Functional Foods


January 11, 2017   /
Elaine M. Hinzey, RD, LDN
Nutrition411 Staff

Quercetin is a flavonoid (a type of phytochemical) that functions as a plant pigment, and is found in teas, onions, red wine, citrus fruits, parsley, olive oil, dark cherries, grapes, apples, leafy-green vegetables, berries, and buckwheat. It is also found in herbs, such as ginkgo and St. John’s wort. Quercetin has antioxidant properties, meaning that it acts as a scavenger in the body to neutralize free radicals that cause damage to cells and DNA.

Quercetin is available in supplement form in doses ranging from 50 milligrams (mg) – 500 mg. However, some people question how well quercetin is absorbed by the human body when taken orally. It is believed to have antioxidant, anti-inflammatory, and antiallergic properties. Quercetin appears to reduce production of inflammatory mediators, such as leukotrienes and histamine. 

Some people claim that quercetin can help protect against heart attacks and strokes by stabilizing blood vessels. Along with resveratrol and catechins, quercetin appears to reduce the risk of atherosclerosis and protect against the damage caused by low-density lipoprotein (LDL) cholesterol. All three of these flavonoids are found in red wine. However, animal studies have used extremely large amounts of flavonoids to garner these effects, and supplements are not available for humans with a comparable level of these phytochemicals. Studies show that quercetin supplementation does reduce blood pressure in people with hypertension. 

Other claims state that quercetin can help ease chronic prostatitis and the neurologic complications of diabetes mellitus. 

In vitro studies have shown that quercetin may have anti-cancer effects, but further research is warranted. Clinical studies have not shown consistent results regarding quercetin and cancer prevention. Most of the research on quercetin and cancer was completed in cell culture or animal studies. This suggests possible helpful effects, but does not prove that such effects would appear in humans. Recent studies have shown that quercetin can slow the growth of cancer cells and fosters apoptosis. Animal studies have shown that quercetin may help to protect against some forms of cancer, particularly colon cancer. Currently, no reliable clinical evidence exists to show that quercetin can prevent or treat cancer in humans. 

Most human studies are population-based and have focused on the role of total flavonoids in the diet, so it is difficult to tell the effects of specific flavonoids, such as quercetin. These studies have shown that people who consume diets high in flavonoids have lower risks of breast, lung, pancreatic, ovarian, endometrial, and other forms of cancer. Studies show that people with interstitial cystitis appear to benefit from flavonoids, and those individuals who took a supplement containing quercetin seemed to have fewer symptoms, but it is not known whether the quercetin or another flavonoid was responsible for this improvement.