Soy protein isolate consumption protects against azoxymethane-induced colon tumors in male rats

Male Sprague–Dawley rats (F2 generation) that had been fed modified American Institute of Nutrition-93G diets formulated with a single protein source of either casein or soy protein isolate for their entire life received azoxymethane once a week for 2 weeks (s.c., 15 mg/kg) starting at age 90 days. Forty weeks later, all rats were euthanized, the colon was examined visually for masses and these were subsequently evaluated histologically. Rats fed the casein diet had a 50% incidence of colon tumors compared with 12% on soy protein-based diets (P<0.05). These results suggest that consumption of soy protein-containing diets may reduce the risk of developing colon tumors.