Quercetin is an antioxidant flavonoid found in many plants that we consume with a regular diet and it is also used as a nutritional supplement.
Like many other antioxidants, it shows promising cancer inhibiting properties in test tube and animal trials. There is limited evidence from animal studies that this flavonoid may reduce the risk of chemically induced cancers, especially of the colon, and may be useful in the prevention of lung cancer.
When treated with a combination of quercetin and ultrasound at 20 kHz for 1 minute duration, skin and prostate cancers showed a 90% mortality rate in vitro within 48 hours with no visible mortality of healthy cells.
This flavonoid has also been identified in vitro as an agent for treatment of breast and ovarian cancers. However, there is no reliable clinical evidence that it can prevent or treat cancer in humans.
Read the abstracts of the studies here.
Important: Some researchers believe that healthy people should not take quercetin as a supplement for cancer prevention because the possibility of high doses causing cancer themselves has not been ruled out completely. In laboratory studies of cells quercetin produces changes that are also produced by compounds that cause cancer. But these studies do not report increased cancer in animals or humans. However there is no need to worry about normal dietary consumption of this antioxidant from foods.
Read the abstract of the study here.
Other in vitro studies showed that quercetin has anti-inflammatory properties, and that together with resveratrol it may exhibit synergistic anti-obesity effect.
Important: Quercetin cannot be recommended for the prevention or treatment of neurodegenerative diseases because at certain levels its protective qualities turn into toxicity.
Researchers from the Division of Pharmacology & Toxicology at the University of Helsinki (Finland) concluded: "...the toxic effects of quercetin occurred at a concentration only 2-fold higher than the one that produced the greatest protection... Finally, single doses of quercetin failed to protect against 6-OHDA toxicity in a unilateral rat model of Parkinson's disease. In conclusion, our data may offer an explanation for the dualistic effect of quercetin reported in the literature. In fact, in most studies suggesting quercetin protection against oxidative stressors, the experimental setting failed to include prolonged exposure, and therefore the toxic effects may have been missed. This study supports previous in vivo studies that cast doubt on the efficacy of quercetin against neurodegenerative diseases due to its delayed toxicity." (Study "Time-dependent protective and harmful effects of quercetin on 6-OHDA-induced toxicity in neuronal SH-SY5Y cells". Ossola B et al.)
As with most of polyphenols, quercetin bioavailability studies show that when ingested it is almost completely metabolized into several non-active phenolic acids, with more than 96% of the ingested amount excreted within 72 hours, which makes it difficult to achieve blood concentrations that showed benefits in lab conditions.
Some of the lab studies used exceedingly high doses of the aglycone quercetin (500 mg per 1 kg (approximately 2.2 lbs.) of body weight), which is not the normal dietary consumption.
Food sources: capers, onions, scallions, buckwheat, tart and sweet cherries, cranberries, black currants, apples, bee pollen, kale, broccoli, raw green peas, asparagus, cherry tomatoes.
For the list of selected foods and quercetin content in them click here.