Minimal Effect of a Low-Fat/High Soy Diet for Asymptomatic, Hormonally Naive Prostate Cancer Patients

Purpose: The effects of a low-fat diet or a low-fat diet with the addition of a soy supplement were investigated in a pilot Phase II study for asymptomatic, hormonally naive prostate cancer patients with rising prostate-specific antigen (PSA) levels. Experimental Design: A two-step intervention was implemented. During step 1 patients were begun on a low-fat diet with a goal to reduce fat intake to 15% of total daily calories. On PSA progression, a soy protein supplement was added to the diet (step 2). The primary end point was PSA reduction by 50%. Secondary end points were PSA doubling time and time to progression (TTP). Serum was analyzed for changes in the sex hormone and insulin-like growth factor (IGF-I) axes. Results: Among 18 evaluable patients, (median follow-up on study 10.5 months), no patient on either step had a PSA reduction by 50% at any time. There was a trend toward a longer PSA doubling time ( P = 0.06) and a prolongation in estimated median TTP of ∼3 months ( P = 0.018) during step 2 compared with step 1 of the study. During step 1, free testosterone levels decreased by 5% ( P < 0.01), and during step 2, IGF-I levels increased by 22% ( P = 0.02). Conclusions: A low-fat diet with the subsequent addition of a soy supplement did not result in a significant decline in PSA levels. The addition of soy protein had a modest effect on TTP. A potentially undesirable effect associated with the administration of soy was an increase in IGF-I serum levels.