Soy Unlikely to Affect Women's Fertility

Eating a diet rich in soy (search) or taking soy supplements probably won't harm a woman's fertility, according to a new study.

Researchers found female monkeys who took soy supplements containing twice the level of the plant estrogens eaten by Asian women for a year did not experience any negative changes in their menstrual cycle or ovarian function that might affect fertility (search).

"Our results suggest that a high-soy diet probably won't compromise fertility in women," says researcher Jay Kaplan, PhD, of Wake Forest Baptist Medical Center, in a news release. The results of the study were presented today at the annual meeting of the American Society for Reproductive Medicine.

Researchers say the study was designed to test a theory that high-soy diets may compromise women's fertility.

A diet high in phytoestrogens (search) has been shown to contribute to a lower rate of breast cancer found in Asian women compared with those in Western countries. One explanation for that is that the plant estrogens found in soy, called isoflavones (search), are in part protective and can increase a woman's menstrual cycle length or reduce ovarian hormones.

The protective effects of phytoestrogens could reduce women's exposure to estrogen and may reduce the risk of breast cancer. But changes in the menstrual cycle may also impair women's fertility.

Soy Doesn't Affect Menstrual Cycle

In the study, researchers looked at the effect of soy supplementation on the menstrual cycles of a group of 96 premenopausal female monkeys over the course of a year. Researchers say monkeys have menstrual cycles similar to those found in women.

Half of the monkeys ate a diet high in soy protein that contained 1.88 milligrams of isoflavones (the human equivalent of about 129 milligrams per day), and the other half got all their protein from animal sources.

The study showed that soy supplementation did not appear to change the monkey's menstrual cycle or affect their hormone levels. In both monkeys and humans, lower-than-normal levels of estrogen can make menstrual cycles more variable and negatively affect fertility.

"Soy treatment did not change any characteristics of the menstrual cycle, including length, amount of bleeding, or hormone levels," says Kaplan. "This suggests that any protection that soy may provide against breast cancer does not come from changes in the menstrual cycle."

Although this study shows that soy supplementation does not affect women's fertility, researchers say more study is needed to look at the effects of soy on the fetus and placenta function.

By Jennifer Warner, reviewed by Brunilda Nazario, MD

SOURCES: Kaplan, J. "High Isoflavone Soy Protein Does not Alter Menstrual Cyclicity or Ovarian Function in Fully Mature, Premenopausal Monkeys," presented at the annual meeting of the American Society for Reproductive Medicine, Philadelphia, Oct. 16-20, 2004. News release, Wake Forest University Baptist Medical Center.