An antioxidant is a substance that slows the rate of an oxidation reaction. Various antioxidants are used to preserve foodstuffs (preventing fatty foods from becoming rancid), and to prevent the deterioration of rubber, synthetic plastics, and many other materials. Some antioxidants act as chelating agents to sequester the metal ions that catalyze oxidation reactions. Others inhibit the oxidation reaction by removing oxygen from free radicals. Naturally occurring antioxidants include β-carotene, glutathione, lutein, lycopene, selenium, vitamin A, vitamin C, and vitamin E; they limit the cell and tissue damage caused by foreign substances, such as toxins and pollutants, in the body.