An antioxidant is a chemical that reduces the rate of particular oxidation reactions in a specific context, where oxidation reactions are chemical reactions that involve the transfer of electrons from a substance to an oxidizing agent, this generally results in different chemicals to the original ones.
Antioxidants are particularly important in the context of organic chemistry and biology.
All living organisms maintain a reducing environment inside their cells, all cells contain complex systems of antioxidants to prevent chemical damage to the cells' components by oxidation.
These antioxidants include glutathione and ascorbic acid and are substrates for enzymes such as peroxidases and oxidoreductases.
Antioxidants are widely used as ingredients in dietary supplements used for health purposes such as preventing cancer and heart disease.
Studies have suggested antioxidant supplements has benefits for health, but several large clinical trials did not demonstrate a definite benefit for the formulations tested, and excess supplementation may even be harmful.
Dietary supplementation has few specific antioxidants compared to a broad diet rich in phytonutrients, which will yield thousands of different polyphenol antioxidants available for metabolism.