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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. 1 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.
What is the Scientific Evidence for Quercetin?
A one-month, double-blind, placebo-controlled trial of 30 men with chronic pelvic pain ( prostatitis ) tested the potential effectiveness of quercetin. 2 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.
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. 3 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.
- Lotito SB, Frei B. Consumption of flavonoid-rich foods and increased plasma antioxidant capacity in humans: cause, consequence, or epiphenomenon? Free Radic Biol Med. 41(12):1727-46.
- Shoskes DA, Zeitlin SI, Shahed A, Rajfer J. Quercetin in men with category III chronic prostatitis: a preliminary prospective, double-blind, placebo-controlled trial. Urology. 54(6):960-3.
- Rodriguez LV, Janzen N, Raz S, et al. Treatment of interstitial cystitis with a quercetin containing compound: a preliminary, double-blind placebo control trial. Presented at: American Urological Association 2001 Annual Meeting; June 2-7, 2001; Anaheim, CA.