An antioxidant is a substance that, by being oxidized itself, protects other chemicals and substances from oxidation. They play a vital role in limiting the damaging effects of the oxidative processes that have been implicated in the development of most degenerative disorders and the aging process.

Oxidation is caused and perpetuated by free radicals (also known as radicals and reactive oxygen species). The unpaired electron common to all free radicals makes these molecules highly reactive and causes them to oxidise other substances. Their targets are many and varied: DNA, cells, proteins, fats, and other molecules are all vulnerable to oxidative attack. Unless free radicals are intercepted by antioxidants the targeted molecules become free radicals themselves. This results in the development of a highly damaging "oxidative cascade" that continues to produce more and more free radicals and leads to an unhealthy condition called "oxidative stress".

The consumption of a wide array of antioxidants is essential for protection against oxidative stress. There are no antioxidants, weak or strong, that can neutralize all free radical species. As some of them are effective against certain free radicals but not others, consuming an assortment of antioxidants ensures that the body is able to tackle all types of damaging radicals. This also enables the antioxidants to work optimally, as many operate synergistically with one another. Moreover, different antioxidants tend to locate preferentially in different types of tissues and cells. For example some antioxidants can cross the blood-brain barrier while others do not have this facility. Certain antioxidants are only effective in lipid-containing areas of the cell, while others work more efficiently in the watery parts of the cell. This is another reason why we should take a wide range of these vital compounds rather than one or two so-called "strong" antioxidants.