Antioxidants are compounds that prevent or slow a chemical change called oxidation. A commonly recognized oxidation reaction is the rusting of iron, in which iron reacts with oxygen to form iron (ferrous) oxide. Similar reactions take place within foods and within the human body itself.
Heat is often released in the process of oxidation.
In the Human Body 
Oxidation that occurs in the human body can lead to improper functioning of the body. Substances known as free radicals, which are highly reactive (have a strong tendency to take part in chemical reactions), undergo oxidation that can result in cell damage. This general process is thought to be a contributor of many undesirable conditions: cancer, aging symptoms, alcohol-induced liver damage, Parkinson's disease, senile and drug-induced deafness, schizophrenia, Alzheimer's and the iron-storage disease hemochromatosis.
While many studies have suggested benefits for antioxidant supplements in laboratory experiments, clinical trials on humans have failed to clearly demonstrate a benefit, while strong antioxidants, such as oxalic acid and phytic acid can have anti-nutritional effects due to their binding to dietary minerals in the gastrointestinal tract and preventing absorption. While antioxidants supplementation is widely hypothesized to prevent the development of cancer, antioxidants may, paradoxically, interfere with cancer treatments.
Nutritional Sources of Antioxidants 
Some foods guard against the negative effects of free radicals in the body, and are commonly referred to as antioxidants. These foods include the following:
- Fruits. Cranberries, blueberries, plums, peaches, blackberries, raspberries, apples, strawberries, red currants, figs, cherries, gooseberries, pears, guava, peaches, oranges, apricots, mangoes, grapes and pomegranates plums, raisins and dates. Dried fruits are especially high in antioxidants (more so than fresh), because the concentration means more antioxidants per serving.
- Vegetables. Artichokes, cabbage, broccoli, asparagus, avocados, beetroot, radishes, potatoes, lettuce and spinach.
- Spices. Cinnamon, turmeric, cumin, curry powder, mustard seed, ginger, pepper, chili powder, paprika, garlic, coriander, onion and cardamom.
- Herbs. Sage, thyme, marjoram, tarragon, peppermint, oregano, parsley, savory, basil and dill weed.
- Cereals. Sorghum bran, crude rice bran, corn flakes, oats and granola.
- Nuts. Pecans, walnuts, hazelnuts, pistachios, almonds, cashews, macademia nuts and peanuts (though not technically a nut).
- Other. Red wine, tea, fungi, essential oils and cocoa.
Vitamin C, vitamin E, and lycopene are noted antioxidants.
In Foods 
In foods, oxidation generally contributes to spoilage, and therefore the canning process preserves food by reducing available oxygen (as well as by killing microorganisms that contribute to spoilage).