Soy Protein in Preventing Recurrent Cancer in Patients Who Have Undergone Surgery for Stage II Prostate Cancer

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