Dietary soybean may be antiestrogenic in male mice.

We tested whether dietary soybeans alter prostatic growth and development of prostatic dysplasia in mice that were treated with a synthetic nonsteroidal estrogen, diethylstilbestrol during the first 3 d after birth. Soybeans were chosen because they contain substantial amounts of isoflavonic estrogens. The presence of estrogenic isoflavonoids in soybean-containing feed was confirmed by measuring the excretion of seven different plant estrogens in the urine of normal adult male mice. Estrogenicity of dietary soybean was confirmed by the growth response in uteruses of immature mice. In addition to their estrogenic effect, antiestrogenic properties of soybeans on uterine growth were observed in the presence of a more potent estrogenic growth stimulator, diethylstilbestrol in feed. In neonatally estrogenized male mice, soybean feeding reduced the prostatic growth inhibition due to diethylstilbestrol and, in preliminary experiments, delayed the development of dysplastic changes in the prostate. The number of animals showing severe dysplasia in prostatic epithelium was significantly lower in 9-mo-old animals given soybean-containing feed from fertilization onwards, but in 12 mo-old animals the difference was less obvious and was not significant. Our findings suggest an antiestrogenic action for dietary soybean in male mice, which may be important for the hormonal regulation of normal as well as neoplastic prostatic growth.