What does it do?
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 estrogen). 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
Quercetin has been used in
connection with the following conditions (refer to the individual
health concern for complete information):
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
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 pharmacist for any health problem and before using any supplements or
before making any changes in prescribed medications. Information expires July 2004.
Contradictory, insufficient, or preliminary
studies suggesting a health benefit or minimal health benefit.