Soy supplements have little benefit for brain function of healthy postmenopausal women, study shows

Soy supplements have little or no benefit for global cognition of healthy postmenopausal women, shows a new study conducted by the researchers of Stanford University School of Medicine and of the USC Keck School of Medicine.

Overall mental abilities, learning, thinking or memory skills do not appear to be ameliorated by taking daily isoflavone-rich soy protein, according to the research published in the June issue of Neurology, the medical journal of the American Academy of Neurology.

Trying to find new strategies to improve cognitive function in aging, the scientists conducted a randomized, controlled, double-blind trial. National Institutes of Health-sponsored Women’s Isoflavone Soy Health Trial was done between 2004 and 2008 to determine the effect of soy isoflavones on the progression of atherosclerosis (hardening of the arteries) and, secondarily, the effect on cognition.

The senior investigators in the Women’s Isoflavone Soy Health Trial include Wendy Mack, PhD, professor of preventive medicine, and Howard Hodis, MD, professor of medicine, with the USC Keck School of Medicine. Other researchers from USC and the University of Hawaii Cancer Center also contributed to the study.

The work was funded by the National Center of Complementary and Alternative Medicine, the Office of Dietary Supplements and the Office of Research on Women’s Health. Solae LLC, based in St. Louis, provided study products without charge.

A total of 350 healthy postmenopausal women, aged 45 to 92 years, were enrolled and 313 of them were included in intention-to-treat analyses. Neuropsychological tests were given to the participants at the start and at the end of the study.

They received either daily 25 g of isoflavone-rich soy protein, comparable to that of traditional Asian diets, or a milk protein-matched placebo. This study was larger and longer than any previous trials on soy use.

After two and a half years, there was no statistically significant between-group difference in global cognition change from baseline (0.42 for soy supplements, and 0.31 for placebo-treated groups).

“There were no large effects on overall cognition one way or another,” said Dr. Victor Henderson, lead author, professor of health research and policy and of neurology and neurological sciences at Stanford University in California.

However, women in the isoflavone-treated group had a greater improvement in visual memory (memory for faces).

This observation could be important, but “the finding needs to be replicated in future studies,” Dr. Victor Henderson said.

“For healthy postmenopausal women, long-term dietary soy isoflavone supplementation in a dose comparable to that of traditional Asian diets has no effect on global cognition but may improve visual memory,” the authors wrote.

In the same time, the study should not discourage women who take soy supplements for other purposes.

“I don’t think they should be disappointed at all. 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,” Dr. Victor Henderson said.

More study is needed for women of reproductive age and for men because the cognitive effects of soy isoflavones might differ, he said.

Soy supplements contain isoflavones, an estrogen-like compound.

Isoflavones might be able to improve overall brain function, it has been thought. The hippocampus, the part of the brain that controls memory, is rich in estrogen beta receptors, and isoflavones can activate these receptors. Some women choose to take soy supplements as an alternative to estrogen.

A previous study, published in a 2004 issue of the Journal of the American Medical Association, a trial which lasted twelve months and involved Dutch women, showed “no significant effect on cognitive endpoints” from daily intake of soy protein.

Also, there were randomized clinical trials on soy’s effect on mental abilities of women that have shown conflicting results regarding its benefits and risks. Improved cognition was seen in some findings, but other research suggested that soy could harm memory.

In the same time there were some worries that soy may increase the risk for breast cancer survivors (due to estrogen-like effects), but new research has partially eased those concerns. A study of women with history of breast cancer suggested those who took high quantities of soy supplements were 25 percent less likely to have a cancer recurrence.

Studies on this subject have generally shown conflicting findings on the role of soy in alleviating hot flashes and other symptoms of menopause, as well in lowering blood pressure or in preventing osteoporosis (bone loss), or in slowing the progression of atherosclerosis, by lowering high cholesterol.

However, some people may benefit from taking soy supplements, because of soy compounds (high protein, fiber and low fat), scientists said.