Soy may raise risk of advanced prostate cancer

Soy is often hailed for its health benefits. But for men, eating soy and other foods rich in isoflavones may not be so favorable; it could increase the risk of advanced prostate cancer.

However, when it comes to the risk of non-advanced prostate cancer — that is, cancer that has not spread beyond the prostate gland — dietary isoflavones appear to have no significant influence.

These are the findings of a new study recently published in the International Journal of Cancer.

Prostate cancer is the most common cancer among men in the United States, after skin cancer.

This year, it is estimated that around 161,360 new cases of prostate cancer will be diagnosed in the U.S., and more than 26,000 men will die from the disease.

Studies have suggested that diet can affect a man's risk of prostate cancer. Last year, for example, Medical News Today reported on a study that linked a regular intake of processed carbohydrates to a greater likelihood of prostate cancer, while other research has associated a high-fat diet with the disease.

The new study — which was conducted by senior author Dr. Jianjun Zhang, of the Fairbanks School of Public Health at Indiana University in Indianapolis, and colleagues — suggests that including isoflavones in the diet may also influence the risk of prostate cancer.

Isoflavones and prostate cancer risk

Isoflavones are a type of phytoestrogen, which are plant-derived compounds that have similar effects on the body as the female sex hormone estrogen.

Soy and soy products — such as miso, tempeh, and tofu — contain the highest concentrations of isoflavones. Other dietary sources include kudzu root and potato beans.

Studies have shown that isoflavones may have varying effects on health. Some have indicated that the compounds may drive breast cancer, while others have suggested that they could benefit some women with breast cancer.

To find out whether dietary or not isoflavones influence the risk of prostate cancer, Dr. Zhang and colleagues analyzed the data of 27,004 men who were part of the Prostate, Lung, Colorectal, and Ovarian Cancer Screening Trial.

The team identified 2,598 prostate cancer cases among the men over a median follow-up period of 11.5 years. Of these cases, 287 were advanced prostate cancer.

As part of the trial, the men completed food frequency questionnaires. The researchers used data from these questionnaires to assess the men's intake of foods rich in isoflavones.

Compared with men who did not have isoflavones in their diet, those who did were found to be at greater risk of developing advanced prostate cancer, or cancer that has spread from the prostate gland to distant sites.

The researchers found no significant link between dietary isoflavones and the risk of non-advanced prostate cancer.

As a result of their findings, Dr. Zhang and team believe that including isoflavones in the diet may affect men's risk of prostate cancer, though further studies are warranted.

"Our study offers novel evidence that dietary intake of isoflavones has different effects on advanced and non-advanced prostate cancer."

Dr. Jianjun Zhang

"This observation," Dr. Zhang adds, "is important for understanding the etiology and prevention of prostate cancer, but needs to be confirmed in more epidemiologic studies among populations with diverse dietary habits."