The nutraceutical industry is reporting that a new study from the University of Minnesota shows that men at high risk of prostate cancer may benefit from increased intake of soy isoflavones. This is good news, because prostate cancer is on the rise worldwide.
This study, appearing in the Journal of Nutrition, adds to an earlier piece of work that claimed to be the first prospective study to report an inverse association between isoflavone and prostate cancer in Japanese men.
The new randomised controlled Minnesota trial investigated the potential of soy isoflavones to increase the excretion of urinary estradiol (E2) and lower ratio of urinary 2-hydroxy estrogens to 16-hydroxyestrone (2:16 OH-E1), two estrogen metabolites suggested to initiate hormone-related cancers.
Researchers recruited 58 men at high risk for developing advanced prostate cancer and supplemented their normal diets with one of three protein isolates: isoflavone-rich soy protein isolate, providing 107 mg isoflavones per day; isoflavone-poor soy protein isolate, providing less than six mg isoflavones per day; or milk protein isolate. All supplements provided 40 grams of protein per day.
After three and six months of supplementation, the researchers report that both soy groups had higher E2 urinary excretion than subjects receiving the milk protein. Moreover, after six months of supplementation, a significantly higher urinary 2:16 OH-E1 ratio was observed amongst individuals receiving the isoflavone-rich soy protein isolate than the milk protein.
Another earlier study (in Cancer Epidemiol Biomarkers) linked isoflavones to potential protection from prostate cancer. It stated that the benefits could be due to the weak estrogenic activity of soy isoflavones, which may act to reduce testosterone levels and inhibit 5 alpha-reductase - an enzyme involved in the metabolism of testosterone.
Another study, animal research published in Biology of Reproduction in 2004 claimed that the metabolite of the soy isoflavone daidzein stopped the effect of the male hormone dihydrotestosterone (DHT), which has been linked to prostate growth and male baldness.