Inclusion Criteria

DISEASE CHARACTERISTICS:

- Diagnosis of stage I or II prostate cancer

- Gleason score 2-6* NOTE: *Patients with a Gleason primary pattern 4 (4 + 1 or 4
+ 2) are ineligible

PATIENT CHARACTERISTICS:

Age:

- 50 to 80

Performance status:

- Not specified

Life expectancy:

- Not specified

Hematopoietic:

- Not specified

Hepatic:

- No known history of hepatic disease

Renal:

- No known history of renal disease

Other:

- Close to ideal body weight (body mass index no greater than 32 kg/m^2)

- No known history of thyroid disease

- No allergy to study agent

- No other prior malignancy except nonmelanoma skin cancer

- No evidence of prostatitis or urinary tract infection

- Fertile patients must use effective contraception

- Omnivorous diet (no vegan or vegetarian diets)

PRIOR CONCURRENT THERAPY:

Biologic therapy:

- No prior or concurrent biologic therapy for prostate cancer

Chemotherapy:

- No prior or concurrent chemotherapy for prostate cancer

Endocrine therapy:

- No prior or concurrent endocrine therapy for prostate cancer

- No concurrent thyroid hormone replacement medications

Radiotherapy:

- No prior or concurrent radiotherapy for prostate cancer

Surgery:

- No concurrent surgery

Other:

- At least 30 days since prior antibiotics

- At least 30 days since prior ingestion of a diet high in soy products

- No other prior or concurrent therapy for prostate cancer

- No concurrent diet high in soy products

- No concurrent nutritional supplements (e.g., retinoids, beta-carotene, and
isoflavones)