Soy and soy foods are rich in protein, fiber, phytochemicals and other nutrients. Soy and soy foods are popular in the United States, however there are some myths that effect consumers' opinion of soy. I asked Dr. Mark Messina, adjunct associate professor of nutrition at Loma Linda University and an expert on soy and nutrition, to help me dispel the myths surrounding soy.
Myth: Soy Estrogens Cause Dementia
If you search the Internet for soy and dementia, you'll likely find a bunch of articles that claim soy causes dementia and even makes your brain shrink. This myth about soy causing dementia began with a study published in 2000. Dr. Messina explains, "Concerns about soy causing dementia are based primarily on an epidemiologic study published in 2000 called the Honolulu-Asia Aging Study (HAAS), a prospective cohort study begun in 1965 which was an offshoot of the Honolulu Heart Program. The primary goal of the heart study was to determine if Westernization of the diet of Japanese men living in Hawaii increased heart disease risk. Near the end of this 30 year study, a cognitive function element to the design was added. That part of the study found that among the 3,734 men aged 71 to 93 years, higher tofu intake was associated with poor cognitive test performance. Furthermore, a similar association of midlife tofu intake with poor late life cognitive test scores was also observed among wives of cohort members, using the husband's intake as proxy for the wife's consumption. These findings were unexpected because at that time estrogen therapy (and therefore perhaps also isoflavones) was thought to exert cognitive benefits."
The problem is nutrition and diet studies can be tricky to interpret. It's difficult to track all the foods people eat every day, so results aren't always accurate. Most of us don't remember what we ate an hour ago much less a day or a week ago. And often the study designs don't take into consideration other foods and factors that can skew the results. Dr. Messina continues, "In this study, tofu consumption 2 to 4 times per week was classified as high intake. However, the HAAS had several design weaknesses. For example, the intake of only 26 foods was assessed. Nowadays, it is common for epidemiologic studies of this type to assess the intake of at least 100 foods. By assessing the intake of so few foods it is very difficult to control for potentially confounding variables. Perhaps tofu intake was associated with cognition because of dietary habits common to tofu consumers and not because of tofu consumption per se. Also, the intake questions pertaining to tofu differed from one time point to the next so the authors had to devise a somewhat convoluted method for classifying men based on different intake responses."
There were just too many design flaws to take the HAAS study as gospel. So what about other research that examines the possible connections between soyfoods and dementia? Dr. Messina describes another study that indicated tofu to be the cause of cognitive problems, but the real culprit was probably formaldehyde. "One other recently published epidemiologic study raised questions about soy and cognition. This was a small Indonesian study carried out in 2 rural sites (Borobudur and Sumedang) and 1 urban site (Jakarta) among mainly Javanese and Sundanese elderly (n = 719, 52 to 98 years of age). High tofu consumption (at least 9 times/week) was associated with worse memory. The authors speculated that isoflavones might be responsible for the association. However, high tempeh (a fermented whole soybean product) consumption was independently related to better memory particularly in participants over 68 years of age. Tempeh is also very high in isoflavones. The paradoxical findings (two soyfoods high in isoflavones having opposite effects) are likely due to the fact that according to the Departments of Public Health at the Universities of Jakarta and Yogyakarta, formaldehyde is added to tofu (but not tempeh) to preserve its freshness. Formaldehyde causes memory problems in animals. Of course, formaldehyde is not used as a food preservative in the United States."
Another study showed no affect of soy on brain function at all. "In contrast to the Hawaiian and Indonesian studies, a cross-sectional study, which included 3999 men and women aged 65 years and older from Hong Kong, found that isoflavone intake was unrelated to cognitive function as assessed by the cognitive part of the Community Screening Instrument for Dementia."
So no published studies successfully back up the claim that soy shrinks your brain or causes dementia. Rather than being harmful, could there possibly be cognitive benefits from eating soy? Maybe for some women. Dr. Messina continues, "Finally, and even more importantly, the 10 clinical trials that have examined the impact of isoflavone-rich products on cognitive function actually suggest that at least in younger postmenopausal women, isoflavones favorably affect several aspects of cognitive function."
"At this point, it is not possible to draw conclusions about the impact of soy or isoflavones on cognitive function although generally speaking, intervention studies - which are suggestive of benefit - carry more scientific weight than epidemiologic studies, which show mixed results."
So what does this mean? If you like soy and soyfoods, go ahead and keep them in your diet. Soy and soyfoods are good for you - don't fall for the fear mongers' cries that soy is dangerous.
Mark Messina, Ph.D. is a Health and Wellness Expert for The United Soybean Board.
Source
Email interview with Mark Messina, PhD. April 2009. The Soy Connection.
