A chemical compound that slows or prevents oxygen from reacting with other compounds. Some antioxidants have been shown to have cancer-protecting potential because they neutralize free radicals. Examples include vitamins C and E, alpha lipoic acid, beta carotene, the minerals selenium, zinc, and germanium, superoxide dismutase (SOD), coenzyme Q10, catalase, and some amino acids, like cystiene. Other nutrient sources include grape seed extract, curcumin, gingko, green tea, olive leaf, policosanol and pycnogenol.