Reporting in the current issue of the scientific journal Hormones and Behavior, the researchers found that in male monkeys, "long-term consumption of a diet rich in soy isoflavones can have marked influences on patterns of aggression and social behavior." Isoflavones are a naturally occurring plant estrogen in soy protein.
"Although considerable attention has been directed at the potentially beneficial effects of isoflavones in reducing the risk of various cancers, osteoporosis, cardiovascular disease and postmenopausal symptoms, less effort has been invested in characterizing neurobehavioral effects," according to the study.
People have the concept that soy is only beneficial, said Jay R. Kaplan, Ph.D., professor of comparative medicine and anthropology, one of the investigators. "There is the thought that what is good for some is good for all and more is better."
But this research points out that not only does the dose make a difference, but so does the sex of the consumer, Kaplan said, adding that the study is consistent with emerging literature showing that soy can have a negative impact on the behavior of male rodents. Previous studies have shown no difference in aggression in females given large doses of soy, Kaplan said.
The study was done over 15 months with adult male monkeys who were divided into three groups and fed different amounts and types of protein. One group had about 125 mg of isoflavones a day. The second group had half that amount, and the third group's protein came from milk and animal sources.
"In the monkeys fed the higher amounts of isoflavones, frequencies of intense aggressive and submissive behavior were elevated," according to the study. "In addition, the propo
Contact: Karen Richardson
Wake Forest University Baptist Medical Center