibiotic use and the global spread of antibiotic resistance, Quercetin may be an effective and safe alternative [40].

8) Protects Against Obesity

Quercetin (1000 mg/day) improved the metabolic profile in 78 obese women with PCOS in one clinical trial. It was used over 12 weeks and reduced an obesity marker (resistin), and well as testosterone and LH [44].

In mice, Quercetin supplementation reduced body weight by nearly 40%, reduced inflammatory cytokines increased in obesity, and boosted fat burning. It also increased anti-inflammatory defense (IL-10) [45].

Quercetin reduced weight gain caused by a high-fat diet and improved insulin sensitivity and glucose tolerance in mice [46].

In rats, it reduces obesity-triggered skeletal muscle wasting by blocking inflammatory receptors and their activity [47].

High dose Quercetin reduced blood glucose levels, improved blood lipids and reduced high blood pressure in obese rats. Quercetin enhanced the sensitivity of fat tissues to glucose (via GLUT4 translocation and AKT signaling), which causes fats to take more glucose from the blood and use it [48, 46].

In cell studies, it reduced fat accumulation and inflammation from obesity [45].

Its effect on harmful genes may explain these benefits. Quercetin turned off obesity genes linked to inflammation (AMPK and MAPK pathways), which stops tissues from storing new fats and triggers the destruction of existing fat cells [49].

9) May Prevent Diabetes

Mice fed a Quercetin-rich diet had lower blood glucose levels and HBA1c. Quercetin blocks the enzymes that degrade complex sugars in the gut after meals. As a consequence, less glucose is absorbed from foods and glucose levels don’t spike [50].

In rats with type 1 diabetes, Quercetin significantly lowered blood sugar levels [51].

Quercetin also improved insulin levels in diabetic mice and reduced the activity of liver and pancreas genes that can trigger diabetes. It may improve pancreas and liver function by preventing damage and recovering normal tissue regeneration [52].

In obese rats, Quercetin supplementation improved insulin resistance [48].

Quercetin increases glucose uptake into fat cells by increasing the number and activity of important glucose transporters. It also sensitizes a pathway that controls insulin release in response to glucose (ERK1/2) (GLUT4) [53, 54, 55].

One cell study analyzed the effects of ten compounds from berries on glucose uptake. Two forms of Quercetin had the strongest effect on glucose uptake into tissues via the AMPK pathway (sugar-bound Quercetin-3-O-glycoside and the free Quercetin aglycone) [56, 57].

By blocking the NF-kb pathway, Quercetin may lower toxins that cause health complications in people with diabetes [58].

10) May Help with Arthritis

In one study, people with Rheumatoid Arthritis who consumed lots of raw berries, fruits, vegetables, nuts, roots, seeds, and sprouts rich in Quercetin and other antioxidants had fewer symptoms [59].

Quercetin supplements (500 mg/day) had no effect on some inflammatory markers (CRP) but did reduce others (TNF-alpha and IL-6) in an 8-week study of 51 women. But in a later 8-week follow up study in 50 women, the same dose did greatly reduce joint stiffness, morning pain, after-activity pain and the inflammatory marker TNF-alpha [60, 61].

In a cellular study, Quercetin triggered the death of inflammatory joint cells that contribute to the development of rheumatoid arthritis. In another study, it also stopped these cells from dividing and prevented neutrophil activation, which worsens the autoimmune response [62, 63+].

Since conventional arthritis drugs (such as methotrexate and paclitaxel) are quite toxic for the whole body, quercetin may be a safe alternative [64].

11) Improves Heart Health

Quercetin (730/day) lowered blood pressure in a trial of 41 people with high blood pressure after 28 days [65].

In a trial of 72 women with type 2 diabetes, Quercetin (500 mg/day) reduced systolic blood pressure after 10 weeks [66].

Lower dose quercetin (100 mg/day) from onion peel extract improved blood lipid profiles, glucose, and blood pressure in a study of 92 smokers. It reduced total and LDL cholesterol while lowering HDL, all of which are important factors for heart health [67].

Quercetin supplements may somewhat protect people exposed to high levels of oxidative stress (such as cigarette smokers) from developing heart disease [68].

12) May Improve Blood Flow

Quercetin’s effects on circulation are closely tied to its heart-protective effects. By its antioxidant action, it protects and expands blood vessels. Healthy blood vessels can relax better and faster to improve blood flow.

In a clinical trial of 10 healthy men, a combination quercetin supplement (200 mg quercetin with EGCG and epicatechin) increased the nitric oxide status. Nitric oxide helps expand the blood vessels to deliver more blood to the body. It also reduced endothelin-1, which narrows blood vessels and counteracts nitric oxide [69].

In one observational study of 120 healthy people, a multivitamin supplement with quercetin increased blood quercetin levels, reduced homocysteine, and GGT – all of which are important for heart health and circulation [70].

Quercetin improves the health of cells that line blood vessels and smooth muscles, which helps reduce blood pressure and keep blood vessels elastic [71].

13) May Reduce Allergies

Lactose, eggs, peanuts, fish, wheat, shellfish, tree nuts, and soy can all trigger an immunoglobulin E (IgE) allergic responses [72].

Quercetin is a potent suppressor of IgE allergic responses and may be used as an alternative remedy for IgE-mediated food allergies. It reduced allergies and balanced the immune response in several cellular and animal studies [73, 74].

Mast cells are activated during an allergic response or in people with mast cell activation syndrome. Overactive mast cells promote inflammation by releasing molecules such as histamine, leukotrienes, cytokines, and other harmful products. Quercetin blocks human mast cell activation [6+].

Some people with allergic hay fever have an over-expression of the histamine H1 receptor (H1R) gene. The more this gene is expressed, the allergic symptoms become severe. Quercetin reduces the expression of this gene [39].

14) Fights Viruses

In a large study of ~1000 people, Quercetin (1000 mg/day) reduced the number of sick days and the severity of common cold symptoms in physically-fit people 40 or older [75].

Quercetin may reduce illness after intensive exercise. Endurance athletes are more likely to catch a cold since they frequently stretch their bodies to exhaustion. Quercetin (100 mg/day) reduced the incidence of upper respiratory infections such as the common cold in a trial of 40 trained athletes. It was given after 3 days of intense exercise over the 2-week exercise recovery period [76].

However, quercetin alone (1000 mg/day) for three weeks before, during, and for two weeks after the 160-km Western States Endurance Run didn’t lower flu incidence in another study [77].

15) May Speed Post-Exercise Recovery

In one trial, Quercetin (1000 mg/day) reduced inflammation in a trial of 30 cyclists following heavy training. It had a stronger effect combined with other antioxidants and anti-inflammatories: EGCG, isoquercetin, EPA, and DHA [78].

Combining several flavonoids may increase their bioavailability immune benefits.

Quercetin (1000 mg/day) reduced post-exercise inflammation and oxidative stress after 2 weeks in young untrained men [79].

16) May Improve Athletic Endurance

Quercetin improved endurance exercise capacity (VO2 max and performance) in clinical trials of over 200 people in total. It was used at 1000 mg/day for an average of 11 days [80, 81].

Quercetin (1000 mg/day) increased endurance and reduced fatigue even without exercise training after just 7 days in a clinical trial of 12 untrained people [82].

In another trial of 10 young men, a single dose of quercetin (1000mg/day) improved muscle performance during and after resistance training [83].

17) May Protect the Liver

Quercetin given before toxic amounts of alcohol protected the liver of rats against oxidative stress. It neutralized harmful products of fat breakdown and increased the production of the master antioxidant Glutathione [84].

Quercetin protected the liver, limited damage and oxidative stress in rats exposed to toxins (aflatoxin) [85].

Acetaminophen can cause serious liver damage in high doses, and overdoses in humans are not rare. Quercetin can reduce liver damage from acetaminophen by neutralizing free radicals. It protected both the kidneys and liver in rats and improved mitochondria health [86].

In rats with liver damage, Quercetin improved liver health, decreased liver scarring, oxidative stress and DNA damage. It also lowered overall inflammation, which helps in liver recovery [87].

Quercetin protected against obesity-induced fatty liver disease in mice by activating the mitochondria and improving energy use in the liver [88].

18) May Protect the Kidneys

Quercetin reduces kidney damage from toxic substance. In rats given the chemotherapy drug cisplatin, quercetin protected the kidney tissue [89].

Additionally, it reduced kidney damage and restored kidney antioxidant enzymes in another rat study [90].

However, these studies used quercetin injections, which have good bioavailability.

19) Might Improve Sexual Function

Oxidative stress is a primary cause of the inability to get or maintain an erection, also known as erectile dysfunction. Quercetin improved erectile dysfunction by reducing levels of oxidative stress and increasing blood-vessel-relaxing nitric oxide in diabetic mice [91].

20) May Help with Asthma

Tightly linked to its balancing effects on Th2 dominance, Quercetin may improve allergic asthma. It relaxed smooth muscles that line the airways in a tissue study, which improves airway flow. It may provide relief of asthma symptoms and decrease the use of conventional, short-term asthma treatments if the same holds true in humans [92].

21) May Protect Your Eyes

Quercetin may reduce cataract risk by acting on important vision pathways (oxidative stress, glycation, and cell signaling) [93].

In rats with cataracts, it reduced eye damage and enhanced lens clarity [94].

Retinal cells found at the back of the eyes act as light receptors and play an important role in setting the color, resolution, and brightness of your vision. Quercetin had a protective effect on retinal cells and increased their survival [95, 95, 96].

22) May Protect Against Radiation

In rats exposed to X-ray radiation, Quercetin reduced skin damage and prevented skin scarring [97].

23) May Protect from Ulcers

In a trial of 40 men, quercetin applied directly to mouth ulcers relieved pain and completely healed the ulcers in half of the cases within 7–10 days [98].

Quercetin protected against stomach ulcers and acid reflux in animal studies. In cells, it also blocked the growth of Helicobacter pylori, the main cause of stomach ulcers. In guinea pigs with H. pylori, high doses of quercetin reduced the infection and the inflammatory response [6+].

Limitations and Caveats

Most of the studies mentioned here were done in animals, although some clinical trials have been carried out too. All of the listed benefits rely on quercetin being absorbed, so it’s questionable if the benefits that lack clinical trials can be applied to humans.

Future Research Perspectives

Quercetin lowers TNF-a, based on both clinical, animal, and human studies. High TNF-a levels are associated with the following diseases:

… and others.

TNF-a is a target of many drug therapies. Antibodies raised against TNF-a are now commercially available under prescription and are proving to be beneficial for many patients.

Since quercetin has a significant impact on TNF-a, it is likely that future research will explore its benefits for a range of health conditions linked to increased inflammation and TNF-a.

Quercetin Dosage & Bioavailability

Typical Dosage

  • The dosage in clinical trials varied between ~100-1,000 mg/day. The most common dose was 500-1,000 mg/day [3].

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 [99]

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 [6].

Once quercetin is in the gut, its bioavailability also depends on how well it’s modified to be made more soluble [6+].

For Dogs

Quercetin is sometimes added to commercially-available dog food. Similar to humans, dogs metabolize quercetin very quickly. Its bioavailability in dogs is also low. Unlike for humans, dogs absorb the rutin form of Quercetin found in apples better than humans [100].

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 [101].

The human dosage could be adapted to dogs if using Quercetin supplements, although the bioavailability remains uncertain.

Based on 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 if you’re unsure how to dose quercetin for 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 [102]
  • Liposomal or nano-quercetin [103, 104, 105]
  • Adding it to foods, such as cereal bars (possibly) [99]
  • Taking quercetin from onion powder instead of from apple peel powder [99]
  • Quercetin 3-glucose as opposed to the free quercetin (in rats) [106]
  • Alcoholic tinctures, estimated to be ~40% bioavailable [6+].
  • Combining it with bromelain, which increases both its bioavailability and anti-inflammatory effects [107]

Resveratrol and ECGC 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 [108].

In cells, the combination blocked colon cancer and increase cancer cell death [108].

All flavonoids act in synergy to increase antioxidant defense. Quercetin increases the bioavailability of ECGC and other antioxidant flavonoids [6+].

Side Effects & Safety

Overall, quercetin is a safe supplement. It doesn’t cause any serious side effects in typical doses [20].

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.

Side Effects

Brain cells

Quercetin was toxic to rat brain cells. Higher concentrations caused more brain cells to die [109].

It’s uncertain how cellular effects and doses could translate to humans.

Homocysteine Levels

In human liver cancer cells, quercetin significantly increased homocysteine levels [110].

The same effect has not been observed in clinical trials.

Thyroid Function

High doses of quercetin and other flavonoids acted as thyroid disruptors in animal studies. People with thyroid problems should use caution [111].

Children and Pregnant Women

A safe quercetin dose for children and pregnant women has not been established or studied.

Quercetin is likely safe if taken through a diet of quercetin-rich foods during pregnancy and childhood. Caution is advised with supplements. Quercetin was mostly safe during pregnancy in animal studies, but it reduced fertility in female mice in one study [112113].

Drug Interactions

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.

On the positive side, it may increase the bioavailability of ECGC and other antioxidant flavonoids [6+].

COMT Expression

COMT, the worrier or warrior gene, helps with methylation and also breaks down important neurotransmitters [114].

People with SNPs that predispose them to low COMT levels should avoid Quercetin, since it has a catechol structure and can block COMT gene expression [115].

Dietary Sources & Supplements

Foods

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 [116].

Food sources of quercetin include [3]:

  • Vegetables such as capers (highest concentration), onions, eggplant, celery, asparagus
  • fruits, especially berries, but also apples and oranges
  • Nuts
  • Black and green tea

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