Soy and Prostate Cancer. What’s the Connection?

Here’s a case of 2+2 = 2. Or, maybe 3. But definitely not 4.
People in Asian countries eat a lot more soy than those of us in Western countries. They also have lower rates of prostate cancer. So, more than a decade ago, scientists started speculating that eating all that soy was responsible for reducing cancer risk. The connection between the two seemed so obvious that the American Cancer Society and the National Cancer Center began recommending that men increase their soy intake. Unfortunately, there has been little evidence to back up the idea that increasing soy is what causes the decreased prostate cancer risk. In fact, according to a just-published study out of the University of Illinois, there’s no connection at all.

Led by Maarten C. Bosland, the team was investigating whether men who’d had a prostatectomy and were are high risk of having the cancer recur, could lower that risk by taking a daily soy supplement. The test subjects started taking the supplements within four months after their surgery and keep up the routine for as long as two years. The researchers measured the subjects’ PSA levels every two months for the first year and quarterly in the second year. Another group of subjects were taking a placebo instead of the soy supplement and had their PSA levels measured as well.
Twenty-eight percent of participants had a cancer recurrence during the study. However, in the soy group, 27% had a recurrence vs. 29% of the placebo group—a difference that is not statistically significant. The soy group’s recurrences appeared a little later than the placebo group’s, but again, the difference was not statistically significant, according to the researchers.
The researchers weren’t ready to completely dismiss any connection between soy intake and reduced cancer risk. And they suggested an interesting topic for future study: “One possible explanation for these discrepant results is that in both 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. 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.”