No significant difference in mental abilities was found between postmenopausal women who took soy supplements and those who did not, according to researchers at the Stanford University School of Medicine and the USC Keck School of Medicine.
The idea of a possible link between soy consumption and cognition has lingered for years, but this research offers no evidence to support any such link.
“There were no large effects on overall cognition one way or another,” said the study’s lead author, Victor Henderson, MD, professor of health research and policy and of neurology and neurological sciences at Stanford.
The 2.5 year-long study, focused on women who were middle-aged and older, was larger and longer than any previous trial on soy consumption.
The results are similar to the largest previous study on this topic — a 12-month trial of Dutch women during which daily soy intake showed “no significant effect on cognitive endpoints.”
Still, there are several randomized clinical trials on soy’s effect on cognition and memory in women that have presented conflicting evidence regarding its benefits and harms.
While improved cognition was seen in some findings, other studies showed evidence that soy could have a negative effect on memory.
Soy has an estrogen-like compound called isoflavones, and some women take soy supplements as an alternative to estrogen.
It has been suggested that isoflavones may possibly boost memory and overall brain function since isoflavones are known to activate estrogen beta receptors in the hippocampus, the part of the brain that is responsible for memory.
Henderson’s desire to figure this out is part of his broader research agenda on finding new ways to enhance cognitive function with age.
For the study, researchers conducted the National Institutes of Health-sponsored Women’s Isoflavone Soy Health Trial, which was done between 2004 and 2008 to determine whether soy isoflavones had an effect on the progression of atherosclerosis and, secondarily, the effect on cognition.
During this research, 350 healthy women ages 45 to 92 were randomly assigned to daily receive either 25 grams of isoflavone-rich soy protein (a dose comparable to that of traditional Asian diets) or a placebo. The participants completed a battery of neuropsychological tests at the beginning of the study and again 2.5 years later.
Researchers looked at the composite of 14 scores and found no significant differences in overall mental abilities during the entire study period between women who took the supplements and those who took placebo.
During a planned secondary analysis, researchers did identify a statistically significant difference in one particular cognitive factor—women in the supplement group showed a greater improvement in visual memory (memory for faces). Henderson said this could be important, but “the finding needs to be replicated in future studies.”
According to Henderson, these results “help provide a firm answer” about soy and overall cognition, and he and his co-authors note in the paper that postmenopausal women shouldn’t pursue a high-soy diet or take supplements for the primary goal of greater mental capabilities.
However, Henderson said the work is not meant to discourage women who eat soy for other reasons.
“I don’t think they should be disappointed at all,” he said. “They should be pleased that there aren’t negative effects on overall cognitive function and that there are potential gains in aspects of memory. If a woman enjoys eating soy and if there may be other health benefits, she should keep doing what she’s doing.”
The researchers add that while these results are reasonably definitive—Henderson said the sample size was large enough that if there were major effects, the researchers would likely have seen them—the cognitive effects of soy isoflavones might differ for women of reproductive age and for men. He believes more research is needed in these populations.
He also emphasized the need for researchers to continue studying ways to improve cognition among older adults, including nutritional approaches, physical and mental activities, and pharmaceutical approaches.
The research is published in Neurology.
Source: Stanford School of Medicine