Just about a year ago, this blog (https://thesoynutritioninstitute.com/can-soy-improve-cognitive-function/) addressed the relationship between soy and cognitive function, a controversial topic given some of the older epidemiologic findings.1,2 The blog post ended by referring to a recent review of the literature that concluded: “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.”3 While that statement remains true, two new studies, one an epidemiologic study and the other an intervention study, suggest the evidence is moving in the direction of soy exerting cognitive benefits.
The aforementioned epidemiologic study included 1,105 Taiwanese participants aged 65 and over who completed the Short Portable Mental Status Questionnaire (SPMSQ), which measured cognitive function.4 In addition to filling out the SPMSQ, all participants provided extensive information on their dietary habits by filling out a food frequency questionnaire and by indicating the foods they consumed within the past 24 hours. Of the more than 1,000 participants, 85.6% reported eating soyfoods at least once per day.
The researchers divided the men and women in this study into three groups according to their soy intake: none, <1 time per day or ³1 time per day. After adjusting for a wide range of variables that could impact cognition, the results indicated that in comparison to not consuming soy, those who ate soy less than 1 time per day were 43% (relative risk, 0.57; 95% confidence interval: 0.32, 1.03) less likely to be cognitively impaired and those eating soy at least daily were 55% (relative risk, 0.45; 95% confidence interval: 0.25, 0.81) less likely to be cognitively impaired. The latter finding was statistically significant. The major limitation of this epidemiologic study is its cross-sectional design.
The primary focus of the intervention study was the relationship between vitamin B12 levels and cognition. Low levels of B12 (together with low folate levels) are a common cause of elevated total homocysteine, which has been shown to be an independent risk factor for cognitive decline and Alzheimer’s disease.5,6 For this Australian research by Zajac and colleagues,7 moderately vitamin B12-deficient participants aged between 45 and 75 years (n = 56) were recruited into a randomized controlled crossover trial. Participants (55% female) consumed daily 50 g whey protein or soy protein isolate for eight weeks. Following a 16-week washout phase, they consumed the alternative supplement. The whey protein provided 6 ug vitamin B12 (for comparison, the B12 RDA is 2.4 ug) and was considered the active treatment whereas the soy protein was devoid of B12 and considered the control treatment.
Consumption of whey protein significantly improved vitamin B12 and folate status but did not result in direct improvements in cognitive function. However, changes in active B12, homocysteine and folate measures during the whey protein treatment, correlated with improvements in cognitive function.
In contrast to whey, among the women in the study, soy protein improved reaction time (p = 0.02) and reasoning speed (p = 0.04). Reaction time measured how quickly participants responded to changes on a computer screen by pressing the keyboard. Reasoning speed assessed the ability to make reasoned decisions concerning distal relationships between stimuli.
Not surprisingly, given that cognitive benefits were observed only in women and that most of the women were postmenopausal, Zajac et al. 7 speculated that the isoflavones in soybeans were responsible for the observed benefits. In support of their speculation is a recent meta-analysis which concluded that soy isoflavones exert a positive effect on a broad range of cognitive abilities in females and that this effect is robust in women as young as 60 years.8
In summary, two studies published within just the past couple of months utilizing different experimental designs provide support for the hypothesis that soy improves cognition, although the evidence is strongest for women being the primary beneficiaries.
- White LR, Petrovitch H, Ross GW, et al. Brain aging and midlife tofu consumption. J Am Coll Nutr. 2000;19(2):242-55.
- Hogervorst E, Sadjimim T, Yesufu A, et al. High tofu intake is associated with worse memory in elderly Indonesian men and women. Dement Geriatr Cogn Disord. 2008;26(1):50-7.
- Soni M, Rahardjo TB, Soekardi R, et al. Phytoestrogens and cognitive function: a review. Maturitas. 2014;77(3):209-20.
- Lin HC, Peng CH, Huang CN, et al. Soy-based foods are negatively associated with cognitive decline in Taiwan’s elderly. J Nutr Sci Vitaminol (Tokyo). 2018;64(5):335-9.
- Duthie SJ, Whalley LJ, Collins AR, et al. Homocysteine, B vitamin status, and cognitive function in the elderly. Am J Clin Nutr. 2002;75(5):908-13.
- Vogel T, Dali-Youcef N, Kaltenbach G, et al. Homocysteine, vitamin B12, folate and cognitive functions: a systematic and critical review of the literature. Int J Clin Pract. 2009;63(7):1061-7.
- Zajac IT, Herreen D, Bastiaans K, et al. The effect of whey and soy protein isolates on cognitive function in older Australians with low vitamin B12: A randomised controlled crossover trial. Nutrients. 2018;11(1).
- Cheng PF, Chen JJ, Zhou XY, et al. Do soy isoflavones improve cognitive function in postmenopausal women? A meta-analysis. Menopause. 2015;22(2):198-206.