Jay R. Kaplan of Wake Forest University School of Medicine and his colleagues studied 96 female monkeys approximately equivalent in age to 30-year-old women. The animals had not eaten soy before the experiment began. For six months, they all ate the same animal-protein-based diet. For the next year, half the monkeys remained on that diet while the other half consumed only soy protein, about twice as much as a typical Asian woman eats. Soy treatment did not change any characteristics of the menstrual cycle, including length, amount of bleeding or hormone levels, Kaplan reports. This suggests that any protection that soy may provide against breast cancer does not come from changes in the menstrual cycle.
Although the researchers determined that eating large amounts of soy doesn't affect fertility, their results indicate that stress levels do have a negative impact. During the study period, the monkeys lived in groups of five or six and formed natural social hierarchies. Animals in subordinate positions were under more stress than those at the top and experienced detrimental changes to their ovarian hormone levels and menstrual cycle patterns. "Our results suggest that a high-soy diet probably won't compromise fertility in women," Kaplan says, "but our results confirmed earlier findings that fertility may be affected by stress levels."