In Brief

While health experts tout the cardiovascular benefits of a soy-rich diet, recent research suggests the effects may be limited when it comes to mental health.

In a study of male and female rats, researchers at the Yerkes National Primate Research Center in Atlanta found that soy supplements decreased anxious behavior in female rats about to go into heat but had no effect on female rats who had just finished cycling. What's more, the soy supplements appeared to increase anxious behavior in male rats. The findings suggest that researchers need to more closely examine the claims of some marketers that soy supplements can improve mood and sexual function, say the study authors, who were lead by neuroscientist Heather B. Patisaul, PhD, now a researcher at CIIT Centers for Health Research in North Carolina's Research Triangle.

"There are a lot of clinical trials under way looking at soy's effects on osteoporosis and cardiovascular functioning, but there aren't any that look at mood or depression or sexual desire in women," says Patisaul. "[This study] drives us to ask the question: Is soy effecting anxiety and depression?"

In the study, the researchers compared the behavior of male and female rats on two different soy-rich diets with control groups on a normal diet. Both soy-rich diets contained soy isoflavones--compounds commonly found in over-the-counter soy supplements that are molecularly and structurally similar to estrogen. But the second soy-rich diet also included the carbohydrates, proteins and other nutrients found in whole soybeans and soy milk.

The researchers placed each rat in the center of an elevated plus-shaped maze. Two of the maze arms were walled and two were open with a drop of more than 1.5 feet to the floor. The researchers then recorded how often and for how long the rats entered the open arms--a bold act that indicates low anxiety--compared with the closed arms. Males on the soy-rich diets spent less time in the open arms and entered the arms less often than the control males--at levels indicating they were significantly more anxious.

Meanwhile, about-to-heat, or proestrous, females on both diets were the least anxious: They entered and spent more time in the open arms than either the finished-cycling or control females. That finding makes sense, says Patisaul, considering that most animals have the highest levels of estrogen during proestrus, and previous research has found that estrogen lowers anxiety.

However, the researchers found that the soy-rich diets didn't increase estrogen levels in the proestrus rats. The researchers conjecture that instead the isoflavones enhanced the anxiety-reducing effects of estrogen already in the proestrus rats--a benefit that didn't extend to other females in the study.

The study appears in this month's Behavioral Neuroscience (Vol. 119, No. 2).