1. Daily soy protein powder consumption for 2 years following radical prostatectomy did not reduce biochemical recurrence of prostate cancer in men at high risk of PSA failure.
Evidence Rating Level: 1 (Excellent)
Study Rundown: Daily soy supplementation taken as a powder for two years was not shown to reduce the recurrence of prostate cancer following radical prostatectomy as measured by PSA levels at regular intervals. It is thought that Asian countries may have lower instances of prostate cancer due to higher levels of soy in the diet. With the growing popularity of supplements, especially various protein powders, this was an informative study for many men. This trial that ended early shows that soy protein powder is not superior to casein-based powder in preventing recurrence. The potential protective mechanism of action of soy isoflavones still should not be fully ruled out. This study examined the role of soy in secondary and tertiary prevention when cancer had already declared itself, but there still may be a role for soy isoflavones in primary prevention of cancers.
In-Depth [randomized study]: This study included 177 patients from 7 centers who were randomized to taking soy protein powder or a casein protein powder (placebo) from 1997-2010. Participants had undergone radical prostatectomy for clinically localized prostate cancer less than 4 months before randomization and had a postsurgery PSA value of less than 0.07 ng/mL. They also fulfilled 1 or more of the following criteria for high risk: preoperative PSA of greater than 20.0 ng/mL, final Gleason score of 8 or greater, established positive surgical margins (but not apical margins), established extracapsular extension (but not in the bladder neck), seminal vesicle invasion, or micrometastases in any removed pelvic lymph nodes. PSA levels were checked at regular intervals, and the primary end points of this trial were the 2-year rate of biochemical recurrence (0.07 ng/mL or higher) and time to recurrence. A blinded interim analysis was conducted once 45 participants had developed recurrence. The trial was ended early because biochemical recurrence of prostate cancer after radical prostatectomy was not shown to be reduced.
By Mike Hoaglin and Brittany Hasty
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