Effect of varying proportions of dietary fat on the development of N-nitrosomethylurea-induced rat mammary tumors.

The influence of diets varying in their relative proportions of dietary fat on N-nitrosomethylurea (NMU)-induced mammary tumorigenesis, was assessed. Animals were initiated on day 50 of age with 25 mg/kg NMU and then placed on casein-based AIN-76A diets containing 5, 10, 16 and 23% corn oil (wt/wt). There were 30 animals/group and the experiment was terminated 22 weeks post-NMU. It was found that animals fed diets containing 16 and 23% corn oil exhibited tumor incidences between 2 and 3 times that of animals fed diets containing 5 and 10% corn oil. No differences in tumor incidence could be detected between the 5 and 10% groups or the 16 and 23% groups, suggesting that a threshold occurs at some point between 10 and 16% fat (or 20 and 33% of total calories as fat). The results of this study suggest that the tumor-promoting effects of dietary fat are manifested in terms of a threshold, rather than a linear dose-response effect.