Effect is stronger in men over age 60 years.
Men who consume large quantities of isoflavone-rich soy food have a decreased risk of localized prostate cancer.
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Researchers in Japan prospectively studied 43,509 men age 45-74 living in half a dozen cities, including Nagasaki, Okinawa, and Tokyo. At baseline, the men completed self-administered questionnaires assessing information on lifestyle factors and medical and smoking histories.
They also responded to a food questionnaire designed to measure their intake of 147 food and beverage items, including various kinds of soy food and miso soup. The soy food included tofu, yushidofu (pre-drained tofu), koyadofu (freeze-dried tofu), aburaage (deep-fried tofu), natto (fermented soybean), and soy milk.
During follow-up from 1995 through 2004, 307 men were newly diagnosed with prostate cancer. Of these, 220 cases were organ-localized, 74 cases were advanced, and 13 were of an undetermined stage.
Results showed that intakes of genistein, daidzein, miso soup, and soy food were not associated with total prostate cancer, but were associated with decreased risk of localized prostate tumors. The negative association was especially significant in men over age 60. In this group, men whose isoflavone intake was in the highest quartile of genistein, diadzein, and soy food had a 48% lower risk of localized prostate cancer compared with men in the lowest consumption quartile.
“To our knowledge this is the first prospective study to report an inverse association between isoflavone and localized prostate cancer in Japanese men, whose intake of soy food is high,” the authors wrote in Cancer Epidemiology Biomarkers & Prevention (2007;16:538-545). Their results support previous studies showing that soy food helps protect against prostate cancer.
Although the investigators say further research is required to fully understand these results, they postulate that the effect of isoflavones may differ according to the stage of cancer. “One mechanism by which isoflavones reduce the risk of prostate cancer seems to involve estrogen receptors in prostate cancer,” they note. “But cancer with…meta-static potential is associated with the complete or partial loss of estrogen
receptor expression.”
