Diets containing whey proteins or soy protein isolate protect against 7,12-dimethylbenz(a)anthracene-induced mammary tumors in female rats

A study was conducted to determine the protective effects of two common die tary proteins, soy protein isolate (soy) and bovine whey, against chemicall y induced mammary tumors in female Sprague Dawley rats. Rats were fed AIN-9 3G diets having casein, soy, or whey as the sole protein source. Rats withi n the same dietary groups were mated to obtain the F-1 and F-2 generations, At age 50 days, F-1 (experiment A) or F-2 (experiment B) female offspring (greater than or equal to 19 rats/group) were p.o. gavaged with 80 mg/ kg 7 ,12-dimethyibenz(a)anthracene, and mammary glands were evaluated when 100% of the casein-fed group developed at least one palpatable tumor, Rats grew well on all three diets, but casein-fed rats gained slightly more body weig ht than soy- or whey-fed rats (P < 0.05). Vaginal opening occurred 1 day ea rlier in soy-fed rats than in casein- or whey-fed rats, but no other differ ences in reproductive and developmental parameters were observed between gr oups. When 50% of the casein-fed rats had at least one mammary tumor, lower tumor incidences (24-34%) were observed in the soy-fed (P < 0.009) and whe y-fed groups (P < 0.001). When 100% of the casein-fed rats had at least one tumor, soy-fed rats had a lower tumor incidence (77%) in experiment B (P < 0.002), but not in experiment A (P < 0.12), and there were no differences in tumor multiplicity. Whey-fed rats had lower mammary tumor incidence (54- 62%; P < 0.002) and multiplicity (P < 0.007) than casein-fed rats in both e xperiments. Our results indicate that diets rich in soy reduce the incidenc e of chemically induced mammary tumors by approximately 20%, Furthermore, w hey appears to be at least twice as effective as soy in reducing both tumor incidence and multiplicity.