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"There are no substantial cognitive effects, positive or negative, from soy protein consumption in women past menopause," said researcher Dr. Victor Henderson, professor of health research and policy and neurology and neurological sciences at Stanford University.
In the study, published June 5 in Neurology, Henderson and his team evaluated 350 postmenopausal women, aged 45 to 92. The researchers randomly assigned the women to take 25 grams of soy protein a day or a milk protein placebo. The soy and placebo were given in powder or bar form.
The study, funded by the U.S. National Institutes of Health, is called the Women's Isoflavone Soy Health Trial. Isoflavones in soy are estrogen-like compounds. Some women choose them as an alternative to hormone therapy to relieve hot flashes and other symptoms of menopause.
Research has produced mixed results about whether soy helps protect so-called "cognitive" health, such as memory and other thinking skills. Some research has even found that soy has a negative effect on thinking skills.
At the study start and then 2.5 years later, the researchers gave the women a battery of tests to evaluate thinking skills. Among the skills tested were verbal memory, visual memory (such as remembering faces), putting letters and numbers in sequence, and other tests. The researchers analyzed results on 313 of the women after some dropped out.
The investigators found no differences in overall mental abilities in the two groups. When they did a secondary analysis, they did find women on soy had a greater improvement in visual memory.
"I think the study is large enough that if there were meaningful overall cognitive effects to be found, we would have found them," Henderson said.
Previous studies have found an effect on a skill known as executive function, which has to do with such tasks as decision making and planning, Henderson noted. The new study did not find that soy improved this skill.
The good news from this study, Henderson said, is that soy had no harmful effect on thinking skills.
Henderson isn't discouraging women from taking soy. He does suggest that if they do take it, not to expect an improvement in memory and other thinking skills from it.
The results are important for women to know, said Pauline Maki, a professor of psychiatry and psychology and director of women's mental health research at the University of Illinois at Chicago. Maki reviewed the new study findings.
"Studies of women who represent the U.S. population show that 17 percent of midlife women use some sort of soy supplements," she said.
"Together with other findings from other clinical trials, we can conclude that soy has little impact on menopausal symptoms and memory," she added.
Some effects of soy are still under study, Maki pointed out. "It remains to be seen whether women show benefits to mood and other aspects of well being with soy," she said. Some clinical trials have shown improvement in mood with soy, she noted. Maki has conducted studies funded by the NIH to compare hormone therapy with alternative botanical therapies.
Other ongoing studies are looking at how social, mental and physical activities might prevent the decline of thinking skills with age.
One recent study, published in the March issue of the Journal of the American Geriatrics Society, found that older adults who participated in a walking program once a week for 90 minutes for three months had improvement in some thinking skills such as word fluency.
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