Taking daily soy supplements doesn't improve thinking and memory skills or keep them from declining in older women, new findings suggest.
The study of 313 postmenopausal women showed those who took 25 extra grams of soy protein each day improved slightly more than other women on a test of visual memory, or remembering faces. But there was no difference in their overall cognitive skills, compared to women who were given regular milk protein.
Studies have typically come to conflicting or disappointing findings on the role of soy in easing hot flashes and other symptoms of menopause, as well as treating high cholesterol and build-up in the arteries.
And there's been some worry that soy may be risky for breast cancer survivors, though new research has partially eased those concerns.
Regardless of any other effects, the nutritional benefits of soy -- including high protein and fiber and low fat -- may make it a good diet addition for some people, researchers said.
"There might be a lot of reasons why healthy middle-aged older women might want to take soy supplements, but cognitive function shouldn't be the driving reason for that," Dr. Victor Henderson, the study's lead author from Stanford University, told Reuters Health.
Researchers have thought certain natural plant-derived chemicals in soy might affect changes that happen during menopause, including disturbances in memory, because of their similarity to estrogen produced by the body.
For the new study, Henderson and his colleagues gave women age 45 to 92 a battery of thinking and memory tests, then randomly assigned them to take 25 grams of soy or milk protein supplements, offered in bar and powder form, each day for two and a half years.
Over the study period, women in both groups tended to improve in their overall thinking and memory skills -- probably because they'd all taken the tests once before.
However, there was no difference in their improvement on the combination of cognitive tests -- or on any individual exam other than visual memory -- based on what type of protein supplement the women were taking, the researchers reported Monday in the journal Neurology.
That was also the case when Henderson's team looked specifically at women younger than 60, who had more recently gone through menopause and may have had more to gain from the extra soy.
It's possible soy might still be beneficial for cognition and health in general, but that starting the supplements after menopause is too late to reap any benefits, according to William Wong, who has studied soy in older women at Baylor College of Medicine in Houston, Texas.
"We are all looking for a magic bullet," he told Reuters Health. "But we don't ever think about, well, women in Asian countries have been taking that product (soy) since birth, essentially. There could be some long-term effect that we don't see in the U.S."
Wong, who wasn't involved in the new study, said one limitation of the research is the relatively small group of women involved and the large variation in age and time since menopause -- which might make it harder to determine a specific group of women that could benefit from soy.
Still, he said his own findings on the effect of soy supplements on bone health and blood pressure have been "discouraging."
One recent study of women with a history of breast cancer, however, did suggest those who ate a lot of soy were 25 percent less likely to have their cancer come back -- despite some concern about the estrogen-like effects of soy in breast cancer survivors.
Henderson's team concluded that postmenopausal women should not take soy with the goal of improving their cognitive skills, but that they can eat a diet high in soy if they choose, without worrying that it may have any negative effects on memory.