Comparative effects of different animal and vegetable fats fed before and during carcinogen administration on mammary tumorigenesis, sexual maturation, and endocrine function in rats.

The purpose of this investigation was to determine whether diets high in animal or vegetable fat affected mammary tumorigenesis when fed to rats only prior to and during the initiation phase of carcinogenesis. Weanling 21-day-old female Sprague-Dawley rats were divided into different dietary treatment groups and were allowed to feed and libitum on one of the following diets: 5% (normal fat) corn oil; 20% (high fat) corn oil; 20% palm oil; 20% beef tallow; or 20% lard. At 52 days of age, all rats were given p.o. 7.5 mg 7,12-dimethylbenz(a)anthracene (DMBA). One week following DMBA administration, all rats were switched to the 5% corn oil control diet and were maintained on this diet for the duration of the experiment. Rats fed a 20% lard diet during the treatment period showed a significant increase in mammary tumor incidence and number 19 weeks after DMBA administration, when compared to all other dietary treatment groups. Rats fed a 20% beef tallow diet during this same time period also demonstrated enhanced mammary tumor development, during the 10- to 19-week time period after DMBA. Mammary tumor development in rats fed 20% corn oil or palm oil diets during this treatment period was similar to that of normal fat controls. Estrogens are potent stimulators of mammary tumor growth and development in rats. Because mammary tumorigenesis was enhanced in rats fed high animal, but not vegetable fat diets, it was possible that estrogens present in animal fat might be responsible for this stimulation. Further studies demonstrated however, that increased mammary tumorigenesis in rats fed diets high in animal fat could not be explained on the basis of endocrine stimulation. Average day of vaginal opening for all groups fed 20% fat diets was similar and occurred earlier than in normal fat controls. In addition, 50- to 65-day-old rats in the different dietary treatment groups showed no differences in basal or surge levels of serum prolactin, luteinizing hormone, or estradiol. Rat diestrus uterine weight also showed no significant differences among dietary treatment groups. Thus diets containing high levels of animal fat caused little if any increased estrogenic activity in rats. In conclusion, high dietary intake of lard and beef tallow, but not vegetable fat, fed from weaning until only 1 week after DMBA administration, significantly enhances mammary tumorigenesis in rats. The mechanism(s) by which animal fat induces this stimulation is not clear, but it does not appear to result from endogenous or exogenous endocrine stimulation.