[Micronutrients in cancer prevention]

Micronutrients are able to influence the process of carcinogenesis by chemoprotection. With regard to preventive medicine, the possible anticarcinogenic effects in the early stages of tumor development, the initiation and promotion stages, require the greatest interest. It is, however, an essential fact that the possible interactions between micronutrients and carcinogenesis are a complex problem that cannot be considered by a simple cause-effect relationship. The basis of our knowledge on a possible protective effect of micronutrients in carcinogenesis are results of in vitro studies and of animal experiments with doses of micronutrients never appearing in the normal nutrition of men. It therefore seems somewhat difficult to transfer the results of those studies to the real-life situation of men. Investigations in humans, mainly epidemiological studies, show controversial results. There has also to be considered that the chemopreventive effect may be different with regard to the single micronutrient. More extensive studies have been performed for beta-carotin or the vitamins A, C, and E as well as for the trace elements selenium and zinc and for calcium. Possible influences of these micronutrients seem to exist mainly in cancers of the breast, lung, gastro-intestinal tract, and prostate. However, chemoprevention may also be produced by various non-nutritive substances, present mainly in food of vegetable origin. Finally it has to be considered that a carcinoprotective effect may not only be induced by enhancing the amount of special micronutrients in the nutrition, but also by avoiding of possible carcinogenic nutrients. The difference between carcinoprotective and carcinogenic is, however, not only a question of quality but not seldom dependent on quantity.