April 4, 2001 -- You've probably heard that adding soy protein to your diet can help lower your cholesterol. Well, tofu lovers, here's more good news: According to new animal research, isoflavones -- the active ingredient in soy products -- also may stave off Alzheimer's disease in postmenopausal women.
"This definitely says good things about eating soy," study author Helen Kim, PhD, tells WebMD. "Our data suggest that soy isoflavones in the diet may have protective actions against certain [brain changes] that are associated with Alzheimer's disease." Kim is a research associate professor in the department of pharmacology and toxicology at the University of Alabama at Birmingham.
In the three-year experiment, groups of female monkeys were fed a diet of only specially prepared food containing soy with isoflavones, soy with the isoflavones taken out, or the isoflavone-free soy plus the widely prescribed estrogen replacement drug Premarin. All 45 monkeys had had their ovaries removed -- an operation that mimics human menopause because it causes estrogen levels to drop significantly.
The researchers then examined the monkeys' brains, looking specifically for modifications to a protein called tau. Two notorious protein markers, commonly called "tangles and plaques," are found in the brains of all Alzheimer's victims. The amyloid "plaques" are sticky deposits of beta-amyloid protein, and the neurofibrillary "tangles" are made up of this modified tau protein.
The monkeys fed the soy isoflavone diet had significantly fewer changes to their tau protein than did monkeys fed the other two diets, Kim tells WebMD. What was really surprising, she says, is that Premarin did not offer a similar benefit.
"Estrogen replacement after menopause is protective against Alzheimer's," Kim says, "and we know from lots of laboratory experiments that estrogen is neuroprotective -- it protects [brain cells]. So there was good reason to believe that the brains of the monkeys on Premarin would be protected."
But Kim further explains that while the compounds found in soy have antioxidant properties, the same is not true of estrogen, and this may be the reason that estrogen replacement didn't seem to work.
"It's curious that they didn't find an effect of Premarin," says Pauline M. Maki, PhD, who reviewed the research for WebMD. "But Premarin has been associated with beta-amyloid ... rather than the tau protein," which is what Kim's team was looking at. Maki is an investigator in the Laboratory of Personality and Cognition at the National Institute on Aging in Baltimore.
That could explain it, says Hauxi Xu, PhD, "but this is very preliminary and definitely requires more careful study before you can conclude anything." Xu, an assistant professor at the Fisher Center for Alzheimer's Research at Rockefeller University in New York, was not involved in the study.
The mechanisms may be mysterious, but the end results are clear, Kim says. Eating soy isoflavones meant fewer tau changes in these monkeys. Now the real question is whether that reduction translates to less dementia in human beings.
"The link between soy consumption and improved [thought] function has not been made," Maki says. "That's not to say that it doesn't exist, it just hasn't been done yet."
Kim agrees. "We cannot make any simple statement about the relation between reduction in tau [changes] and future ... impairment" in thinking or cognition, Kim says. Experts do know, however, that rats that have had their ovaries removed "don't learn as well and don't remember as well. And giving them either pure estrogen or soy isoflavones prevents these deficits," she says.
While the jury may be out on the long-term benefits of soy on cognitive, or thinking, skills, the experts agree that you really can't go wrong adding soy to your diet. "Soy protein in and of itself is good for you," Kim says.
No longer a health food store exclusive, soy is now available at your local supermarket, and there's a formulation to please every taste. Options include crunchy soy nuts, flavored and unflavored soy milk to drink on its own or over cereal, soy-based meat substitutes, and even tofu-based frozen treats.
"You can do a lot by simply buying a block of tofu," Kim says. "You can substitute it for the ricotta in lasagna, for some of the cream cheese in cheesecake, and the really soft kind can be blended with fruit into delicious smoothies," she says. Dietary guidelines currently suggest that adults eat 20-25 g of soy protein daily.
Kim's preliminary findings appeared in the journal Biofactors.The current study is ongoing, with 36 of the monkeys still under investigation. "We haven't looked at [soy's effect on] beta-amyloid yet, but that's definitely something we're going to do," she says.