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The effect of feeding soybean protein isolate (SBP) diet or soybean protein isolate diet supplemented with 0.7% DL-methionine (SBP + Met) on mammary tumor progression was investigated. Sprague-Dawley female rats were fed from weaning a 20% casein (CAS) diet supplemented with 0.3% DL-methionine (AIN-76) and injected via jugular vein with N-nitrosomethylurea (NMU, 40 mg/kg body weight) at 7 wk of age. Five weeks after NMU treatment, animals were divided into the three isoenergetic, isoprotein diet groups: CAS (25 rats); SBP (26 rats) and SBP + Met (25 rats). First palpable mammary tumors were evident 8, 9 and 13 wk and the mean latency period was 13.30 +/- 1.23, 16.70 +/- 1.32 and 17.82 +/- 1.28 wk after NMU treatment in the CAS, SBP + Met and SBP diet groups, respectively. Tumor incidence was 80% in the CAS group compared with 42.3% in the SBP group (P = 0.01). Methionine supplementation increased tumor incidence to 64%. Total number and total weight of tumors was greater in the CAS group compared with either SBP + Met or SBP groups: 41 vs. 28 or 21 tumors and 97.28 g vs. 27.87 or 32.46 g, respectively. These data indicate that SBP diet, low in methionine content, fed 5 wk after carcinogen exposure significantly repressed mammary tumor progression. Methionine supplementation increased the number of animals with tumors but not the mean tumor weight.

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