Ability of Dietary Fat to Overcome the Resistance of Mature Female Rats to 7,12-Dimethylbenz(a)anthraceneinduced Mammary Tumorigenesis1

a!. (13) treated Sprague-Dawley rats of various ages with a single dose of 3-methybcholanthrene and reported that rats between 50 and 65 days of age were most susceptible to tumorigenesis. Similar results were obtained by Dao (7) with DMBA2 in Wistar-Furth rats. The tumor incidence varied from 80% in rats given DMBA at 50 days of age to less than 5% in rats given DMBA at 150 days of age. There are other reports that indicate that age at carcinogen administration also affects the proportion of carcinomas versus fibroadenomas (18) as well as the frequency of spontaneous tumor regression (8). An enhancement of mammary tumorigenesis by dietary fat in rats treated with DMBA has been confirmed by several inves tigators (3, 5, 12, 15) as well as by our laboratory (14). In view of the observation that adult rats are more resistant to the development of DMBA-induced mammary tumors, it would be of great interest to determine if the promoting effect of a high fat diet is manifested in rats that are fed the diet since weaning and given DMBA at 150 days of age. The temporal relationship between the introduction of a high-fat diet and DMBA admin istration should also be explored so as to better delineate the action of dietary fat on the carcinogenic process. Experiments described in the present study are thus designed to provide some insight on the interdependence of dietary fat and age as modifiers of mammary oncogenesis.