According to a report from Japan in the March issue of “Cancer Epidemiology, Biomarkers & Prevention”, dietary isoflavones may reduce the risk of initially developing localized prostate cancer, but appear to be associated with advanced disease when prostate cancer does occur.
This would explain the prior studies in which the consumption of soy by Japanese men seems protective for prostate cancer. In this report, Dr. Norie Kurahashi of the National Cancer Center, Tokyo said, “Isoflavone intake from traditional Japanese food throughout life may be protective for incidence of prostate cancer, but we can not recommend intake of isoflavones from supplements to persons who do not consume isoflavones regularly, because isoflavones may increase the risk of advanced prostate cancer.”
Dr. Kurahashi investigated the association between isoflavone intake and risk of prostate cancer in a prospective study of Japanese men, who generally consume large amounts of soy products and have a low incidence of prostate cancer. This study involved some 43,500 men followed from 1995 through 2004. During that time, only 307 of them were newly diagnosed with prostate cancer.
Consumption of isoflavones (principally genistein, but also daidzein and soy foods) correlated with a decreased risk of localized prostate cancer. Genistein and daidzein, however, were associated with an increased the risk of advanced prostate cancer in the men who did develop localized prostate cancer.
“We suggest that isoflavones delay the progression from latent cancer to clinically significant prostate cancer in Japanese who consume isoflavones regularly throughout life,” Dr. Kurahashi said. “However, we do not know when or how isoflavones affect latent or localized prostate cancer development and whether isoflavones can be used in the treatment or chemoprevention of this cancer.”
There have not been any studies that look at the role of isoflavones once the cancer has moved on to the advanced stage. The evidence does raise a red flag for men with local disease as isoflavones might promote the development of advanced disease. Considering this, I would urge caution in consuming isoflavones (soy, soymilk and related products and supplements) if you have either local or advanced disease.
Joel T Nowak MA, MSW