Response

Extract

We thank Dr. Cohen for his interest in our work and for raising the issue of fiber versus fat in affecting mammary tumorigenesis. Two aspects of our paper concerned Dr. Cohen: 1) the use of fiber to adjust the caloric content of the high-and low-fat diets, and 2) the low 7,12-dimethylbenz[ a ]anthracene (DMBA)-induced mammary tumor incidence. Rodents efficiently regulate their daily caloric intake. If they are fed a diet high in fat and calories compared with a diet low in fat and calories, their feed intake is significantly reduced. This is particularly true during pregnancy. We found that pregnant rats consume 21 gof a diet containing 24.6 g corn oil100 g feed (caloric density, 4.8 kcal g) and 32 gof a diet containing 5 g corn oil100 g feed (caloric density, 3.8 kcal) (34% difference in daily food intake). The daily caloric intake is significantly higher (21%) in the low-fat group (122 kcal) than in the high-fat group (101 kcal) (P<.001) (17% difference in daily caloric intake). Similar findings have been reported by others ( 1 ) . Reflecting the difference in daily caloric intake, the serum estradiol (E ₂ ) levels do not differ in pregnant rats kept on nonisocaloric high-and low-fat diets. These findings may explain the failure to observe a difference in circulating estrogens in adult animals kept on the non-isocaloric high-and low-fat diets. To ensure the same daily caloric intake, we fed pregnant rats isocaloric diets. The high caloric density of the high-fat diet is compensated for by adding noncaloric fiber to the diet. Thus, the high-fat diet is a high-fatfiber diet, not a high-fat low-fiber diet. Examples of how fiber affects caloric density are given in Table 1. Because fiber increases fecal excretion of estrogens ( 2 ) , we expected the serum E ₂ levels to be only moderately different between the pregnant rats kept on the isocaloric high-fatfiber and low-fatfiber diets. We found that the high-fat-fed animals, either pregnant or not, have a 30%-100% higher circulating E ₂ content than the low-fat-fed animals ( 3 , 4 ) . Thus, despite the high fiber content of the high-fat diet (which would reduce serum E ₂ ) and the low fiber content of the low-fat diet (which would increase serum E ₂ ), we still detect significantly higher E ₂ levels in the rats fed a high-fat diet.