Quercetin belongs to a class of water-soluble plant coloring substances called bioflavonoids. Bioflavonoids have strong antioxidant effects when they are studied in the test tube, and this is the basis for some of the health claims attached to them. However, growing evidence suggests that bioflavonoids do not in fact act as antioxidants in human beings.24 Nonetheless, as widely available plant substances, they are considered possible semi-nutrients, substances that are not essential for life but might help promote optimal health.
Quercetin is not an essential nutrient. It is found in red wine, grapefruit, onions, apples, black tea, and, in lesser amounts, in leafy green vegetables and beans. However, to get a therapeutic dosage, you'll have to take a supplement.
Quercetin supplements are available in pill and tablet form.
A typical dosage is 200 to 400 mg 3 times daily. A special type of quercetin, quercetin chalcone, is claimed to be absorbed better, but there is little reliable evidence to prove this.
Quercetin is widely marketed as a treatment for allergic conditions such as asthma, hay fever, eczema, and hives. These proposed uses are based on test-tube research showing that quercetin prevents certain immune cells from releasing histamine, the chemical that triggers an allergic reaction.1,2 Quercetin may also block other substances involved with allergies.3 However, this evidence is extremely preliminary, far too preliminary to rely upon at all. There is as yet no direct evidence that taking quercetin supplements will reduce your allergy symptoms.
A different proposed use of quercetin does have some meaningful supporting evidence: prostatitis. This condition is an inflammation or infection of the prostate gland. The condition causes chronic pain and difficulty with urination and is sometimes called chronic pelvic pain syndrome. Conventional treatment for this condition is often unsatisfactory. One small double-blind, placebo-controlled study has found preliminary evidence that quercetin might help (see next section).4
As noted above, it has been suggested that quercetin’s antioxidant properties might make it helpful for preventing heart disease and strokes.6-10 However, the evidence that it works is highly incomplete. Keep in mind that other powerful antioxidants such as vitamin E and beta-carotene have been ineffective for preventing these conditions. There is limited evidence, however, from a single, small double-blind trial that quercetin might have the separate effect of lowering blood pressure when it is high.25
An animal study found that quercetin might protect rodents with diabetes from forming cataracts.16 Another intriguing finding from test-tube research is that quercetin seems to prevent a wide range of viruses from infecting cells and reproducing once they are inside cells. One study found that quercetin produced this effect against Herpes simplex, polio virus, and various respiratory viruses, including influenza.17,18 However, such studies are too indirect to tell us whether humans taking quercetin supplements can hope for benefits against diseases caused by those viruses.
A one-month, double-blind, placebo-controlled trial of 30 men with chronic pelvic pain ( prostatitis) tested the potential effectiveness of quercetin.4 Participants received either placebo or 500 mg of the supplement twice daily. The results showed that people who received quercetin experienced a statistically significant improvement in symptoms (such as pain), but those given placebo did not improve.
While these are promising results, the study was small and cannot be regarded as definitive. Furthermore, researchers failed to provide the usual statistical evaluation required for such studies (a statistical analysis that directly compares the results in the treatment group against those in the placebo group). Thus, further study will be necessary to discover whether quercetin is actually effective for prostatitis.Interstitial Cystitis
People with interstitial cystitis experience pain and discomfort in the bladder that is reminiscent of a bladder infection, but without the actual presence of such an infection. In a 6-week double-blind, placebo-controlled study, 20 people received either placebo or a supplement containing quercetin and other bioflavonoids.5 The results appeared to indicate better results in the quercetin group. However, this study has only been presented as an abstract and it is not clear from the writeup whether the results were statistically meaningful.
Quercetin appears to be quite safe. However, concerns have been raised that, under some circumstances, it might raise cancer risk. Quercetin "fails" a standard laboratory test called the Ames test, which is designed to identify chemicals that might be carcinogenic. Nonetheless, a bad showing on the Ames test does not definitely mean a chemical causes cancer. Most other evidence suggests that quercetin does not cause cancer and may, in fact, help prevent cancer.19,20,21 Still, one highly preliminary study suggests that quercetin combined with other bioflavonoids in the diet of pregnant women might increase the risk of infant leukemia.22 On this basis, pregnant women should probably avoid quercetin supplements. Maximum safe dosages for young children, nursing women, or people with serious liver or kidney disease have not been established.
Evidence suggests that use of quercetin supplements can elevate urine and blood levels of the substance homovanillic acid.23 While this itself should be harmless, lab tests for homovanillic acid are used to diagnose a rare, dangerous condition called neuroblastoma, and for this reason, use of quercetin supplements could potentially cause a false positive diagnosis of this condition.
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21. ElAttar TM, Virji AS. Modulating effect of resveratrol and quercetin on oral cancer cell growth and proliferation. Anticancer Drugs. 1999;10:187-193.
22. Strick R, Strissel PL, Borgers S, et al. Dietary bioflavonoids induce cleavage in the MLL gene and may contribute to infant leukemia. Proc Natl Acad Sci. 2000;97:4790-4795.
23. Weldin J, Jack R, Dugaw K, et al. Quercetin, an over-the-counter supplement, causes neuroblastoma-like elevation of plasma homovanillic acid. Pediatr Dev Pathol. 2004;6:547-551.
24. Lotito SB, Frei B. Consumption of flavonoid-rich foods and increased plasma antioxidant capacity in humans: cause, consequence, or epiphenomenon? Free Radic Biol Med. 2006;41:1727-1746.
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Last reviewed September 2014 by EBSCO CAM Review Board
Last Updated: 9/18/2014