The antioxidant paradox: Damage and defence

Over the last three decades, a considerable body of evidence has accumulated to suggest that a key factor in the development of both cancer and coronary heart disease is damage to tissues or plasma lipoproteins caused by free radicals. The majority of radicals that damage tissues are reactive oxygen species, and compounds that can quench potentially damaging radical chain reactions are therefore generally referred to as antioxidants. Epidemiological studies have shown a negative correlation between intakes of antioxidants (especially vitamin E and -carotene) and the incidence of cancer and coronary heart disease, and a positive correlation between markers of radical damage and disease. At the molecular level, there are sound theories to explain how radical damage can lead to cancer and coronary heart disease, and how antioxidants may provide protection. However, the results of intervention trials of vitamin E and -carotene have been, at best, disappointing; indeed, many trials have shown increased death among people taking supposedly protective antioxidant supplements. This is the antioxidant paradox.