Male rats born to mothers whose diet during pregnancy included genistein, a chemical found in soybeans, never ejaculated while trying to mate with females, the Johns Hopkins researchers found.
The male rats whose mothers ate genistein during pregnancy also had lower testosterone levels and a larger prostate as adults and, at puberty, smaller testes, compared with rats whose mothers had none of the chemical in their diets, the study says.
Hopkins researchers stressed the findings do not indicate genistein has a similar effect in humans, but say they plan further research they hope will answer that question.
"We found that these males were de-masculinized," says researcher Sabra L. Klein. "This raises questions as to the effects that [genistein] can have on the developing fetus."
The study, reported in the April issue of the Journal of Urology, divided male baby rats into three groups with a dozen rats in each.
The mothers of one group consumed no genistein. Another group's mothers consumed a low dose, which the researchers said would be comparable to the human equivalent of a Western diet with moderate soy consumption. The third group's mothers received a high dose, approximating the human equivalent of high-soy diets.
Mothers who consumed genistein continued to do so during lactation, but Klein says it's uncertain whether male babies could be harmed by soy in breast milk or infant formula.
Klein -- a researcher at the Hopkins School of Public Health, which teamed with the Johns Hopkins Children's Center on the study -- adds that genistein appeared to have the greatest effect on the developing fetus.
"We speculate that the exposure in utero is probably where there is the greatest effect because during this is a critical time for sexual development of the fetus," Klein says.
Genistein had no effect on sperm counts, and researchers say it's unclear precisely why the chemical caused abnormal reproductive organs and sexual dysfunction.
Klein says the effects could result from low levels of testosterone, one of the androgens, which are necessary for the normal sexual development of males.
Researchers remain unsure whether genistein acts as an estrogen, a group of hormones responsible for female sexual development, or an anti-androgen, blocking the sex hormones necessary for a male to develop a normal reproductive system.
The effects of genistein continued long after the rats had been exposed, suggesting exposure to the chemical during development of a human male's reproductive system could cause long-term damage, the study concluded.
As soy-based foods and vegetarian diets grow more popular, some experts suggest pregnant women to talk to their doctors about their diets and potential dangers of genistein.
For her part, Klein says, "What we hope to do is increase awareness because I think there's often an assumption that if something is natural, it's good for you, and you don't need to understand anything about safe doses."
Retha Newbold, a developmental biologist in the Environmental Toxicology Program at the National Institute for Environmental Health Sciences, says the Hopkins research marks the latest animal study to link genistein consumed by mothers to male reproductive abnormalities.
For adults other than pregnant women, soy may be safe, Newbold says. "But," she adds, "for a fetus or neonate or young infant, we just don't know that, and I would just prefer to err on the side of being safe. We just don't have enough information."
Nancy Chapman, executive director of the Soyfoods Association of North America, says no further study is necessary to prove the safety of soy.
"The trial is being done and has been done for generations in Japan and China and other cultures that have consumed soy," Chapman says.
And there's no evidence linking such high-soy diets to reproductive system damage or sexual dysfunction, she says.
Chapman also says basing conclusions on consumption of genistein, not soy itself, is invalid because other substances in soy offset potential negative effects on males.
Klein, however, responds that genistein likely would have the same effects whether fed in soy or separately.
And Newbold says researchers use genistein instead of soy itself in studies partly because the amount of the chemical in soy varies widely, depending on factors including growing conditions.