- Polyphenolic flavonoid
For Patients & Caregivers
Bottom Line: Quercetin has not been shown to treat cancer or other diseases.
Quercetin belongs to a family of compounds called bioflavonoids, which are largely responsible for the bright colors and medicinal activities of many plants. Quercetin is the most common bioflavonoid that people consume, and is the most active of the bioflavonoids in laboratory experiments. It is known to act as an antioxidant, neutralizing free radicals that can cause cellular and DNA damage. Quercetin is thought to have anti-inflammatory properties by inhibiting the release of substances that mediate the inflammatory response, such as histamine. Presently, considerable laboratory data support the concept of quercetin as an anticancer compound, but it is still unclear from clinical trials whether this effect occurs in the human body.
Becasue of its antioxidant effects, quercetin may interfere with the actions of certain chemotherapy drugs.
- To treat allergies
Laboratory studies show an anti-inflammatory effect of quercetin, including inhibition of histamine release. Clinical trials have not been conducted.
- To prevent and treat cancer
Laboratory studies indicate anti-cancer activity of quercetin against a wide range of cancer cell types. But human data are lacking.
- To treat heart disease
One study showed that quercetin, in combination with red wine extract, lowered LDL oxidation (which may contribute to atherosclerosis) in healthy volunteers. However, it is unclear how much of this effect was due to quercetin alone, and other similar studies have not found the same effect.
Chronic prostatitis (chronic pelvic pain syndrome)
Thirty men with category III chronic prostatitis (chronic pelvic pain syndrome) took part in a study about the effects of quercetin. They were randomly assigned to take either 500 mg of quercetin or a placebo pill by mouth twice a day for one month. Men who took quercetin reported significant improvements in quality of life and symptoms (67%) compared to those on the placebo pill (20%). This study also suggested that bromelain and papain can increase the absorption of quercetin. But larger studies are needed to confirm these results.
For Healthcare Professionals
Quercetin is a dietary flavonoid found in fruits and vegetables including apples, black, green and buckwheat tea, onions, red grapes, cherries, raspberries, citrus fruits. It is also found in some popular medicinal plants including ginkgo biloba and St. John’s Wort (5) and is used widely for its antioxidant effects.
In vitro data indicate that quercetin has anti-inflammatory(13)(14)and chemopreventive effects (15). However, quercetin can also act as an anti-apoptotic agent (5).
Studies in animal models have shown its ability to potentiate the antitumor effects of doxorubicin in liver cancer cells, while protecting normal liver cells (16). Quercetin also demonstrated neuroprotective property and induced anti-depressant effects (17); it was also shown to exert pro-oxidant effects by decreasing serum homocysteine levels (18).
Long-term supplementation with 1000 mg/day quercetin resulted in wide-ranging metabolic effects in a study of healthy subjects (19). Further research is needed to understand the implications of these effects.
Quercetin was shown to exacerbate estrogen-induced breast tumors in rats (12). But human data are lacking.
Due to its antioxidant effects, quercetin may interfere with the actions of certain chemotherapy drugs.
Quercetin constitutes the major bioflavonoid in the human diet. Its antioxidant effects are due its phenolic group, which reacts with free radicals to form the more stable phenoxy radicals (1). Quercetin also exerts anti-inflammatory (13) and chemopreventive (15) properties. It also has been shown to have membrane-stabilizing capabilities and inhibits aldose reductase and low-density lipoprotein oxidation (8). The anti-cancer effects of quercetin are via down regulation of mutant p53 proteins; G1 phase arrest (1); tyrosine kinase inhibition (10); and down regulation of cell survival, proliferative and anti-apoptotic proteins (15). Preclinical data support the concept of quercetin as an anti-cancer compound (15). However, clinical studies that support these uses are few and the results are mixed (7)(9).
Following oral administration, quercetin glycosides are absorbed from the gut. These glycosides may then undergo hydrolysis in the enterocyte via b-glucosidases before draining into the portal vein. Absorption rate from dietary sources is influenced by the position and chemical nature of the glycoside in combination with the various compounds in the food matrix (1).
Quercetin is found predominantly in plasma in the form of its conjugates (e.g., quercetin glucuronides and/or sulfates) and small amounts of unconjugated quercetin aglycone. Maximum plasma concentrations are achieved within the first two hours of administration. This suggests that the absorption site is in the upper gut compartment, and may rule out intestinal bacterial degradation (2)(3).
Previous pharmacokinetic studies using intravenous administration suggest that quercetin is quickly eliminated in humans, with an approximate elimination half-life of less than two hours. Several studies report that quercetin is present in urine as a conjugate of glucuronic acid and sulfate groups (2).
Shoskes D, et al. Quercetin in men with category III chronic prostatitis: a preliminary prospective, double-blind, placebo-controlled trial. Urology 1999;54:960-3.
Thirty men with chronic pelvic pain syndrome received either placebo or 500 mg of quercetin orally twice a day for 1 month. 67% of the patients taking quercetin compared to 20% of patients from the placebo group had an at least 25% improvement of symptoms. A follow-up unblinded, open-label study suggested that bromelain and papain can enhance the absorption of quercetin.