Soy protein isolate increases urinary estrogens and the ratio of 2:16α‐hydroxyestrone in men at high risk of prostate cancer

Abstract

In epidemiological studies, significantly lower excretion of urinary estradiol (E2) and lower ratio of urinary 2-hydroxyestrone to 16α-hydroxyestrone (2:16 OH-E1) has been reported in prostate cancer cases compared to controls. Although soy supplementation has been shown to increase the ratio 2:16 OH-E1 in women, no studies have investigated the effects of soy supplementation on estrogen metabolism in men. The objective of this randomized controlled trial was to determine the effects of soy protein isolate consumption on estrogen metabolism in men at high risk of prostate cancer. Fifty-seven men supplemented their habitual diets with 40 g protein in soy protein isolate (SPI+) (containing 107 mg isoflavones/d), alcohol-extracted soy protein isolate (SPI-) (containing < 6 mg isoflavones/d), or milk protein isolate (MPI). The urinary estrogen metabolite profile was measured by gas chromatography-mass spectrometry. Both soy groups had higher E2 excretion than MPI at 3 and 6 months. After 6 months of supplementation, the SPI+ group had a significantly higher urinary 2:16 OH-E1 ratio than the MPI group. These results were not influenced by equol-excretor status, although at baseline, equol-excretors had higher 2:16 OH-E1 concentrations than non-excretors. Increased urinary E2 excretion and 2:16 OH-E1 ratio in men consuming soy protein isolate are consistent with studies in women and suggest that soy consumption may be beneficial in men at high risk of progressing to advanced prostate cancer.