Antioxidants are compounds that inhibit oxidation, a process that may lead to cellular damage and contribute to diseases such as cancer, diabetes, and heart disease.
Onions are an excellent source of antioxidants and contain at least 17 types of flavonoids.
Red onions, in particular, contain anthocyanins, plant pigments in the flavonoid family that give red onions their deep color. These may protect against diabetes and certain types of cancer.