Isoflavones reduce prostate cancer risk

Epidemiological studies show that incidences of prostate cancer are lower in countries with high soy intake compared to countries with low soy intake. Mortality from prostate cancer is nine times lower in Japan than in the United States. But in Japan the incidence of prostate cancer is rising, probably because the Japanese diet becomes westernized. Isoflavones have been suggested as the component responsible for this protective action. A previous study conducted at the Nara Medical University in Japan suggested that equol (produced by gut bacteria from daidzein) has preventive effect on prostate cancer.

A new study conducted at the Nara Medical University investigated the association between prostate cancer risk and isoflavone intake in a phase II, randomized, double-blind, placebo-controlled trial. Phase II trials are performed on larger groups of volunteers and are designed to assess how well the drug works. This study was conducted on 158 Japanese men, aged between 50 and 75 years which had a negative prostate biopsy. About 50% of the men were equol producers.

The researchers found that treatment with isoflavones had no influence on the level of Prostate-Specific Antigen (PSA), an indicator for the presence of prostate cancer. Biopsies conducted on 89 patients showed that isoflavones intake reduced the incidence of prostate cancer, but that this difference was not statistically significant. However, if they only considered patients aged 65 years or more, the incidence of cancer in the isoflavone group was significantly lower than that in the placebo group. The study concluded that the obtained results support the value of isoflavone for prostate cancer risk reduction. Limitations of this study are the small number of patients and the short duration of the trial. Although the obtained results indicate a protective effect of isoflavones against prostate cancer, we find it strange that such effect can be observed over a period of only one year, considering that the progression of prostate cancers is very slow. The researchers recommend conducting large-scale phase III studies of the effect of isoflavone tablets in men with different hereditary factors and living conditions.


Miyanaga et al. A Prostate Cancer Chemoprevention Study: An Investigative Randomized Control Study Using Purified Isoflavones in Men with Rising PSA. Cancer Sci. 2011 Oct 11.
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