Antioxidants that reduce the pro-oxidant or oxidative stress causing action of various reactive oxygen species and free radicals also have a dark side.

For example, vitamin C or ascorbic acid that acts as an antioxidant when it reduces oxidizing substances such as hydrogen peroxide also reduces metal ions that generate free radicals through the Fenton reaction.
 

The relative importance of the antioxidant and pro-oxidant activities of antioxidants are of importance. Vitamin C of course has more antioxidant activity than pro-oxidant activity.

The Fenton reaction

The Fenton reaction is as follows:

2 Fe3+ + Ascorbate → 2 Fe2+ + Dehydroascorbate

When antioxidant enzymes deviate from physiological antioxidant activity they may have a dramatic effect on the resistance of cells to oxidant-induced damage to DNA and cell killing. The Fenton reaction is responsible for DNA damage produced under oxidative stress.

Beneficial and detrimental effects of antioxidants

The development of a beneficial or a detrimental cellular response by a nutrient depends on the nutrient's antioxidant or pro-oxidant characteristics. This depends on the product or the cellular oxygen environment.

Nutrients such as carotenoids, tocopherols or ascorbate derivatives will demonstrate an antioxidant or pro-oxidant characteristic depending on the redox potential of the individual molecule and the inorganic chemistry of the cell.

Most antioxidant nutrients that act as chemopreventives prevent excessive cell growth. When an inappropriate pro-oxidant activity develops in normal cells, the reactive oxygen metabolites generated could damage the DNA and cellular membranes. Thus the labile redox character of each nutrient must be considered in terms of the extracellular and intracellular microoxygen environment.

Several animal and human studies have shown the beneficial and detrimental effects of dietary antioxidants.