Soy likely doesn’t affect fertility, according to research in monkeys

New research shows that the plant estrogens in soy don’t impair fertility in monkeys. The study was designed to test a theory that high-soy diets can compromise fertility in women.

The results, from Wake Forest University Baptist Medical Center and Emory University School of Medicine, were reported today at the annual meeting of the American Society for Reproductive Medicine in Philadelphia, Pa. “Our results suggest that a high-soy diet probably won’t compromise fertility in women,” said Jay Kaplan, Ph.D., lead researcher, from Wake Forest Baptist. “But our results confirmed earlier findings that fertility may be affected by stress levels.”

Women in Asian countries where a lot of soy is consumed have dramatically lower rates of breast cancer than women in the United States. One explanation is that plant estrogens, called isoflavones, increase menstrual cycle length or reduce ovarian hormones – both which would reduce lifetime exposure to estrogen. However, these changes in the menstrual cycle could also impair fertility.

In a study of monkeys, which have menstrual cycles similar to those of women, Kaplan and colleagues tested the hypothesis that the estrogen in soy can affect menstrual cycles. “Our study was designed to determine whether a soy supplement containing twice the level of plant estrogen consumed by Asian women would alter any aspect of the menstrual cycle or ovarian function in monkeys,” he said.

For one year, half of the monkeys were fed a high-soy diet and half got their protein from animal sources. All monkeys were evaluated during this period for changes in ovarian hormones and menstrual cycles. “Soy treatment did not change any characteristics of the menstrual cycle, including length, amount of bleeding or hormone levels,” said Kaplan. “This suggests that any protection that soy may provide against breast cancer does not come from changes in the menstrual cycle.”

He said consumption of a high-soy diet probably would not compromise fertility, although further research is warranted to evaluate effects of soy on placenta function and on the fetus. The study did confirm earlier findings by Kaplan – that high levels of stress can affect ovarian function.

The monkeys in the study were housed in groups, where they naturally form a social hierarchy. In previous research, Kaplan found that the stress of being subordinate in the group impairs ovarian function, which means that lower levels of estrogen are produced. In both monkeys and people, reduced levels of estrogen can make menstrual cycles more variable and affect fertility.

In the current study, subordinate monkeys had reductions in ovarian hormones and changes in the menstrual cycle pattern that were observed in the earlier research.

Other researchers involved in the work were Thomas Clarkson, D.V.M., from Wake Forest Baptist, S. L. Berga, M.D., from Emory University School of Medicine, and M. E. Wilson, Ph.D., from the Yerkes National Primate Research Center.