A US team reports that long-term consumption of an isoflavone-rich diet made male monkeys prone to aggression and impacted their social behaviour. They say that it could have an adverse effect on the behaviour of men.
Soy is the most widely used botanical by pre- and postmenopausal women but its use by men is also growing as research continues to show the benefits on heart health. In the US soy sales have grown from $940 million in 1990 to a projected $4 billion this year.
And there is recent evidence to suggest that isoflavones, a naturally occurring plant oestrogen in soy protein, may also help fight prostate cancer.
But reporting in April's issue of Hormones and Behavior (volume 45, issue 4, pp278-284), researchers from Wake Forest University Baptist Medical Center warn that: "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 characterising neurobehavioral effects."
Investigator Dr Jay Kaplan added that people tend to believe that "what is good for some is good for all and more is better."
However the research demonstrates that, not only does the dose make a difference, but so does the sex of the person consuming isoflavones.
Kaplan added that the study is consistent with emerging literature showing that soy can have a negative impact on the behaviour of male rodents. Previous studies have shown no difference in aggression in females given large doses of soy, however.
For the 15-month study, researchers divided adult male monkeys into three groups and fed them 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 behaviour were elevated," said the researchers. "In addition, the proportion of time spent by these monkeys in physical contact with other monkeys was reduced by 68 per cent, time spent in proximity to other monkeys was reduced 50 per cent and time spent alone was increased 30 per cent."
Isoflavone levels of 125mg 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 US.
"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 said. "The present findings suggest that careful attention will be required to balance beneficial and potentially adverse effects."