Quercetin is a chemical pigment commonly found in the bark of many plants. It is an important dietary flavonoid and is rich in tea, onions, apple peel, and red wine. Studies show that quercetin can fight certain types of cancer cells. Quercetin is a flavonoid antioxidant found in plant nutrients such as vegetables, tomatoes, strawberries and broccoli. Quercetin deficiency can lead to stress and depression, prostate in men and exacerbation of asthma.