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
Quercetin Dosage & Bioavailability
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. Unlike 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.
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 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 likely 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 .
Dietary Sources & Supplements
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
Quercetin, an antioxidant flavonoid, is found in many plant-based foods. The best food sources are capers, onions, berries, and tea. It has poor oral bioavailability (around 2% of ingested quercetin is absorbed into the bloodstream), but taking it alongside fatty foods or oils or in alcoholic tinctures may increase the amount that is absorbed.
The safety profile of quercetin is incomplete, and the effect of large quantities of quercetin on vulnerable populations is unknown. In cells, it blocks certain CYP enzymes, which means that it may theoretically interact with drugs that are metabolized by those enzymes.