Effects of soy vs casein protein diet and obesity on DMBA‐induced mammary tumor development

Abstract

We reported that intact obese Zucker rats fed a standard chow diet had increased susceptibility to DMBA‐induced mammary tumors compared to lean Zucker rats. The objective of this study was to investigate the effects of long‐term high‐isoflavone soy (high‐isof soy) vs casein protein intake and obesity on mammary tumor development. Five‐week‐old female Zucker rats (45 obese fa/fa and 54 lean) were used. After one week of acclimation (age 42 days), rats were randomly assigned to the following diet groups; 1) Lean, casein; 2) Obese, casein; 3) Lean, high‐isof soy and 4) Obese, high‐isof soy. Rats had ad lib access to water and diet with either casein or high‐isof soy protein isolate. All rats were orally gavaged at age 50 days with 65 mg/kg DMBA. Rats were weighed and palpated twice weekly and sacrificed 155 days post‐DMBA treatment. Obese soy‐fed rats gain significantly (P<0.001) more weight than obese casein rats. The lean high‐isof soy‐fed rats had slightly higher (P = 0.057) body weight than lean casein‐fed rats. At end of the experiment, 69% of the Lean casein rats developed mammary tumors compared to 50% in lean high‐isof soy group (P<0.08). In the obese group, 76% of the high‐isof soy‐fed rats developed mammary tumors compared to 15% (P<0.05) of obese casein‐ fed rats. These results suggest that intact obese versus lean Zucker female rats fed semi‐purified diets are protected against DMBA‐induced mammary tumor on the casein but not high‐isof soy diet. (Supported by USDA and ABI to RH)