Quercetin is one of the many substances produced by plants called flavonoids. Scientists are becoming more interested in flavonoids as studies suggest they play an important role in the health benefits provided by fruits and vegetables, according to the Linus Pauling Institute. Quercetin belongs to a subclass of flavonoids called flavonols.
Function
Quercetin functions as an antioxidant, antihistamine and an anti-inflammatory agent, according to the University of Maryland Medical Center.
Sources
The primary dietary sources of quercetin are green tea, red wine, and fruits and vegetables, including onions, apples, citrus fruits, kale and broccoli. It's also found in foods that are generally high in flavonoids, including grapes, blueberries and blackberries. Quercetin can be obtained as a supplement without a prescription.
Side Effects
Even though quercetin supplements are considered safe, mild side effects such as nausea and headache may occur. You should not consume more than 1g a day without first consulting your physician because excessive amounts can cause kidney damage, according to the University of Maryland Medical Center. Women who are pregnant or breastfeeding, and people with kidney disease, should not take quercetin supplements.
Research
A paper titled "Quercetin Reduces Blood Pressure in Hypertensive Subjects," which was published in the November 2007 issue of the Journal of Nutrition, showed that 730mg of quercetin given to participants for 28 days successfully reduced their high blood pressure. Test tube studies indicate quercetin has the potential to treat allergies, asthma, hay fever and hives by preventing cells from releasing the histamines that are responsible for allergic reactions. Studies that examine the effect of flavonoids on cells in the lab suggest that quercetin affects cell growth and death. This might prevent cancer and explain quercetin's ability to reduce inflammation, according to the Linus Pauling Institute. However, as of April 2010, studies in animals and people have produced mixed results.
Warning
Flavonoids interfere with the absorption of iron and might reduce the absorption of vitamin C. Information from LPI states that just one cup of tea or cocoa with a meal might decrease the absorption of any iron in the food by 70 percent.
Considerations
Quercetin supplements should not be taken at the same time as some medications. Always consult your physician before using quercetin if you take blood thinners, corticosteroids or cyclosporine. It might increase the effect of blood thinners, cause corticosteroids to stay in the body longer and interfere with the body's ability to absorb cyclosporine.
The amount of quercetin recommended for adults varies according to needs. If you're supplementing your diet, the recommended dose is 100 to 250mg taken three times a day, according to UMM. Your physician might prescribe a larger dose to help treat allergies, prostatitis or interstitial cystitis.
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