Chemical In Soy Alters Reproductive Organs In Male Rats

While these findings do not indicate that genistein has a similar effect in humans, researchers say the increasing popularity of soy and soy-based foods, such as tofu and some infant formulas, may warrant further research to determine if genistein exposure in the womb and during breast-feeding influences human reproductive development.

In the study, described in the April issue of the Journal of Urology, pregnant female rats were randomly assigned to one of three regimens: a genistein-free diet, a diet supplemented with a low dose of genistein, and a diet with a high dose of genistein.

Male offspring were exposed to genistein indirectly through maternal consumption during pregnancy and lactation.

When the genistein-exposed offspring matured, researchers found the males had smaller testes and a larger prostate gland compared to unexposed rats. Although their sperm counts were normal, exposed adult males had lower testosterone levels and were also less likely to ejaculate when presented with the opportunity to mate with a female.

"The effects of genistein continued long after the rats were exposed, leading us to believe that exposure to this plant-derived estrogen during reproductive development can have long-term detrimental effects in males," said the study's lead author, Amy B. Wisniewski, Ph.D., a researcher at the Johns Hopkins Children's Center.

"Genistein may act as an estrogen or an anti-androgen, blocking the function of endogenous androgens – the sex hormones necessary for males to develop a normal reproductive system – and ultimately leading to the reproductive abnormalities and sexual dysfunction we saw in the exposed rats," added study co-author Sabra L. Klein, Ph.D., of the School of Public Health. "However, additional research is needed to determine if this is the case."

Whether the long-term effects of genistein on the reproductive development of male rats are caused by exposure during gestation, lactation, or both also requires further investigation, Wisniewski said.

The study was funded by The Brady Urological Institute of The Johns Hopkins Hospital. In addition to Wisniewski and Klein, other co-authors include John P. Gearhart, M.D., and Yegappan Lakshmanan, M.D., of the Johns Hopkins Children's Center and The Brady Urological Institute.