Protective effect being studied in prostate cancer patients at high risk of recurrence after prostatectomy.

DENVER—Consuming 20 g of soy daily for up to two years is safe and may be beneficial for men at high risk for biochemical failure after radical prostatectomy, according to preliminary findings from an ongoing study.

“There may be a protective effect,” said investigator Maarten C. Bosland, DVSc, PhD, a Professor in the Department of Pathology at the University of Illinois at Chicago.

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“We also found that the soy protein isolate was extremely well-tolerated. There have been very few side effects. Soy is known to cause or to aggravate existing constipation, an effect we have seen in a few men. However, we told them to drink more water and the problem went away.”

At the 2009 American Association for Cancer Research Annual Meeting here, Dr. Bosland presented findings from 131 men who completed two years of intervention with either soy protein supplements or placebo.

To be eligible for the ongoing study, the subjects need to have a Gleason sum of 8 or greater, extracapsular extension, seminal vesicle invasion, positive surgical margins, positive lymph nodes, and/or a preoperative PSA level of 20 ng/mL or greater. Over the two years, 30 men (23%) had confirmed PSA recurrence. Because the study is blinded we do not yet know the rate in the treatment and placebo arms separately, Dr. Bosland said. The expected recurrence rate in the placebo group is 30%.

Besides transient constipation, which occurred in a few men, Dr. Bosland’s group observed other treatment-related adverse events. Soy consumption did not affect iron status parameters (ferritin, hemoglobin, RBC counts, and hematocrit) or levels of serum thyroid hormones (thyroid-stimulating hormone, tri-iodothyronine [T3], thyroxine [T4], and free T3 and T4). Evidence from animal studies suggest that soy isoflavones could adversely affect thyroid function, Dr. Bosland said, and soy can negatively affect ion status in infants on soy formula.

Some men dropped out of the study or declined to participate because they read on the Internet that soy is contraindicated for preventing prostate cancer or has harmful effects, Dr. Bosland said.

This kind of misinformation was harming the trial and he hoped urologists would help educate patients about this issue and consider referring them for the study.