Enrollment into the study will occur over 3 years, with each participant taking part in the study for a total of 2 years. Participants will be randomly assigned to one of two groups; the first group will receive a 200 mg dose of soy isoflavones daily and the second group will receive placebo daily. There will be 10 study visits: screening, study entry, randomization at Month 1, six follow-up visits at Months 2, 4, 8, 12, 16, 20, and a final visit at the end of active participation at Month 24. At each study visit, participants will have blood drawn, provide urine samples, answer questionnaires, and have mammograms and bone density tests.