In a study of :Soy's health benefits may not extend to reduced anxiety" Posted in American Psychological Association April 2005, Vol 36, No. 4, by comparing 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.
They concluded 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.