Effects of dietary fat on age-dependent sensitivity to mammary carcinogenesis.

Two groups of female Fischer rats were fed either a high-fat, low-carbohydrate (HF) diet or a low-fat, high-carbohydrate (LF) diet throughout the experiments, beginning from weaning. Each group of rats was divided into 4 subgroups and given an i.v. dose (50 mg/kg body wt.) of N-methyl-nitrosourea (NMU) at 35, 50, 90 or 130 days of age, respectively. Mammary tumor yield showed an age-dependent response in both groups. Although mammary tumor incidence decreased in rats as their age increased, irrespective of whether they were fed a high- or low-fat diet, the tumor incidence was significantly higher in rats fed the high-fat diet at any age. Thus, the combined effect of dietary fat and age-dependent sensitivity on mammary carcinogenesis caused more than a 4-fold difference in mammary tumor incidence rates between the 2 groups of rats.