THE EFFECT OF DIETARY-FAT ON THE PROMOTION OF MAMMARY AND COLON-CANCER IN A DUAL-ORGAN RAT CARCINOGENESIS MODEL

Female Sprague-Dawley rats (n = 270) were randomly divided into nine s ubgroups (30 rats/group) and fed diets with fat contents ranging from 5% to 45% of total energy, in which fat was isocalorically substituted for carbohydrates. At Week 3, the rats were given a single injection of methylnitrosourea (MNU, 50 mg/kg body wt). At Weeks 4 and 5, rats w ere given an injection of azoxymethane (AOM, 15 mg/kg body wt sc). The rats were maintained on their respective experimental diets until the y were sacrificed Nine weeks after the injection of MNU, 10 rats from each group were killed and their mammary tissue was examined for tumor s and their colons for aberrant crypt foci (ACF) formation. The result s showed no significant increase in the number of colonic ACF or mamma ry tumors at 11 weeks among individual groups consuming 5-25% energy f rom fat or among groups consuming 30-45% energy from fat. Data analysi s indicated that the effects of the two diets (5-25% energy from fat a nd 30-45% energy from fat) on ACF formation as well mammary tumor deve lopment were statistically different. Continuation of the diets for 32 weeks led to the development of colonic and mammary tumors. Analysis of the combined group turner incidence data clearly showed that the co lonic and mammary tumor incidence in groups fed 30-45% of energy from fat was significantly higher than that observed among groups fed 5-25% of energy from fat. Linear regression analysis of the colonic ACF at 11 weeks, colonic tumor incidence at 32 weeks, and mammary tumor incid ence at 11 and 32 weeks showed good linear correlation, demonstrating the potential value of A CF and mammary turners at 11 weeks for evalua ting the carcinogenic risk associated with different diets. The main c onclusion of the experiment is that the incidence of colon and mammary cancers increased rapidly when the dietary levels of fat were increas ed from 15% to 30% of calories and that there was no significant influ ence on the colon and mammary cancer risk beyond 30% of calories from fat.