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 proportion of time spent by these monkeys in physical contact with other monkeys was reduced by 68 percent, time spent in proximity to other monkeys was reduced 50 percent and time spent alone was increased 30 percent."
Isoflavone levels of 125 mg per day are higher than amounts consumed by many Asians, who typically eat more soy than other populations. But, the isoflavone levels are comparable to levels found in many dietary supplements sold in the United States.
The FDA approved a statement in 1999 that said, "25 grams of soy protein a day, as part of a diet low in saturated fat and cholesterol, may reduce the risk of heart disease." Soy sales have grown from $940 million in 1990 to a projected $4 billion this year. Kaplan said that soy is the most widely used botanical by pre- and postmenopausal women.
"To the best of our knowledge, the present study may be the first to demonstrate that long-term consumption of isoflavones can alter patterns of agonistic and social behavior in primates," the researchers reported. "The present findings suggest that careful attention will be required to balance beneficial and potentially adverse effects."
Other researchers in the study included Michael R. Adams, D.V.M., professor of pathology, and Thomas C. Register, Ph.D., associate professor of pathology, at Wake Forest Baptist, as well as two researchers from Lehigh University's Department of Biological Sciences.
The research was funded by NIH grants and a grant from the HF Guggenheim Foundation.
About Wake Forest University Baptist Medical Center: Wake Forest Baptist is an academic health system comprised of North Carolina Baptist Hospital and Wake Forest University School of Medicine. It is licensed to operate 1,282 acute care, psychiatric, rehabilitation and long-term care beds and is consistently ranked as one of "America's Best Hospitals" by U.S. News & World Report.