Increased intake of soy isoflavones significantly reduced the risk of prostate cancer amongst Japanese men by as much as 50 per cent//, says a new study.
"To our knowledge, this is the first prospective study to report an inverse association between isoflavone and localized prostate cancer in Japanese, whose intake of soy food is high," wrote lead author Norie Kurahashi from Japan's National Cancer Center.
Over half a million news cases of prostate cancer are diagnosed every year world wide, and the cancer is the direct cause of over 200,000 deaths. More worryingly, the incidence of the disease is increasing with a rise of 1.7 per cent over 15 years.
"The incidence of prostate cancer is much lower in Asian than Western populations… Although isoflavones have been suggested to show a preventive effect against prostate cancer in animal experiments, the results of epidemiologic studies are inconsistent," stated the researchers in the journal Cancer Epidemiology Biomarkers & Prevention.
The study recruited 43,509 Japanese men (average age 57) with a generally high soy isoflavone intake. Dietary assessment was performed using a validated 147-item food frequency questionnaire.
During the five years of follow-up, 307 men were newly diagnosed with prostate cancer (74 cases were advanced), and 220 cases were organ localized. While no relationship between total prostate cancer risk and dietary intakes of genistein, daidzein, miso soup, and soy food was observed, all four food items were associated with significant risk reduction for localised prostate cancer.
The highest intake of soy isoflavones (at least 32.8 milligrams of genistein per day) had a 40 per cent reduced risk compared to those with the lowest intake (less than 13.2 mg/d), the researchers found.
"These results were strengthened when analysis was confined to men ages greater than 60 years, in whom isoflavones and soy food were as
sociated with a dose-dependent decrease in the risk of localized cancer, with relative risks for men in the highest quartile of genistein, daidzein, and soy food consumption compared with the lowest of 0.52, 0.50, and 0.52, respectively," stated the researchers.
In contrast, increased isoflavone intake was found to enhance the risk of advanced prostate cancer, said Kurahashi.
The mechanism behind the benefits was linked to the weak oestrogenic activity of soy isoflavones, which may act to reduce testosterone levels and inhibit 5á-reductase - an enzyme involved in the metabolism of testosterone.
"Given that Japanese consume isoflavones regularly throughout life, we do not yet know the period during which the effects of isoflavones on prostate cancer are preventive," concluded the researchers.
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