Antioxidants - friend or foe?

Particularly in the field of advertising and in popular scientific journals, antioxidants such as β-carotene (from carrots) or lycopenes (from tomatoes) and the vitamins C and E are one-sidedly portrayed as substances capable of offering protection against cancer or ageing because they diminish or prevent the effects of free radicals. However, free radicals should by nomeans be considered as being exclusively destructive. On the contrary, processes such as energy metabolism (in the respiratory chain) and the defence of neutrophilic granulocytes against pathogens and foreign bodies depend on the formation of free radicals [1], [2]. For this reason alone, it has to be expected that a supply of antioxidants may also cause considerable undesired effects to the skin and the organism. In addition, “under certain conditions, some anti-oxidants can also exhibit a pro-oxidant mechanism of action” [3], e.g. β-carotene [3] or vitamin C [4], with the question “When is an antioxidant not an antioxidant?” [4]. Many people with above-average consumption of fruits and vegetables undoubtedly demonstrate a lower risk of developing lung cancer. However about 10 years ago, completely unexpectedly two studies on highly dosed β-carotene supplementation had to be abandoned because the risk of lung cancer in smokers had unforeseeably increased following β-carotene supplementation. Further studies in vitro and in vivo were interpreted as showing β-carotene to act as an anticarcinogen, whereas its oxidation product acted as a carcinogen, possibly related to the instability of the β-carotene molecule in the free radical-rich environment in the lungs of cigarette smokers [5]. In a large-scale, multicenter, double-blind, placebo-controlled clinical trial on prevention, 864 persons, whose colon polyps had been removed, received 25 mg of β-carotene or placebo combined with 1000mg vitamin C + 400mg vitamin E or placebo on a daily basis. After four years, the following observations could bemade concerning supplementation of β-carotene and the development of colon polyps: