When weanling rats consumed a partially purified diet containing 9 to 27 per cent of casein, 57 out of 66 developed mammary tumors (86 per cent incidence). The daily intake of 2-acetylaminofluorene ranged between 1.8 and 2.1 mg/rat. On a corresponding intake of the carcinogen, diets containing 40 or 60 per cent of casein produced a marked reduction in mammary tumor incidence (three of 25 rats or 12 per cent incidence). Increased intakes of food and carcinogen exceeding 2.1 mg. daily largely overcame the protective effects of the high protein diets and resulted in a high incidence of mammary tumors (20 of 26 rats or 77 per cent incidence). Irrespective of level of carcinogen and feed intake, the high protein diets promoted generally improved well-being and prolonged the survival period from an average of 28 weeks to over 40 weeks.
Suggestive evidence was obtained indicating that high casein diets were also partially protective against the induction of ear duct and liver tumors.
Desoxyribonucleic acid or yeast nucleic acid, added to a 12 per cent casein diet, had no influence on tumor induction. The supplementation of a 60 per cent casein diet with vitamin B12 and folacin decreased the average survival period from 42 weeks to 32 weeks.
Published with the approval of the Director of the Alabama Agricultural Experiment Station. This work was supported in part by a grant from the Nutrition Foundation, Inc., New York City.
The preliminary results of this study were presented at the scientific meetings of the American Association for Cancer Research, Inc., Atlantic City, N.J., April 16–18, 1950 (Cancer Research, 10:215, 1950).