Inclusion Criteria

DISEASE CHARACTERISTICS:

- Diagnosis of prostate cancer

- Clinically localized (T1c or T2) disease

- At high risk for recurrence, as defined by ≥ 1 of the following:

- Preoperative PSA > 20.0 ng/mL

- Seminal vesicle invasion

- Extracapsular extension (excluding the bladder neck)

- Positive surgical margins (excluding apical margins)

- Micrometastases in any removed pelvic lymph nodes

- Final Gleason score of ≥ 8

- Must have undergone radical prostatectomy for prostate cancer within the past 4
months

- Must have an undetectable PSA (< 0.07 ng/mL) at baseline, as measured by Tosoh PSA
assay

- No clinical evidence of locally recurrent or metastatic disease

PATIENT CHARACTERISTICS:

- No significant intake of soy (i.e., more than once a week) at baseline including, but
not limited to, any of the following:

- Vegetarians who regularly consume soy products (e.g., tofu)

- Individuals with customary Asian dietary habits, including regular intake of soy
products

- Individuals who use soy-based milk replacements

- No anemia, iron deficiency problems, or subclinical iron deficiency at baseline

- No diabetes

- No thyroid disease

- No requirement for a sodium-free diet

- No substantive tendency to be constipated (i.e., ≥ grade 2 constipation experienced
regularly)

- No medical problem that would preclude the consumption of the soy containing beverage
powder, including allergies against soy (or milk protein)

- No concurrent major disease, including major mental disease or major substance abuse
problems

- No significant side effects from medication

PRIOR CONCURRENT THERAPY:

- See Disease Characteristics

- No concurrent radiotherapy or hormonal therapy

- No other concurrent adjuvant therapy for prostate cancer