Soy is unlikely to damage fertility in women, report the researchers, who investigated the theory that its positive impact on breast cancer risk may have other, unwanted effects.
Their study, carried out on monkeys, also offers insight into how the plant food impacts breast cancer risk.
Women in Asian countries where a lot of soy is consumed have dramatically lower rates of breast cancer than women in the United States and Europe. Isoflavones, a kind of plant oestrogen found in soy, are thought to be play a role in this lower incidence, either by increasing menstrual cycle length or reducing ovarian hormones - both of which would reduce lifetime exposure to oestrogen. However, these changes in the menstrual cycle could also impair fertility.
But a new study, presented at the annual meeting of the American Society for Reproductive Medicine in Philadelphia, fails to confirm these concerns.The team from the Wake Forest University Baptist Medical Center and Emory University School of Medicine tested the hypothesis on monkeys, which have menstrual cycles similar to those of women.
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.
"Our study was designed to determine whether a soy supplement containing twice the level of plant oestrogen consumed by Asian women would alter any aspect of the menstrual cycle or ovarian function in monkeys," said lead researcher Jay Kaplan.
"Soy treatment did not change any characteristics of the menstrual cycle, including length, amount of bleeding or hormone levels," he explained. "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 foetus.
The findings are positive for the booming soy products industry, which has seen increasing interest in the use of isoflavones in supplements toalleviate menopausal symptoms, maintain bone health and improve heart health.
In 2002, demand for soy-derived isoflavones in the US was estimated to be worth $45 million, according to the Freedonia group, which forecast a 22 per cent yearly increase to $125 million, or one million pounds, in 2007.