• Change in nutritional intake [ Designated as safety issue: No ]
• Change in anthropometric measurements (height, weight, BMI) at baseline and post study [ Designated as safety issue: No ]

• Change in nutritional intake
• Change in anthropometric measurements (height, weight, BMI) at baseline and post study

RATIONALE: Soy isoflavones may reduce the risk of some types of cancer. It is not yet known if isoflavones are effective in preventing the development of prostate cancer.

PURPOSE: This randomized phase III trial is studying the effectiveness of isoflavones in preventing further development of cancer in patients who have stage I or stage II prostate cancer.

OBJECTIVES:

• Determine the effectiveness of isoflavones in producing a change in risk parameters, such as decrease in free testosterone, increase in sex-hormone-binding globulin and estradiol, and decrease in tumor progression and volume, as measured by decreasing prostate-specific antigen in patients with stage I or II prostate cancer.

OUTLINE: This is a randomized, double-blind study. Patients are stratified according to Gleason score (2-4 vs 5-6). Patients are randomized to 1 of 2 treatment arms.

• Arm I: Patients receive oral isoflavones twice daily and an oral multivitamin once daily for 12 weeks.
• Arm II: Patients receive oral placebo twice daily and an oral multivitamin once daily for 12 weeks.

PROJECTED ACCRUAL: A total of 148 patients will be accrued for this study within 3 years.

• Dietary Supplement: multivitamin
• Dietary Supplement: soy isoflavones

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)