An antioxidant is a substance that counters the negative effect of oxygen.

In foods anti-oxidants prevent the oxidation of different food components. Oxidation of foods results for example in an undesirable brown colour and/or off-odours and off-flavours known as rancidity. Antioxidants are substances that react with oxygen and thus counteract the negative effects. Examples are vitamin C (E300) and vitamin E (E308).

In the body antioxidants protect key cell components by neutralizing the damaging effects of "free radicals," natural byproducts of cell metabolism. Free radicals form when oxygen is metabolized, or burned by the body. They travel through cells, disrupting the structure of other molecules, causing cellular damage. Such cell damage is believed to contribute to aging and various health problems. Example of anti-oxidants which are active in the body are the vitamins A, C and E, and polyphenols (a group of plant chemicals, present in for example tea and fruits).