Quercetin CasNo.: 6151-25-3 is a type of flavonoid antioxidant that’s found in plant foods, including leafy greens, tomatoes, berries and broccoli. It’s technically considered a “plant pigment,” which is exactly why it’s found in deeply colored, nutrient-packed fruits and veggies. Considered one of the most abundant antioxidants in the human diet, quercetin plays an important part in fighting free radical damage, the effects of aging and inflammation. While you can get plenty of quercetin from eating a healthy diet, some people also take quercetin supplements for their strong anti-inflammatory effects.