Taking supplements of soy isoflavones may help men at high risk of prostate cancer, suggests a new study by researchers from the University of Minnesota.

The study, published in the October 2007 issue of Journal of Nutrition, confirmed early studies that showed an inverse association between isoflavones and prostate cancer in Japanese men,

In the United States, 219,000 men are expected to be diagnosed with prostate cancer and 27,000 will die from the disease this year, according to the National Cancer Institute.

This new randomized controlled trial was meant to examine the effect of soy isoflavones on the excretion of urinary estradiol (E2) and ratio of urinary 2-hydroxy estrogens to 16-hydroxyestrone (2:16 OH-E1), which are believed to trigger hormone-related cancers.

This effect had been already observed early in studies of women, but no one had reported the effect in men before.

In the trial, Jill Hamilton-Reeves and colleagues assigned supplements of proteins with or without soy isoflavones to 58 men at high risk for developing advanced prostate cancer.  

Three protein isolates used in the trial were 40 grams of soy protein with 107 mg isoflavones each day, or with less than six mg of isoflavones and 40 grams of milk protein isolate.

After three-month supplementation, both groups on soy proteins experienced increased excretion of E2 and urinary 2:16 OH-E1 ratio was increased, an observation that is in agreement of early findings in women. But after six months of supplementation, only urinary 2:16 OH-E1 ratio was increased.

The researchers wrote "(the results) suggest that soy consumption may be beneficial in men at high risk of progressing to advanced prostate cancer as a result of effects on endogenous estrogen metabolism."

One study published in Cancer Epidemiol Biomarkers Prev. established a link between isoflavones and potential protection from prostate cancer probably due to the weak estrogenic activity of soy isoflavones, which may reduce production of male hormone testosterone and inhibit an enzyme involved in metabolism of testosterone.

Another study published in 2004 in Biology of Reproduction suggested daidzein, a metabolite of soy isoflavones, has an effect on the male hormone dihydrotestosterone (DHT), which was linked to prostate growth and male baldness.