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SAN DIEGO — Soy may help protect against the onset of Alzheimer’s disease, especially in postmenopausal women, according to research presented Tuesday at the 221st national meeting of the American Chemical Society, the world’s largest scientific society.

A three-year animal study shows that plant-based estrogens found in soy, called phytoestrogens, appear to reduce the number of protein changes in the brain that are associated with Alzheimer’s disease. The irreversible brain disorder, which affects about 4 million people in the United States, is an increasing national health problem, according to the National Institute of Health. Nearly half of all people 85 and older have Alzheimer’s.

Estrogen is known for its ability to reduce a woman’s risk of heart disease and osteoporosis, and there is epidemiological evidence that suggests it can protect against Alzheimer’s disease, according to Helen Kim, of the University of Alabama.

Kim conducted the study to see if certain isoflavones — phytoestrogens — found in soy, “could act like estrogen and protect against biochemical events associated with Alzheimer’s disease.” The study was done with aged female monkeys that had their ovaries removed. They are considered to be animal models for human menopause, she noted.

During the three-year study, funded by the National Institutes of Health, 45 monkeys were fed one of three diets — soy with isoflavones, soy without isoflavones and Premarin, a commonly prescribed estrogen replacement for women. Fifteen monkeys were in each diet group. The scientists then examined the brain tissue of three monkeys in each group for some of the protein changes that are markers of Alzheimer’s disease.

This is the first time that soy isoflavones have been examined for estrogenic activity against Alzheimer’s in the postmenopausal brains of monkeys, according to Kim.

“Consistent with our expectation that the soy phytoestrogens would have ‘beneficial’ effects, we found fewer Alzheimer’s disease-linked tau protein changes in the brains of monkeys that received soy isoflavones,” said Kim. Improperly functioning tau proteins in the brains of people with Alzheimer’s disease can cause the skeletal structure of nerve cells to collapse. Kim says her findings are statistically significant.

She suspects that isoflavones and real estrogens, such as those in Premarin, may both be beneficial for the brain but work through different mechanisms.

A health claim approved for food labels by the U.S. Food and Drug Administration states that a daily diet containing 25 grams — two to three servings — of whole soy protein is enough to reduce the risk of heart disease.

“I would expect the same levels of soy intake to translate to protection in the brain,” Kim said. She cautions against consuming “lite” soy, however, because the protective isoflavones could be removed during processing.