Health Care Professional Information
Quercetin is a flavonol that constitutes the major bioflavonoid sources in the human diet. The glycoside form is readily available in dietary plants such as teas, onions, apple and buckwheat (5). Quercetin is thought to have antioxidant, anti-inflammatory and anti-allergic properties. In vitro data suggest quercetin may have anti-cancer effects (1), but more research is needed to explore this potential.
Quercetin was shown to exacerbate estrogen-induced breast tumors in rats (12).
Teas, onions, apples, buckwheat
- Cancer prevention
- Cancer treatment
- Cardiovascular disease
Mechanism of Action
Quercetin constitutes the major bioflavonoid in the human diet. Its antioxidant activity is due to the reactivity of its phenolic group, which reacts with free radicals to form the more stable phenoxy radicals (1). Quercetin is thought to have anti-inflammatory and anti-allergy properties. The proposed mechanism of action is inhibition of lipoxygenase and cyclooxygenase resulting in reduced production of inflammatory mediators (e.g., leukotrienes and histamine). Quercetin appears to inhibit cyclooxygenase to a greater degree than lipoxygenase. It also has been shown to have membrane-stabilizing capabilities and to inhibit aldose reductase and low-density lipoprotein oxidation (8). Significant antiviral activity has been shown in vitro and in vivo. Proposed anti-cancer mechanisms of action include down-regulation of mutant p53 proteins; G1 phase arrest (1); tyrosine kinase inhibition (10); estrogen receptor binding; inhibition of heat shock proteins; and RAS protein expression inhibition (1). Presently, considerable in vitro data support the concept of quercetin as an anti-cancer compound. 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 bacteria 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 conjugates of glucuronic acid and sulfate groups (2).
No adverse effects have been reported with single oral doses up to 4 grams.
Papain and Bromelain: May assist the absorption of Quercetin in the intestine (6).
Quinolone antibiotic: Quercetin may compete for DNA gyrase binding sites on bacteria (5).
CYP 3A4: Quercetin was shown to significantly inhibit the constitutive CYP3A4 activity (11).
Literature Summary and Critique
Dosage (Inside MSKCC Only)
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- Lamson DW, Brignall MS. Antioxidant and cancer III: quercetin. Altern Med Rev 2000;5:196-208.
- Graefe EU, et al. Pharmacokinetics and bioavailability of the flavonol quercetin in humans. Int J Clin Pharmacol Therapy 1999;37:219-33.
- Erlund I, et al. Pharmacokinetics of quercetin aglycone and rutin in healthy volunteers. Eur J Clin Pharmacol 2000;56:545-53.
- Sampson S, et al. Flavonol and flavone intakes in US health professionals. J Am Diet Assoc 2002;102:1414-20.
- Herr, SM. Herb-Drug Interaction Handbook. Chuch Street books. 2nd ed. Nassau NY 2002.
- 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.
- Janssen K, et al. Effects of the flavonoids quercetin and apigenin on hemostasis in healthy volunteers: results from an in vitro and dietary supplement study. Am J Clin Nutr 1998;67:255-62.
- Chopra M, et al. Nonalcoholic red wine extract and quercetin inhibit LDL oxidation without affecting plasma antioxidant vitamin and carotenoid concentrations. Clin Chem 2000;46:1162-70.
- Beatty ER, et al. Effect of dietary quercetin on oxidative DNA damage in healthy human subjects. Br J Nutr 2000;84:919-25.
- Ferry DR, et al. Phase I clinical trial of the flavonoid quercetin: pharmacokinetics and evidence for in vivo tyrosine kinase inhibition. Clin Cancer Res 1996;2:659-68.
- Sergent T, Dupont I, Van der Heiden E, et al. CYP1A1 and CYP3A4 modulation by dietary flavonoids in human intestinal Caco-2 cells. Toxicol Lett. 2009 Dec 15;191(2-3):216-22.
How It Works
Bottom Line: Quercetin may be helpful in relieving the symptoms of prostatitis, but there is no proof that it can treat cancer, heart disease, asthma, colitis, or any other medical condition.
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 and anti-allergy properties by inhibiting the release of substances that mediate the inflammatory response, such as histamine. A study in healthy volunteers suggests that quercetin inhibits low-density lipoprotein (LDL) oxidation, which may be involved in the development of atherosclerosis and cancer. Significant antiviral activity has been shown in laboratory and animal experiments. 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.
- To treat allergies
Extensive laboratory studies show an anti-inflammatory effect of quercetin, including inhibition of histamine release, but there is no proof from clinical trials that this effect occurs in humans.
- To prevent and treat cancer
Extensive laboratory studies show anti-cancer activity of quercetin against a wide range of cancer cell types, but there is no proof from clinical trials that it can prevent or treat cancer.
- 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. There is no other proof that quercetin can treat heart disease.
- To reduce inflammation (in conditions such as asthma, inflammatory bowel disease, psoriasis, and colitis)
Extensive laboratory studies show an anti-inflammatory effect of quercetin, but there is no proof from clinical trials that quercetin can treat these conditions.
Chronic prostatitis (chronic pelvic pain syndrome)
Thirty men with category III chronic prostatitis (chronic pelvic pain syndrome) took part in a study of 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. More men taking quercetin reported significant improvements in quality of life and symptoms (67%) as compared to men taking the placebo pill (20%). This study also suggested that bromelain and papain can enhance the absorption of quercetin. Because the sample size of this clinical trial is relatively small, larger studies are needed to substantiate these results.
Prevention of DNA damage
The effect of dietary quercetin on DNA damage was measured in a study with 36 healthy volunteers. Fifty percent of the volunteers were given a low-quercetin diet for 14 days, followed by a seven-day “washout” period of normal diet, and 14 days of high-quercetin diet (one onion cake and one glass of black tea daily). The other volunteers were given the high-quercetin diet first and the low-quercetin diet second. DNA damage in white blood cells was measured before and after each diet period. Volunteers were found to have the same levels of DNA damage after eating both the high- and low-quercetin diets, indicating that quercetin has no short-term effects on DNA damage in white blood cells. This study is small and does not examine the long-term effects of a high-quercetin diet.
Do Not Take If
- You are taking quinolone antibiotics (Quercetin may lessen their effect).
- No side effects have been reported with single oral doses of up to 4 grams.
- The fruit extracts papain and bromelain may help increase the absorption of Quercetin in the intestine.
- Quercetin was shown to exacerbate estrogen-induced breast tumors in rats.
Last updated: September 28, 2012