An antioxidant is a chemical that prevents the oxidation of other chemicals. In biological systems, the normal processes of oxidation (plus a minor contribution from ionizing radiation) produce highly reactive free radicals. These can readily react with and damage other molecules: in some cases the body uses this to fight infection. In other cases, the damage may be to the body's own cells. The presence of extremely easily oxidisable compounds in the system can "mop up" free radicals before they damage other essential molecules.

The following vitamins have shown positive antioxidant effects:

  • Retinol (Vitamin A or beta-carotene) has been discovered to protect dark green, yellow and orange vegetables and fruits from solar radiation damage, and is thought to play a similar role in human body. Carrots, squash, broccoli, sweet potatoes, tomatoes, kale, collards, cantaloupe, peaches and apricots are particularly rich sources of beta-carotene.
  • Ascorbic acid (Vitamin C) is a water-soluble compound that fulfills this role, among others, in living systems. Important sources include citrus fruits (like oranges, sweet lime, etc.), green peppers, broccoli, green leafy vegetables, strawberries, raw cabbage and tomatoes.
  • Vitamin E (tocopherol) is fat soluble and protects lipids. Sources include wheat germ, nuts, seeds, whole grains, green leafy vegetables, vegetable oil and fish-liver oil.
  • Selenium is best obtained through foods, as large doses of the supplement form can be toxic. Good food sources include fish, shellfish, red meat, grains, eggs, chicken and garlic. Vegetables can also be a good source if grown in selenium-rich soils.

Several food additives (including ascorbic acid and tocopherol-derived compounds) are used as antioxidants to help guard against deterioration of food.

Other antioxidants are enzymes. These include glutathione peroxidase, superoxide dismutase and catalase.

Much damage is done by free radicals in mitochondria as a byproduct of oxidative phosphorylation. Superoxide radicals are generated which can damage mitochodrial DNA and mitochondrial membranes. Unlike DNA in the cell nucleus, mitochondrial DNA has only a few DNA-repair enzymes and the DNA is not protected by histones.

Many antioxidants, however (including vitamic C and vitamin E) can't get into mitochondria for various reasons (e.g. because too hydrophilic to cross mitochondrial membranes or too hydrophobic to cross the cytoplasm). A group of scientists in Russia (led by V. Skulachev) has created a custom antioxidant (A Skulachev ion forms the point of the molecule and penetrates the mitochondrial membrane; the antioxidising part is attached behind it) that can enter the mitochondria and stays there preventing damage to DNA.'

Although there is little doubt that antioxidants are a necessary component for good health, there is considerable doubt as to the most beneficial antioxidant and as to the optimal amount for results. Due to the complex nature of the interactions of antioxidants with the body, it is difficult to interpret the results of many experiments designed to test such things.

For example, recent studies are suggesting that at high levels, antioxidant vitamins may prove to have pro-oxidant effects: increasing the formation of free radicals.

The benefits of antioxidants were examined during the Age-Related Eye Disease Study.


  • Halliwell B. 1999. Antioxidant defence mechanisms: from the beginning to the end (of the beginning). Free Radical Research 31:261-72.

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