Genistein in Preventing Breast or Endometrial Cancer in Healthy Postmenopausal Women

DISEASE CHARACTERISTICS: Healthy participants Papanicolaou test (pap smear) normal within the past 13 months Mammogram normal within the past 13 months No history of breast cancer Not at high-risk (5-year risk < 1.9%) for breast cancer according to NCI's Breast Cancer Risk Assessment Tool Hormone receptor status: Not specified PATIENT CHARACTERISTICS: Age 45 to 70 Sex Female Menopausal status Postmenopausal Last spontaneous menstrual bleeding > 12 months ago Performance status ECOG 0-2 Life expectancy Not specified Hematopoietic WBC ≥ 3,500/mm^3 Platelet count ≥ 100,000/mm^3 Hepatic Bilirubin ≤ 2.0 mg/dL ALT and AST < 2 times normal No significant abnormality of the liver by physical exam Renal Creatinine < 2.0 mg/dL Cardiovascular No significant cardiac disease No New York Heart Association class III or IV heart disease No significant abnormality of the heart by physical exam Pulmonary No significant abnormality of the lung by physical exam Other Body mass index < 35 Follicle-stimulating hormone > 27 mIU/mL Thyroid or endocrine function test normal Alcohol intake ≤ 2 drinks/day or ≤14 drinks/week Not pregnant No intermediate equol values (≥10 ug/L to ≤ 20 ug/L) on soy challenge No history of seizures No significant abnormality of the spleen or other abdominal organs by physical exam No neurologic abnormality by physical exam No significant metabolic abnormality on the biochemical screen No history of substance abuse or addiction No tobacco use No diets containing > 20 mg of genistein/day or > 40 mg isoflavone/day No known intolerance to soy No other serious medical illness No active malignancy or malignancy initially diagnosed within the past 2 years except curatively treated nonmelanoma skin cancer PRIOR CONCURRENT THERAPY: Biologic therapy Not specified Chemotherapy More than 2 years since prior chemotherapy No concurrent chemotherapy Endocrine therapy More than 3 months since prior hormonal or estrogen therapy More than 3 months since prior tamoxifen or other selective estrogen-receptor modulators More than 1 month since prior supplements containing phytoestrogens or that have estrogenic side effects (soy isoflavones or PC-SPECS) No concurrent thyroid medication Other concurrent endocrine medication allowed provided medication was initiated ≥ 3 months before study entry AND participant has been on a stable regimen for the past 3 months Radiotherapy Not specified Surgery No prior hysterectomy or oophorectomy Other More than 3 months since prior antibiotics