July 17, 2013
Soy Supplements Don’t Reduce, Delay
Prostate Cancer Recurrence

Chicago—About half of men diagnosed with prostate cancer use dietary supplements, including soy protein, in hopes of reducing or delaying cancer recurrence.

A new study found, however, that daily consumption of a beverage powder supplement containing soy protein isolate for 2 years had little or no effect in keeping the cancer from coming back, in comparison with men receiving a placebo.

The study, led by researchers from the University of Illinois in Chicago and published recently in the Journal of the American Medical Association, was ended because of the lack of treatment effects.

“Prostate cancer is the most frequently diagnosed malignancy and the second most frequent cause of male cancer death in the United States and other Western countries but is far less frequent in Asian countries,” background information in the article notes. “Prostate cancer risk has been inversely associated with intake of soy and soy foods in observational studies, which may explain this geographic variation because soy consumption is low in the United States and high in Asian countries.”

The authors point out that laboratory studies have suggested that isoflavones and other compounds in soy can prevent prostate cancer development and that “a substantive fraction (48%-55%) of men diagnosed as having prostate cancer use dietary supplements including soy products.” The hypothesis, however, had not been tested in randomized studies with cancer as an endpoint, they add.

In response, the researchers conducted a randomized trial, including 177 men, from July 1997 to May 2010 at seven U.S. medical centers. Supplement intervention was begun within 4 months after surgery and continued daily for up to 2 years, with prostate-specific antigen (PSA) measurements made at 2-month intervals in the first year and every 3 months afterwards. Biochemical recurrence of prostate cancer was defined as development of a PSA level of ≥0.07 ng/mL.

Among study participants, 87 were randomized to receive a daily serving of a beverage powder containing 20 g of soy protein isolate, while 90 were given beverage powder containing the placebo, calcium caseinate.

In total, 28.3% of participants reached a PSA level of ≥0.07 ng/mL within 2 years of entering the trial, with 27.2% (22) of the participants in the intervention group developing confirmed biochemical recurrence compared to 29.5% (23) of men receiving placebo. The lack of treatment effects led to the early discontinuation of the trial at a point where 81 men were in the intervention group and 78 in the placebo group.

“Among participants who developed recurrence, the median time to recurrence was somewhat shorter in the intervention group (31.5 weeks) than in the placebo group (44 weeks), but this difference was not statistically significant,” the authors write.

"The findings of this study provide another example that associations in observational epidemiologic studies between purported preventive agents and clinical outcomes need confirmation in randomized clinical trials,” according to the research report. “Not only were these findings at variance with the epidemiologic evidence on soy consumption and prostate cancer risk, they were also not consistent with results from experiments with animal models of prostate carcinogenesis, which also suggest reduced risk.”

Researchers suggest that one possible explanation for the differing results is that, in earlier epidemiologic studies and animal experiments, “soy exposure typically occurred for most or all of the life span of the study participants or animals; there are no reports of such studies in which soy exposure started later in life.”

“Thus, it is conceivable that soy is protective against prostate cancer when consumption begins early in life but not later or when prostate cancer is already present,” according to the authors. “If this is the case, chemoprevention of prostate cancer with soy is unlikely to be effective if started later in life, given the high prevalence of undetected prostate cancer in middle-aged men."

U.S. Pharmacist Social Connect