Diet can influence the process of carcinogenesis at various stages: first, by affecting the initial stages of carcinogenesis (primary prevention), by preventing the malignant transformation of precursor cells (secondary prevention), and by preventing the recurrence of the disease following a recovery (tertiary prevention) (51).
No form of a diet can prevent cancer with certainty, and epidemiological evidence of a correlation between diet and cancer is still incomplete; yet it is possible to establish dietary recommendations that might reduce the risk of developing cancer (26).
The positive health benefits of a diet rich in fruits and vegetables are now common knowledge. However, the question of what the underlying mechanisms may be is still the subject of ongoing research. Experimental research, in particular, has shown that there are many biologically plausible explanations for the anticancer properties of plant-derived food. However, the protective effect cannot be traced back to individual ingredients. According to currently available scientific findings, it is rather the food pattern—the selection, preparation, and amount of food—that seems to be essential for influencing the risk of cancer and other chronic diseases. It seems as if the effects associated with individual food ingredients are adding up and thus determine the risk of developing cancer. However, it is still not clear which mechanisms of action are relevant in humans (20).
A great number of substances that occur naturally in foods are regarded as protective factors against the development of cancer. These include antioxidants (e.g., p-carotene, and the vitamins A, C, and E), calcium, selenium, zinc, fiber, secondary plant metabolites, and lactic acid. Also considered are riboflavin and folic acid (61).
When considering individual nutrients in our food, we should be aware of the fact that they always exist in a close relationship with other nutrients. For example, a low intake of fat is correlated with a reduced risk of breast cancer, on the one hand, but associated with a reduced level of fat-soluble vitamins, on the other. We need to gather results that can be properly interpreted. When assessing certain types of diet, it makes sense to define specific indicator substances (markers) that, either alone or integrated, are known to have a preventive effect against the development of cancer. These markers should be detectable in human blood and tissues and be correlated in a quantifiable manner with the respective food items. The following indicator substances are often used:
• in vegetables: p-carotene (as well as vitamin A)
• in vegetable oils: vitamin E
• in fruits, especially citrus fruits: vitamin C.
These markers can be used for interpreting epide-miological studies as well as for analyzing direct interactions in animal experiments and in-vitro assays. A final proof of their isolated mechanisms of action, however, is only possible through targeted dietary intervention studies (6).
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