Quercetin is a flavonoid, a substance found in fruits, flowers, and vegetables that, among other functions, gives them their color. The average daily intake of flavonoids in the United States is between 150 and 200 mg. In general, flavonoids have been found to have both antioxidant and anti-inflammatory properties, useful in preventing or treating many health conditions. The thousands of flavonoid compounds can be placed into a handful of categories—one of which is quercetin. Quercetin can decrease allergic reactions and may be beneficial in treating other inflammatory responses, such as canker sores, hives, asthma, and arthritis. Other conditions for which quercetin may be beneficial include diabetes mellitus, dysentery, gout, heart disease, infections, inflammatory bowel disease, cataracts, atopic dermatitis, and psoriasis.

Recently, research has focused on prevention of carcinogenesis. Quercetin may be helpful in the prevention of skin carcinogenesis and has demonstrated antitumor activity in patients with ovarian cancer and hepatoma. Further clinical studies are needed to confirm these results.

Fruits and vegetables, particularly citrus fruits, apples, onions, parsley, tea, and red wine.

Quercetin is a flavonoid. Flavonoids are found in fruits and vegetables, are ascorbic-acid-related substances, and are compounds that are in human diets as glycosides. Research suggests, however, that many of the biologic effects are associated with aglycone, a substance that is released in the intestine, rather than with the compound bound in the glycoside.

• Quercetin is available commercially in several strengths in powder or capsule form.
• Quercetin is often packaged with bromelain as an anti-inflammatory agent.
• Flavonoid-rich extracts include those from grape seed, bilberry, Ginkgo biloba, and green tea. Quercetin is one of the many flavonoids these substances contain.

• Allergies and hives: Research shows that quercetin inhibits histamine release from basophils and mast cells.
• Asthma: Quercetin has been shown to inhibit histamine release, as with allergies.
• Canker sores: Quercetin may be beneficial in the treatment of canker sores by increasing the number of ulcer-free days and by producing mild symptomatic relief.
• Diabetes mellitus: Quercetin can reduce the accumulation of sorbitol in lens tissues, and thus may be beneficial in preventing cataracts, retinopathy, neuropathy, and other complications of diabetes mellitus. Flavonoids, including quercetin, promote insulin secretion, increase intracellular vitamin C levels, protect blood vessels, prevent easy bruising, and support the immune system, all of which are beneficial to individuals with diabetes.
• Dysentery: Quercetin and tocopherol in combination were found to restore the immune homeostasis and to normalize clinical indices in patients with dysentery.
• Gout: Quercetin inhibits uric acid production, as well as the manufacture and release of inflammatory compounds.
• Heart disease: Although studies are not conclusive on the benefits of quercetin in the treatment of coronary disease, it has been shown that individuals with very low intakes of flavonoids are at higher risk.
• Infection: Quercetin inhibits infectiousness and/or replication of such viruses as herpes simplex type 1, polio virus type 1, parainfluenza virus type 3, and respiratory syncytial virus.
• Psoriasis: Quercetin inhibits lipoxygenase.
• Rheumatoid arthritis: Quercetin may be beneficial by inhibiting mast cell degranulation, thus reducing tissue destruction.

• 100 to 250 mg tid is thought to be an effective general supplementation dose.
• 250 to 600 mg per day, divided in several doses, is effective in reducing histamine levels and allergy symptoms.
• 200 to 400 mg of quercetin should be taken with bromelain between meals tid for the treatment of gout. (Bromelain both enhances the absorption of quercetin and provides anti-inflammatory effects of its own.)
• 200 to 400 mg of quercetin should be taken approximately 20 minutes before each meal for the treatment of chronic hives. Acute symptoms of allergies or outbreaks of hives can be treated with 800 mg qid.

No side effects have been associated with quercetin.

No problems with quercetin have been demonstrated.

In vitro, quercetin (1 and 10 mM) potentiated the effects of adriamycin on MCF-7 ADR resistant human breast cancer cells by inhibiting growth in a dose-dependent manner (Scambia et al. 1994). Concomitant treatment with quercetin (20 mg/kg) and cisplatin (3 mg/kg) in mice implanted with human large-cell lung cancer reduced tumor growth to a greater degree than cisplatin alone (Hofmann et al. 1990). More research is needed to determine if quercetin would be a useful adjunct to chemotherapy.

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