Quercetin is a dietary flavonoid found in fruits and vegetables including apples, black, green and buckwheat tea, onions, red grapes, cherries, raspberries, citrus fruits. It is also found in some popular medicinal plants including ginkgo biloba and St. John’s Wort ⁽⁵⁾ and is used widely for its antioxidant effects. In vitro studies indicate that quercetin has antioxidant, anti-inflammatory ^(13) (14) and chemopreventive effects ⁽¹⁵⁾. However, it can also act as an anti-apoptotic agent ⁽⁵⁾. Studies in animal models have shown its ability to potentiate the antitumor effects of doxorubicin in liver cancer cells, while protecting normal liver cells ⁽¹⁶⁾. Quercetin also demonstrated neuroprotective and antidepressant effects ⁽¹⁷⁾, and exerted pro-oxidant effects by decreasing serum homocysteine levels ⁽¹⁸⁾.

In clinical studies, long-term quercetin supplementation resulted in wide-ranging metabolic effects in healthy subjects ⁽¹⁹⁾. But concomitant intake of quercetin did not attenuate postprandial metabolic responses such as lipemia and insulinemia, induced by high fat and carbohydrate foods ⁽²²⁾; nor did it affect an increase in the concentration of alpha linoleic acid, and its conversion to omega-3 fatty acids, which is associated with cardiovascular benefits ⁽²³⁾.

In other studies, quercetin was reported to attenuate the severity of muscle weakness caused by eccentric exercise ⁽²⁴⁾; to reduce oxidative damage following eccentric exercise ⁽²⁵⁾; and to enhance neuromuscular performance during and after resistance training ⁽²⁶⁾. In women with rheumatoid arthritis, supplementation affected improvements in clinical symptoms and disease activity ⁽²⁷⁾.

Findings from a systematic review indicate that quercetin obtained from a typical diet may not decrease the risk of ovarian cancer ⁽²¹⁾. Quercetin was also found to exacerbate estrogen-induced breast tumors in a murine model ⁽¹²⁾. Further research is warranted.