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Soy milk and PSA levels in men with progresive prostate cancer

Posted Mar 01 2010 12:00am

Over the years there has been a lot of discussion, but few hard facts, about the impact of soy consumption on PSA levels in men with progressive prostate cancer.

Kwan et al. have now published data from a small, prospective, Phase II study on the effects of soy milk consumption in prostate cancer patients with biochemical failure after radiotherapy.  They initially enrolled 34 patients with a rising PSA after radiation therapy for localized prostate cancer. The patients were instructed to consume 500 ml of soy beverage daily for 6 months. Five of the 34 enrolless withdrew before completing 1 month of soy (for reasons apparently unrelated to soy consumption). All remaining 29 subjects were included in the analysis.

The results of this study are reported as follows:

  • The mean consumption of the assigned quantity of soy beverage was 93 percent.
  • Mild gastrointestinal upset (in 11/29 patients or 38 percent) was the commonest side effect, but did not affect overall soy consumption.
  • PSA levels “showed a declining trend” in 4/29 patients (13.8 percent).
  • There was a > 100 percent prolongation of PSA doubling time in 8/29 patients (27.6 percent).
  • PSA doubling time decreased by ≥ 50 percent in 5/29 patients (17.2 percent).

The authors conclude that, over a period of 6 months, daily consumption of a soy beverage was well tolerated and was associated with a declining trend in PSA level or > 2 times prolongation of PSA doubling time in 41 percent of subjects.

So first of all let us recognize that a high soy diet does not reduce PSA levels or extend PSA doubling times for everyone. More than 17 percent of the men in this trial actually saw their PSA doubling time shorten while one study, and a short PSA doubling time is a known signal of increased risk for progression of prostate cancer in men after first-line treatment.

On the other hand, over 40 percent of men did appear to benefit from the soy diet, with PSA levels either falling buy a small amount or at least rising less slowly over time.

The key question now is going to be: Can we get the same result if we re-do this trial, but with about 100 patients, randomized to receive either soy milk or some form of dietary placebo that mimics soy milk but has no known impact on prostate cancer. The follow-up question will be: If the answer to the first question is “yes,” then can we distinguish in advance between those patients whose PSA seems to respond to soy milk “therapy” and those whose PSA continues to rise (either at the same pace or faster)

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