Soy diet and prostate cancer risk: Good news and bad news

A soy-rich diet offers mixed results on its impact on the risk of prostate cancer, according to a large study from the National Cancer Center in Japan.

The results showed that while consumption of isoflavone chemicals from soy foods lowered risk of localized prostate cancer, such consumption elevated the risk of advanced prostate cancer.

Authors of the prospective study of 43,509 men found that the effects of isoflavones on prostate cancer progression may depend on the disease stage. Participants (age range, 40 to 69 years) were questioned about the consumption of 147 foods and then followed from 1995 to 2004. By 2004, 307 men were diagnosed with prostate cancer (74 [24%] advanced cases, 218 [71%] cases confined to prostate organ, and 15 [5%] of undetermined origin).

Based on the findings, researchers estimated that risk of developing localized prostate cancer was 50% lower in those who consumed the most isoflavones, compared with those who ate the least. They also found that the protective effect of isoflavone-rich food was highest in men over the age of 60 years. Researchers concluded that isoflavones may be protective for localized disease only in men over age 60, and may not have a protective effect in younger men. They also found that risk of advanced prostate cancer was double in men who consumed at least two bowls of soy-rich miso soup per day, compared with men who ate one bowl or less, a finding the researchers were unable to explain.

“Consumption of isoflavones from traditional Japanese food throughout life may protect against the incidence of prostate cancer, but we cannot recommend the use of isoflavones from supplements for people who do not regularly consume these chemicals because the relationship between isoflavones and the risk of advanced prostate cancer is not yet clear,” said lead author Norie Kurahashi, MD.

The study was published in the journal Cancer, Epidemiology, Biomarkers and Prevention (2007; 16:538-45).