Soy supplements and prostate cancer–no effect

Tofu is displayed in a supermarket in Santa MonicaOver the past few years, some research has suggested that there might be a correlation between high soy consumption and lowered prostate cancer risk. Most of the research suggesting the link was observational or based on animal studies on the role of soy supplements on the risk of prostate cancer, or on the risk of recurrence of the cancer after treatment.

According to the American Cancer Society, there is some logic to a potential causal relationship between soy consumption and risk of prostate cancer:

Plant estrogens (phytoestrogens) were first identified in the early 1930s. At that time it was discovered that soybeans, willows, dates, and pomegranates contained compounds that were much like estrogens. Scientists began studying the role isoflavones play in reducing breast cancer risk in the 1960s. In a 1981 prospective study in Japan, researchers found that daily intake of miso, a soybean paste, was linked to lower death rates from stomach cancer in more than 260,000 men and women. Around that time, other studies on soy began to be published in the United States. In October 1999, the U.S. Food and Drug Administration (FDA) agreed to allow health claims about soy’s role in reducing heart disease on food products containing soy protein.

In a study published in the Journal of the American Medical Association, researchers tested the hypothesis that soy might have an effect on the recurrence of prostate cancer in a randomized clinical trial. According to the results of the study, prostate cancer survivors who took daily soy supplements did not have lower risks of the disease returning compared to men who took a placebo. These findings solidly contradict the observations from epidemiological studies along with laboratory and animal that had some effect on the risk of prostate cancer. 

This investigation was a randomized, double-blind trial conducted from July 1997 to May 2010 at seven US centers which compared daily consumption of a soy protein supplement vs placebo in 177 men at high risk of recurrence after radical prostatectomy. According to the paper, soy supplements commenced within four months after surgery and continued for up to two years. Prostate-specific antigen (PSA) measurements were made at two-month intervals in the first year and every 3 months in subsequent years. The researchers were looking to see if participants developed what’s known as a biochemical recurrence, which is defined as a PSA of 0.07 ng/mL or more.

The results demonstrated that the hazard ratio (the ratio of the hazard rates between two different experimental groups) was approximately 0.96, meaning that the risk of recurring prostate cancer was no different between the soy group and the placebo group. The hazard ratio was considered statistically significant. Moreover, the period of time until the cancer returned was shorter in the group of men taking the soy supplement: 31.5 weeks compared to 44 weeks for those in the placebo group, which might indicate that it’s actually ill-advised to increase soy consumption.

When reviewing research with an eye towards “science based medicine,” different types of studies have different levels of scientific value. Evidence from a randomized clinical trial, as long as it was designed to be experimentally powerful (for example, including enough patients to get proper statistical relevance, or formulating the clinical trial to be actually random and blinded), is the ultimate study that allows us to confirm or refute a hypothesis, even one that is supported by high levels evidence of correlation in epidemiological research. 

In reviewing the article, I was surprised that it only included 177 subjects, 87 in the soy group and 90 in the placebo group, which is small for a study that lasted nearly 13 years. It is possible that some fell out of the study for any number of reasons (including death from the cancer or just random events). However, the hazard ratio of 0.96 with statistical significance provides me with some level of assurance that this study is an authoritative one.

I often hear people complain that “one day soy is good for you, the next day it does nothing.” Actually, the evidence here strongly suggests that soy never worked. It’s when individuals (mostly the popular press) decide that a weak animal study is convincing evidence that “X will do Y.” It’s not until we have randomized clinical trials along, repeated in different ways, that we can conclude one way or another. We had a hypothesis, based on observational studies, that soy might reduce the recurrence of prostate cancer. But the gold standard clinical trial fails to support that hypothesis. So, there’s no evidence, at this point in time, that soy has any truly clinical effect.

There are too many nutritional supplement beliefs that are based on weak observational studies, or worse yet, on laboratory or animal research. Of course, eating soy may actually do harm (and this study shows that) with respect to prostate cancer, and there is some weak evidence that losing weight and/or exercising may have a more profound effect on reducing prostate cancer risk.

So far, vitamin E, selenium and soy have been demonstrated to have no effect on prostate cancer. Here’s the thing, when you read that some supplement is going to actually do something about cancer, be very skeptical before you waste any money on it.

Key citations:

The Original Skeptical Raptor
Chief Executive Officer at SkepticalRaptor
Lifetime lover of science, especially biomedical research. Spent years in academics, business development, research, and traveling the world shilling for Big Pharma. I love sports, mostly college basketball and football, hockey, and baseball. I enjoy great food and intelligent conversation. And a delicious morning coffee!