Blog | Monday, July 15, 2013

QD: News Every Day--Soy supplements didn't reduce risk of prostate cancer recurrence after prostatectomy

Daily use of soy protein isolate among men who had undergone radical prostatectomy did not reduce or delay development of biochemical recurrence of prostate cancer compared to men who received placebo, according to a study.

Prostate cancer risk has been inversely associated with intake of soy and soy foods in observational studies, but this randomized trial undercuts the association. Between 48% to 55% of men diagnosed as having prostate cancer use dietary supplements, including soy products, researchers noted.

Researchers conducted the randomized trial on 177 patients from July 1997 to May 2010 at seven U.S. centers. Supplement intervention was started 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 thereafter. Participants were randomized to receive a daily serving of a beverage powder containing 20 g of protein in the form of either soy protein isolate (n=87) or as placebo, calcium caseinate (n=90).

Results appeared in the July 10 issue of JAMA.

The trial was stopped early for lack of treatment effects at a planned interim analysis with 81 participants in the intervention group and 78 in the placebo group. Overall, 28.3% of participants developed biochemical recurrence (defined as development of a PSA level of ≥0.07 ng/mL) within 2 years of entering the trial. Twenty two (27.2%) of the participants in the intervention group developed confirmed biochemical recurrence, whereas 23 (29.5%) of the participants receiving placebo developed recurrence.

Researchers noted that one reason for soy's purported effects may be because in many animal and human trials, soy exposure typically occurred for most or all of the life span of the subjects. Soy may protect against prostate cancer when consumption begins early in life but not later or when prostate cancer is already present. 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.

Researchers wrote, "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. 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."