Quercetin is a polyphenol, one of a number of water-soluble plant pigments called flavonoids (meaning class of plant secondary metabolites known for their antioxidant activity) that are largely responsible for the color of many flowers, fruits and vegetables. High concentrations of quercetin are found in apples, onions, tea and red wine . Other sources of quercetin include olive oil, grapes, broccoli, cauliflower, cabbage, dark cherries and dark berries such as blueberries, blackberries and bilberries. The average U.S. citizen eating a normal, healthy diet including fruits and vegetables consumes approximately 25-50 mg of quercetin/day. Quercetin and other flavonoids (also referred to as bioflavonoids) cannot be produced in the human body.