A flavonoid found in apples, green tea and a number of other foods. In at least one study, people who had diets high in quercetin had a 58% lower chance of developing lung cancer than those with lower quercetin intakes. A nutritionist may suggest quercetin supplements or adding more foods containing quercetin to the diet to help prevent recurrence of cancer.
The most studied flavonoid because it is among the most abundant; a more potent antioxidant than vitamin E, according to some research. Onions are the richest source; it's also found in wine and tea. (Many sources say "onions, tea, wine and apples" because these were the main dietary sources in a major Netherlands study.) Among other functions, it may block carcinogens as well as slow the growth and spread of cancer cells. It also may prevent the conversion of nitrites in the stomach to compounds that become building blocks for carcinogens. Quercetin appears to survive the heat of cooking, and about 5% to 10% of the quercetin from onions is absorbed by the body.
Quercetin is considered to be the main flavonoid in the diet. People who have the highest intakes of quercetin-containing foods were found to have a lower risk for asthma, lower mortality from heart disease, and lower lung cancer incidence.