Red Onions are full of sulfur compounds that protect the body from ulcers and various cancers. They can also fight bacteria in the urinary tract. The most important of these compounds is called quercetin - an antioxidant compound that could provide protection against cancer, heart disease, and allergies. Red onions, in particular, contain anthocyanins — special plant pigments in the flavonoid family that give red onions their deep color. Multiple population studies have found that people who consume more foods rich in anthocyanins have a reduced risk of heart disease.