The role of non-nutritive dietary constituents in carcinogenesis.

Very low levels of a large variety of non-nutritive chemicals occur in the diet. Among those that occur naturally are metabolites of molds (for example, mycotoxins) and bacteria (for example, nitrosamines) and natural constituents of plants (for example, pyrrolizidine alkaloids). Many of these are occasional contaminants, whereas others are normal components of relatively common foods. Some compounds (for example, aflatoxin, nitrosamines, and hydrazones) have been found to be carcinogenic in laboratory animals and mutagenic in bacterial and other systems, thereby posing a potential risk to humans. However, there have been very few definitive epidemiologic studies. Therefore, further investigations are necessary to determine the significance of these experimental findings for humans. It is apparent that many foods naturally contain substances with mutagenic properties and that some substances found in foods can enhance or inhibit the mutagenic activity of other compounds. Furthermore, mutagens can be formed during the cooking or processing of foods. However, caution is needed in the interpretation of these findings. Although mutagens by definition are "suspect" carcinogens, many mutagens detected in foods have not been adequately tested for carcinogenicity, and therefore their significance for human health cannot be fully assessed. With the exception of studies on non-nutritive sweeteners like saccharin and cyclamate, or those on nitrate and nitrite, very few epidemiologic studies have been conducted to examine the effect of food additives on cancer risk. Of the few direct food additives that have been tested and found to be carcinogenic in animals, all except saccharin have been banned from use in the food supply. Minute residues of a few indirect additives that are known either to produce cancer in animals (for example, vinyl chloride and acrylonitrile) or to be carcinogenic in humans (for example, vinyl chloride) are occasionally detected in foods. Thus far, the increasing use of food additives does not seem to have contributed significantly to the overall cancer risk for humans. However, the relatively short duration of use of many of these substances and the inadequacy of the data base preclude definitive conclusions. Very low levels of a large and chemically diverse group of substances--environmental contaminants (for example, residues of pesticides, polycyclic aromatic hydrocarbons, and traces of toxic metals)--may be present in foods.(ABSTRACT TRUNCATED AT 400 WORDS)