Oxidation is a chemical reaction in which molecules combine with oxygen to form different molecules that include oxygen. Oxidation can happen very slowly, as with iron gradually rusting, or very quickly, as with a hydrogen molecule H2 combining explosively with an oxygen atom O to form a water molecule H2O, or in between, as with wood burning.
An antioxidant is any substance that can regulate oxidation. If air were pure oxygen, the wood in your fireplace would burn much hotter and faster, though not as violently as hydrogen burning. But 80% of air is nitrogen, which does not participate in oxidation but inhibits it by limiting the amount of oxygen available to the combustion process. As such nitrogen functions as an antioxidant.
Antioxidants can be a mixed blessing. Normally you appreciate the role of nitrogen in preventing the wood from burning too violently. However a boy scout trying to start a fire outdoors might be tempted to blame his difficulty on nitrogen, without which getting the fire going would be much easier.
Your body extracts energy from the fuels you feed it by oxidizing them. The need to regulate oxidation is even more important in the body, which depends on keeping a complex array of processes in balance.
One complication is that pure oxygen is not the only oxidant. The hydroxyl radical OH, one byproduct of oxidation, can itself oxidize, for example combining with an atom of hydrogen to produce water, H2O. The OH molecule was produced by incomplete combustion of hydrogen; when OH oxidizes hydrogen it is simply completing the combustion. While the body does not contain hydrogen gas per se, it does contain many important molecules whose oxidation by OH is along similar lines, for example DNA molecules that OH radicals can cut into pieces like a knife. Antioxidants limit the ability of such rogue oxidants to damage your body's important molecules.
Among the most important antioxidants in the body are vitamins A, C, and E, each of which plays one or more important roles as an antioxidant. These are by no means the only antioxidants useful to the body. Among those relevant to diet are various alkaloids of vegetable origin, in particular caffeine (found in coffee and to a lesser extent in tea) and theobromine (found in chocolate). These tend to inhibit the production of the hydroxyl radical OH. However the presence of metal ions can catalyze (passively stimulate) oxidation by these substances, whichever therefore can act both to inhibit and stimulate oxidation. Overall however the inhibiting effect of these alkaloids tends to dominate, making them valuable as antioxidants.
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