Soy a Double-Edged Sword for Prostate Cancer

Japanese researchers have concluded that the intake of isoflavones, derived largely from soyfoods, decreased the risk of localized prostate cancer but increased the risk of advanced prostate cancer. The large study, conducted by National Cancer Center, Tokyo, looked at the relationship between the traditional soy-rich Japanese diet and development of prostate cancer in 43,509 Japanese men. Japanese men eat significantly more soyfoods than their Western counterparts, and the incidence of prostate cancer is much lower there than in Western countries.

Published in the March issue of Cancer Epidemiology, Biomarkers & Prevention, Soy Product and Isoflavone Consumption in Relation to Prostate Cancer in Japanese Men suggests that isoflavones effect on prostate-cancer development might differ according to the disease stage. Previous, smaller epidemiological studies have reached differing conclusions about soys protective effects on prostate cancer, so the research team questioned thousands of men age 40 to 69 about their consumption of 147 foods, including miso soup (which uses fermented soybeans), natto (also uses fermented soybeans) and tofu. Miso, natto and tofu account for about 90% of the mens consumption of the estrogen-mimicking isoflavones daidzein and genistein, according to the researchers. Intakes of genistein, daidzein, miso soup and soyfood were not associated with total prostate cancer, but decreased the risk of localized prostate cancer. At the same time, the study found positive associations between isoflavones and advanced prostate cancer.

The cancer-fighting properties of isoflavones were strongest in men over 60; the more isoflavones they ate, the less risk they had of developing localized prostate cancer. Isoflavone may be protective for localized prostate cancer only in men aged more than 60 years, and may not have a protective effect in the early stage of prostate cancer in younger men, said the studys researchers.

It is believed that isoflavones may delay the progression of only latent prostate cancer. They may be unable to provide a positive effect once tumors lose estrogen-receptor beta expression and become aggressive. The scientists theorize isoflavones could potentially increase the risk of progression, possibly by reducing serum testosterone, or because advanced and localized prostate cancer may have different tumor subtypes, with different reactions to isoflavones.

In addition, the opposing findings could be a result of errors in food measurement, or because of the small number of participants who developed advanced prostate cancer, says one of the studys authors, Dr. Norie Kurahashi, of the Epidemiology and Prevention Division of the National Cancer Center. The present findings provide no clear understanding of when or how localized cancer will develop to aggressive cancer, and of the related effect of isoflavones, she says. Given that Japanese consume isoflavones regularly throughout life, we do not know the period during which the effects of isoflavones on prostate cancer are preventive, and further research, including well-designed clinical trials, is required to find that out,

Until additional studies are done, the researchers recommend eating foods with isoflavones rather than taking supplements. 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, Kurahashi says.