The color of the onion is key. Red onions have the highest amount of quercetin, while white onions have very little. People who eat onions, especially red ones, as a regular part of their diet not only are eating the food for flavor, but are getting great health benefits as well. These foods are known as functional foods. Other sorts of anti-oxidants exist in many other fruits and vegetables.
Carotene is found in carrots and cantaloupes, and lycopenes are found in tomatoes and watermelons.
Any produce that is brightly colored and vibrant usually has very high amounts of these anti-oxidants. Sulfur compounds, isothiocyanates and sulforophanes, are found in deep green vegetables such as broccoli leaves and sprouts, and cabbage and Brussels sprouts. These phytochemicals each have a role in raising levels of protective enzymes that fight against cancer. This whole area of science is known as chemoprotection. Sulfur compounds, or glucosinolates, are found in deep green vegetables such as broccoli leaves and sprouts, cabbage and Brussels sprouts.