Quercetin is one of the most researched bioflavonoids in laboratory experiments and has also been shown to have anti-inflammatory and anti-carcinogenic properties.
As research continues on this compound, scientists have also added neuroprotective, anti-allergy and antidepressant effects to its growing list of possible benefits.
Quercetin is naturally found in fruits and vegetables (it gives many vegetables and fruits their color) including apples, black and green tea, onions, red grapes, cherries, raspberries and citrus fruits; the highest concentrations are found in onions and red grapes.
Quercetin helps fight free radicals. Free radicals are atoms or molecules that are highly reactive with other cellular structures because they contain unpaired electrons. As a result of this unpaired electron, free radicals seek out and take electrons from other molecules, which frequently cause damage to the second molecule.
There are many types of radicals, but those of most concern to human health are derived from oxygen, and are known collectively as reactive oxygen species.
When there is a buildup of reactive oxygen species in cells, this may cause damage to cells, and may even cause cell death.
Many studies demonstrate quercetin’s high total antioxidant capacity. The results of one study, published in a 2011 issue of Advances in Experimental Medicine and Biology, showed quercetin’s ability to reduce LPS-induced reactive oxygen species to near normal levels in human acute monocytic leukemia cells (2).
Quercetin acts as an anti-inflammatory. The effects of bioflavonoids, including quercetin, on a variety of inflammatory processes and immune functions have been extensively reviewed. Tumor necrosis factor alpha (TNF-α) is one of the major pro-inflammatory cytokines (cell signaling molecules that aid cell to cell communication in immune responses) involved in the development of chronic inflammatory diseases.
The findings of a study published in Clinical and Vaccine Immunology, showed that quercetin can effectively inhibit the cytokine TNF-α.
Animal experiments also support its anti-inflammatory effect. According to a 2016 review, quercetin improved the inflammatory response induced by carrageenan and a high-fat diet in mice.
In addition, it decreased clinical signs of arthritis compared to untreated controls in rats with chronic arthritis (3).
Researchers note, however, that although quercetin exhibited an anti-inflammation effect in vitro (cells) and in vivo (animals), some studies in humans did not totally support these results. The effect in which quercetin acts as an inflammation fighter in humans, needs to be further verified.
While more studies are needed, these early results show this compound’s potential in being a new flavonoid-based neutrapharmaceutical agent for the treatment of various inflammatory diseases (4).
Quercetin may help prevent certain cancers. The medical community has long been emphasizing the importance of eating a healthy diet rich in fruits and vegetables as a way to help minimize one’s cancer risk. Quercetin and other flavonoids are believed to play an important role in preventing cancer.
In the last decades, several anticancer properties of quercetin have been found, including cell signaling, pro-apoptotic (cancer cell destroying), anti-proliferative and growth suppression. There are also studies showing potential synergistic effects when quercetin is combined with chemotherapy or radiotherapy.
Studies in animal models have shown its ability to boost the antitumor effects of doxorubicin (a chemotherapy drug) in liver cancer cells, while protecting normal liver cells (5).
Researchers are also looking into quercetin’s role in protecting healthy cells from the side effects of chemotherapy and radiation (6).
According to the Memorial Sloan Kettering Cancer Center website, considerable laboratory data supports the concept of quercetin as an anticancer compound, but it is still unclear from clinical trials whether this effect occurs in the human body (7).
Quercetin has allergy-fighting potential. Quercetin has several anti-allergic properties, including stimulation of the immune system, antiviral activity, inhibition of histamine release and suppression of the antigen-specific immunoglobulin E (IgE) allergic response.
These mechanisms of action contribute to the anti-inflammatory and immunomodulating properties of quercetin, making it a potentially effective treatment for bronchial asthma, seasonal allergies and nut and food allergies (8).
Quercetin may help lower blood pressure. Studies report that quercetin supplementation reduces blood pressure in hypertensive rodents. Studies demonstrating the ability of supplementation to lower blood pressure in hypertensive humans, however, are lacking.
In a 2007 study, researchers set out to evaluate if this compound had an effect in humans. They found that supplementation (730mg/day for 28 days) reduced systolic, diastolic and mean arterial pressure in stage 1 hypertensive subjects (9).
In a clinical trial of 72 women with type 2 diabetes, 500 mg of daily supplementation decreased systolic blood pressure (top number) significantly compared to placebo. Systolic blood pressure measures the pressure in the blood vessels when the heart beats. Changes in diastolic blood pressure between the groups were not significant (10,11).
Quercetin appears to be safe when taken short-term. It has been safely used in some studies in amounts up to 500 mg twice daily for 12 weeks. It is not known if long-term use or higher doses are safe.
Some side effects with oral use may include headache, numbness and tingling. When given intravenously, nausea, vomiting and shortness of breath have been reported (12).
High doses of quercetin may cause kidney damage.
Recommended dosages range from 100 to 250 mg/three times daily to 400 to 500 mg/three times daily. Further studies will help determine the proper dosage.
Quercetin on its own is believed to have poor absorption. It is often suggested to supplement with other bioflavonoids such as resveratrol, genistein or green tea catechins to increase the potency.
Many studies also note a high range of differences between people who ingest the same amount of quercetin, suggesting a large degree of variability is possible with supplementation.
Quercetin is a plant-derived compound known primarily for its potent antioxidant properties. Quercetin is naturally found in fruits and vegetables, with some of the highest concentrations found in onions and red grapes.
This compound has been well researched and in addition to its ability to fight free radicals, it has also been shown to have anti-inflammatory, cancer-fighting and allergy-relieving properties.
Quercetin on its own is believed to have poor absorption; it is often suggested to supplement it with other bioflavonoids to increase its potency.