As the U.S. population ages, increasing numbers of Americans are concerned about making sure their later years are healthy ones. Health involves numerous components but one that has received more attention in recent years is cognitive function. Everyone wants to know what they can do to maintain cognitive abilities and to prevent the onset of dementia. Recent studies suggest that soyfood intake could help in this regard, which is a somewhat astounding development when considering the results of early work in this area.
Back in the year 2000, results of the Honolulu-Asia Aging Study indicated that higher midlife tofu consumption was independently associated with indicators of cognitive impairment and brain atrophy in late life.1 The study involved men, but a post-hoc analysis showed that the results for men also applied to the spouses of the men in this study. Male tofu intake was used as a surrogate for their wife’s intake. Not surprisingly, this study generated a huge amount of press and led to headlines such as “Tofu shrinks brains.” The operating hypothesis at the time was that the isoflavones in soy were exerting an anti-estrogenic effect in the brain. In 2000, there was growing speculation that estrogen might reduce risk of developing dementia and Alzheimer’s disease.
There were a number of very important limitations to the Honolulu-Asia Aging Study. For one, the primary outcome was coronary heart disease, not cognitive function. The latter outcome was added long after the study began. In addition, the food frequency questionnaire (FFQ) included questions about only 26 foods, which pales in comparisons to more 100 questions modern FFQs typically include. Also, questions about tofu intake varied over the course of the study so a new tofu intake category had to be created. Despite these limitations, this study significantly challenged the image of soy as a healthful food.
Over the past 20 years, quite a bit of research into the cognitive effects of soyfoods has been conducted but with very mixed results. However, the most recent epidemiologic study to examine this relationship, which was published in 2018, showed surprisingly beneficial effects of soy. The participants in this study comprised 403 men and 373 women. Multivariate-adjusted odds ratios (95% confidence intervals) for cognitive impairment with a 1 standard deviation increase in total bean, total soybean and total soy isoflavone intakes were 0.48 (0.28, 0.81; p=0.006), 0.51 (0.32, 0.83; p=0.007), and 0.55 (0.32, 0.93; p=0.026), respectively, in women. In contrast, no benefits were reported for men.
A small Korean case-control study also published this year that included both men and women, found that a dietary pattern characterized by high intake of seafood, vegetables, fruits, bread, snacks, soy products, beans, chicken, pork, ham, egg, and milk, had a marked decreased risk of mild cognitive impairment compared to those who did not. Obviously, the benefits associated with this dietary pattern can’t be specifically ascribed to soy.2 More importantly, overall, it would be difficult to conclude that the epidemiologic data support the hypothesis that soy intake is associated with cognitive benefits.3
What about the clinical data? A 2015 meta-analysis of 10 placebo-controlled randomized controlled trials of soy isoflavone supplementation that involved 1,024 postmenopausal women who were studied for 6 weeks to 30 months, found isoflavones favorably affected cognitive function and visual memory.4 A subsequently published analysis found that supplementation with either soy isoflavone improved executive function and memory domains of cognitively normal older adults in half of the included studies, mostly with medium effect sizes.5
Perhaps the best way to sum up the data is the way Soni and colleagues3 did after their comprehensive review of the clinical and epidemiologic data: “The evidence to date is not sufficient to make any recommendations about the association between dietary intake of soy isoflavones and cognition in older adults.” That is, a decision to add or not add soy to the diet should not be based on its possible effect on cognition. Given the amount of research already conducted, it is unlikely that additional research will change the current scientific consensus any time soon.
- White LR, Petrovitch H, Ross GW, et al. Brain aging and midlife tofu consumption. J Am Coll Nutr. 2000;19:242-55.
- Shin D, Lee KW, Kim MH, Kim HJ, An YS, Chung HK. Identifying dietary patterns associated with mild cognitive impairment in older Korean adults using reduced rank regression. Int J Environ Res Public Health. 2018;15.
- Soni M, Rahardjo TB, Soekardi R, et al. Phytoestrogens and cognitive function: a review. Maturitas. 2014;77:209-20.
- Cheng PF, Chen JJ, Zhou XY, et al. Do soy isoflavones improve cognitive function in postmenopausal women? A meta-analysis. Menopause. 2015;22:198-206.
- Thaung Zaw JJ, Howe PRC, Wong RHX. Does phytoestrogen supplementation improve cognition in humans? A systematic review. Ann N Y Acad Sci. 2017;1403:150-63.