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Many men with prostate cancer try soy products, but there is no hard evidence that they thwart a return of the disease, the researchers said.
"If one eats soy every day after surgery for prostate cancer, one does not reduce the risk of recurrence," said the study's lead researcher, Dr. Maarten Bosland, a professor of pathology at the University of Illinois at Chicago.
"A lot of men think that soy might be beneficial, but this study shows that it's not," he said. On the other hand, soy posed no adverse side effects. "It's safe to take soy, but you won't benefit from it for your prostate cancer," Bosland added.
All of the men studied had an increased risk of having cancer recur because the surgery -- called a radical prostatectomy -- hadn't removed all the cancerous cells, Bosland noted.
For the report, published in the July 10 issue of the Journal of the American Medical Association, more than 150 men were assigned to drink either a powdered soy protein drink or an inactive placebo beverage daily for two years. At the end point, no significant difference was seen between the two groups in terms of cancer recurrence.
But one expert thinks the study was too small to be definitive.
"It raises a question whether soy might have been helpful and it was missed because it was such a small study," said Dr. Anthony D'Amico, chief of radiation oncology at Brigham and Women's Hospital in Boston.
"I am not sure I would tell people not to take soy based on this because there may be a benefit and there is no harm," D'Amico said.
Bosland, however, said a larger study wouldn't show any different result. "If there were an effect, it would take thousands and thousands of men to figure that out," he said. "But that effect would be so small as to be totally insignificant -- clinically irrelevant."
In Asia, where people eat a lot of soy starting at birth, the rates of prostate cancer are much lower than they are in the United States, D'Amico noted. "That's probably the most convincing data of what soy can do," he said.
Whether starting to consume soy later in life would have the same benefit isn't clear, he noted.
While laboratory studies have shown that soy, which contains beneficial isoflavones, has anti-cancer properties, the researchers noted that this is the first study to test soy supplementation's effect on prostate cancer recurrence in humans.
To do so, they randomly assigned 177 post-surgical patients to start taking the soy protein drink or placebo within four months after their cancer surgery.
Levels of prostate specific antigen (PSA), a marker of prostate cancer, were measured periodically.
The trial was stopped early because there was no benefit to the men taking the soy drink.
Overall, 28.3 percent of the men taking the supplement had a recurrence of their prostate cancer within two years -- 27.2 percent of the soy group and 29.5 percent of the placebo group.
Men taking the soy supplement developed their cancer sooner than the men taking placebo (31.5 weeks versus 44 weeks), but this small difference was not statistically significant, the researchers added.
According to the American Cancer Society, one in six U.S. men will develop prostate cancer in their lifetime.
Copyright © 2013 HealthDay. All rights reserved.
SOURCES: Maarten Bosland, D.V.Sc., Ph.D., professor of pathology, University of Illinois at Chicago; Anthony D'Amico, M.D., Ph.D., chief, radiation oncology, Brigham and Women's Hospital, Boston; July 10, 2013, Journal of the American Medical Association