Isoflavone-rich soy protein isolate suppresses androgen receptor expression without altering estrogen receptor-beta expression or serum hormonal profiles in men at high risk of prostate cancer

Author: Hamilton-Reeves JM, Rebello SA, Thomas W, Slaton JW, Kurzer MS
Publication: J Nutr. 2007 Jul;137(7):1769-75

Steroid hormones such as androgens have been associated with prostate cancer risk. Consumption of soy products have been linked with lower prostate cancer risk and may act through altering hormones or hormone receptor signalling. American scientists at the University of Minnesota examined the potential protective effect of soy protein isolate, with low and high levels of isoflavones, on prostate cancer risk in men at high risk for developing advanced prostate cancer.

Fifty-eight men were divided in three groups depending on type of protein and isoflavones intake: 40 g/d soy protein isolate with 107 mg/d isoflavones, alcohol-washed soy protein isolate containing almost no isoflavones (group 2) and milk protein isolate with no isoflavones. The men received these proteins twice daily for 6 months. At baseline, after 3 months and 6 months serum samples were collected and analyzed for circulating hormones. Prostate biopsy samples were taken at baseline and after 6 months and were analyzed for androgen receptor and estrogen receptor-beta expression.

The researchers found that consumption of soy protein isolate and isoflavones for six months significant lowered expression of androgen receptor, but did not influence on estrogen receptor-beta expression or circulating hormones. Consumption of low-isoflavones soy protein isolate increased serum estradiol and the effect on the androgen receptor was intermediate to that of high isoflavones protein isolate and milk protein isolate, suggesting that very low levels of isoflavones, or other components in soy protein isolate, could have an effect on androgen receptor. Only men who consumed no soy protein isolate or isoflavones, but dairy milk protein isolate, showed a significant increase in androgen receptor from baseline.

The study concluded that soy protein isolate consumption suppressed androgen receptor expression in the prostate and could be beneficial in preventing prostate cancer.