Quercetin

What is it?

Quercetin belongs to a class of water-soluble plant pigments called flavonoids.

Quercetin acts as an antihistamine and has anti-inflammatory properties. As an antioxidant, it protects LDL cholesterol ("bad" cholesterol) from becoming damaged. A variety of evidence indicates that quercetin possesses potent antioxidant properties. Cardiologists believe that damage to LDL cholesterol is an underlying cause of heart disease. Quercetin blocks an enzyme that leads to accumulation of sorbitol, which has been linked to nerve, eye, and kidney damage in those with diabetes. However, no human research has demonstrated these actions of quercetin in people with diabetes patients.

Quercetin is considered a phytoestrogen (i.e., a plant substance with similar functions as that of oestrogen). Some phytoestrogens are believed also to have antiestrogenic effects that might lead to reduced risks of certain cancers. Quercetin was found to have this antiestrogenic activity by inhibiting breast cancer cells in a test tube.¹

In a double-blind trial, 67% of people taking quercetin had an improvement of prostatitis symptoms, compared with a 20% response rate in the placebo group.²

Where is it found?

Quercetin can be found in onions, apples, green tea, and black tea. Smaller amounts are found in leafy green vegetables and beans.

Quercetin has been used in connection with the following conditions (refer to the individual health concern for complete information):

Science Ratings

Health Concerns

Prostatitis (nonbacterial prostatitis, prostadynia)

Oedema (water retention)

Reliable and relatively consistent scientific data showing a substantial health benefit.
2Stars

Contradictory, insufficient, or preliminary studies suggesting a health benefit or minimal health benefit.
1Star

For a herb, supported by traditional use but minimal or no scientific evidence. For a supplement, little scientific support and/or minimal health benefit.

Who is likely to be deficient?

No clear deficiency of quercetin has been established.

How much is usually taken?

Some doctors recommend 200–500 mg of quercetin taken two to three times per day. Optimal intake remains unknown.

Are there any side effects or interactions?

No clear toxicity has been identified. Early quercetin research suggested that large amounts of quercetin could cause cancer in animals.³ Most,⁴ ⁵ ⁶ but not all,⁷ current research finds quercetin to be safe or actually linked to protection from cancer.

Quercetin has been shown to cause chromosomal mutations in certain bacteria in test tube studies.⁸ Although the significance of this finding for humans is not clear, some doctors are concerned about the possibility that birth defects could occur in the offspring of people supplementing with quercetin at the time of conception or during pregnancy.

Since flavonoids help protect and enhance vitamin C, quercetin is often taken with vitamin C.

Are there any drug interactions?
Certain medicines may interact with quercetin. Refer to drug interactions for a list of those medicines.

References
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1. Miodini P, Fioravanti L, di Fronzo G, Capelletti V. The two phyto-oestrogens genistein and quercetin exert different effects on oestrogen receptor function. Br J Cancer 1999;80:1150–5.

2. 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 1999;54:960–3.

3. Ishikawa M, Oikawa T, Hosokawa M, et al. Enhancing effect of quercetin on 3-methylcholanthrene carcinogenesis in C57B1/6 mice. Neoplasma 1985;43:435–41.

4. Hertog MGL, Feskens EJM, Hollman PCH, et al. Dietary flavonoids and cancer risk in the Zutphen elderly study. Nutr Cancer 1994;22:175–84.

5. Castillo MH, Perkins E, Campbell JH, et al. The effects of the bioflavonoid quercetin on squamous cell carcinoma of head and neck origin. Am J Surg 1989;351–5.

6. Stavric B. Quercetin in our diet: from potent mutagen to probably anticarcinogen. Clin Biochem 1994;27:245–8.

7. Barotto NN, López CB, Eyard AR, et al. Quercetin enhances pretumourous lesions in the NMU model of rat pancreatic carcinogenesis. Cancer Lett 1998;129:1–6.

8. Stoewsand GS, Anderson JL, Boyd JN, Hrazdina G. Quercetin: a mutagen, not a carcinogen in Fischer rats. J Toxicol Environ Health 1984;14:105–14.

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The information presented in Healthnotes is for informational purposes only. It is based on scientific studies (human, animal, or in vitro), clinical experience, or traditional usage as cited in each article. The results reported may not necessarily occur in all individuals. For many of the conditions discussed, treatment with prescription or over the counter medication is also available. Consult your doctor, practitioner, and/or chemist for any health problem and before using any supplements or before making any changes in prescribed medications. Information expires March 2007.

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