We have previously described the necessary parameters for conducting the mouse bioassay for detecting estrogenic activity in animal diets, including a standardized method for removing, trimming, and weighing the uterus, age and strain of test pups, and methods of statistical evaluation. Statistical evaluation (t-test) of bioassay data has shown that uterine weight alone, without calculated uterine: body weight ratio, is adequte to determine a statistically significant (P.05) difference between control diets containing 0, 4, or 6 ppb DES. We have also employed the mouse bioassay using the CD-1 mouse to compare the estrogenic activity of semipurified, certified, standard and open formula rodent diets. Mice fed semipurified diet (AIN-76), modified semipurified diet containing 4% corn oil, or a natural ingredient breeder diet (Purina #5015), demonstrated significant (P less than .05) increases in uterine weight at 5, 7, and 9 days post weaning, when compared to other natural ingredient diets. This level of estrogenic activity was equal to or greater than the activity observed in a natural ingredient maintenance diet (Purina #5002), containing 4 or 6 ppb of DES. These results suggest: 1) significant differences exist in the level of estrogenic activity in some commercially available diets; 2) the importance of the diet when performing or comparing fertility, reproduction, organogenesis, toxicological or estrogenic studies; 3) a standardized diet with minimal estrogenic activity may be desirable for some studies.
