Antioxidants are molecules that prevent or slow down the breakdown of other substances by oxygen. In biology, antioxidants are scavengers of small, reactive molecules known as free radicals and include intracellular enzymes such as superoxide dismutase (SOD), catalase and glutathione peroxidase. Antioxidants can also be extracellular originating as exogenous cofactors such as vitamins. Nutrients functioning as antioxidants include vitamins, for example ascorbic acid (vitamin C), tocopherol (vitamin E) and vitamin A. Trace elements such as the divalent metal ions selenium and zinc also have antioxidant activity as does uric acid, an endogenous product of purine metabolism. Free radicals are molecules with one or more unpaired electrons, which can react rapidly with other molecules in processes of oxidation. They are the normal products of metabolism and are usually controlled by the antioxidants produced by the body or taken in as nutrients. However, stress, aging, and environmental sources such as polluted air and cigarette smoke can add to the number of free radicals in the body, creating an imbalance. The highly reactive free radicals can damage nucleic acids and have been linked to changes that accompany aging (such as age-related macular degeneration, an important cause of blindness in older people) and with disease processes that lead to cancer, heart disease, and stroke.
The last few years have witnessed an explosion of information on the role of oxidative stress in causing a number of serious diseases, and there appears to be a potential therapeutic role for antioxidants in preventing such diseases. For example, recent epidemiological studies have shown that a higher consumption of vitamin E and to a lesser extent B-carotene is associated with a large decrease in the rate of coronary arterial disease. The most effective dose of vitamin E is apparently 400-800 mg/day. Other studies have shown that a diet rich in fruits and vegetables leads to a marked decline in the cancer rate in most organs with the exception of blood, breast, and prostate. Antioxidants play a major role in this. It is now abundantly clear that toxic free radicals play an important role in carcinogenesis. In several studies high cancer rates were associated with low blood levels of antioxidants particularly vitamin E. Similarly, vitamin C is thought to protect against stomach cancer by scavenging carcinogenic nitrosamines in the stomach.
The brain is particularly vulnerable to oxidative stress. Free radicals play an important role in a number of neurological conditions including stroke, Parkinson's disease, Alzheimer's disease, epilepsy and schizophrenia. Some other diseases in which oxidative stress and depletion of antioxidant defence mechanisms are prominent features include hepatic cirrhosis, pre-eclampsia, pancreatitis, rheumatoid arthritis, mitochondrial diseases, systemic sclerosis, malaria, neonatal oxidative stress, and renal dialysis.
Studies have suggested that the antioxidants occurring naturally in fresh fruits and vegetables are very beneficial and protect against excessive oxidative stress. There is still some question as to whether antioxidants in the form of dietary supplements are equally beneficial. Some scientists assert that regular consumption of such supplements interferes with the body's own production of antioxidants.
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