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In a randomized, double-blind trial that enrolled 177 men at high risk of recurrence after radical prostatectomy for prostate cancer, daily consumption of a soy protein isolate supplement for 2 years did not reduce biochemical recurrence of prostate cancer (defined as development of a PSA level ≥0.07 ng/mL).
The trial was conducted from July 1997 to May 2010 at seven centers in the United States. Participants were randomized to receive a daily serving of a beverage powder containing 20 g of either soy protein isolate (n=87) or placebo (calcium caseinate; n = 90). Supplement intervention was started within 4 months after surgery and continued for up to 2 years. Prostate-specific antigen measurements were made at 2-month intervals in the first year and every 3 months thereafter.
The trial was stopped early because of lack of treatment effects at a planned interim analysis. Overall, 28.3% of participants developed biochemical recurrence of prostate cancer within 2 years; there were 22 (27.2%) recurrences in the intervention group and 23 (29.5%) in the placebo group. The resulting hazard ratio for active treatment was 0.96 (95% CI, 0.53–1.72; log-rank P = 0.89).