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)