The effect of the quality of dietary protein on the post-initiation development of aflatoxin B1-initiated putatively preneoplastic foci in Fischer 344 rat liver was compared with the effect of the quantity of dietary protein. Feeding wheat gluten, a low-quality protein, during the post-initiation period (between the end of aflatoxin B1 dosing and the death of the rats) inhibited the development of γ-glutamyltransferase-positive foci when compared with that in animals fed high-quality protein (casein) diets during the same period. Lysine supplementation of wheat gluten during the postinitiation period enhanced the γ-glutamyltransferase-positive response to a level comparable with that of the high-quality protein. These results suggest that one can inhibit the development of foci either by decreasing the quantity of protein intake and holding the quality of the protein constant or by decreasing the quality and holding the quantity constant. [J Natl Cancer Inst 81:1241–1245, 1989]

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