Inclusion Criteria:

• Performance status 0, 1, or 2 on ECOG scale
• Acceptable hematopoietic, hepatic, and renal function (WBC≥3500 µL, platelet count ≥ 100,000/ml, serum creatinine < 2.0 mg/dl, serum bilirubin ≤ 2.0 mg/dl, transaminases < two times normal limits)
• Post-menopausal status - FSH level > 27 mIU/ml, no spontaneous bleeding > 1 year
• Normal mammogram within the last 13 months
• Normal Papanicolaou Test within the last 13 months

Exclusion Criteria:

• Serious intercurrent medical illnesses or history of seizure
• Significant cardiac disease (New York Heart Association Class III or IV)
• Abnormalities on the physical examination screen including significant abnormality of the heart, lungs, liver, spleen, or other abdominal organs, or neurologic abnormality Abnormalities on the biochemical screen that could be metabolically significant
• Individuals with intermediate equol values (> 10 ug/L to < 20 ug/L) on soy challenge
• Use of antibiotics within the last 3 months
• Use of hormone/estrogen therapy or SERMs within the last 3 months
• Abnormal thyroid or endocrine function tests; or current use of thyroid medication
• BMI at or above 35
• History of complete hysterectomy or oophorectomy
• High risk of breast cancer (5-year risk of >1.9%) based on NCI's Breast Cancer Risk Assessment Tool
• A history of a malignancy active or initially diagnosed within 2 years other than curatively treated non-melanoma carcinoma of the skin
• History of substance abuse or addiction
• Alcohol intake of greater than 2 drinks/day or 14 drinks/week
• Tobacco use
• Diets containing more than estimated intake of 20 mg genistein/day or 40mg isoflavone/day AND unwilling/unable to reduce soy intake to below this level during the study period
• Use of supplements containing phytoestrogens or that have estrogenic side effects within 1 month of study
• Pre-menopausal or pregnant
• Inability to read and comprehend the Informed Consent
• Life expectancy < 6 months
• History of breast cancer
• Known soy intolerance
• On chemotherapy within the last 2 years
• Inability to obtain an endometrial biopsy