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With the combined global trends of increased population growth and population aging, researchers continue to forecast a worldwide surge in the number of people living with dementia in the next several decades, from an estimated 57 million cases in 2019 up to almost 153 million cases by 2050.¹ Aging is a unique path for every person. Experiencing mild forgetfulness may be one potential result of biological aging; however, chronic or progressive deterioration of cognitive function is not an inevitable consequence of getting older. As we age, healthy lifestyle practices are important in the prevention of many diseases and conditions. For example, ensuring an adequate intake of nutritious foods may help us achieve and maintain not only optimal physiological health but also cognitive health.

Previous meta-analyses have highlighted the fiber and phytonutrient-rich Mediterranean diet as one potential proactive approach to support a healthy brain as we age,^2,3 and recent studies continue to echo these results. A 2022 randomized controlled trial (n=224) found that a “green” Mediterranean diet (rich in polyphenols from foods such as plants, green tea, and walnuts and low in red and processed meat) may be neuroprotective for age-related brain atrophy.⁴ Most recently, a cohort study investigated one specific component of a healthy diet, fiber, and explored its impact on dementia development in a general population.⁵ The results were promising.

Dementia Prevention: A Focus on Dietary Fiber

The prospective cohort study, published in February 2022, followed 3,379 Japanese individuals for a median 19.7 years, assessing the relationship between dietary fiber intake and the development of disabling dementia.⁵ The participants were between 40 and 64 years of age and healthy at the study’s inception, and 24-hour dietary recall surveys conducted by trained dietitians were used at that time (1985-1999) to gather fiber intake estimates. For more specific analysis, researchers further classified dementia cases into those with or without a history of stroke and fiber intake amounts into total, soluble, and insoluble fibers. Hazard risk ratios were calculated after adjustment for the potential confounders of age, sex, smoking status, alcohol consumption, total energy intake, and fish, meat, and sodium intake.⁵

Reported results included the following:⁵

• Researchers observed a total of 670 cases of disabling dementia from the total cohort during the follow-up years.
• Overall, total fiber intake was inversely associated with risk of dementia.
• Compared to the lowest quartile of dietary fiber intake, the hazard ratios for participants with the second, third, and highest quartiles of dietary fiber intake showed reduced dementia risk respectively at 0.83 (0.67–1.04), 0.81 (0.65–1.02), and 0.74 (0.57–0.96).
  • While all of the reported hazard ratios reflected a reduced dementia risk, the highest fiber intake reflected the only statistically significant results.
• Researchers found that the inverse association between fiber intake and dementia risk was more evident for soluble fiber intake.
  • The reported median intake amounts for soluble fiber in each quartile were 1.7g (Q1), 2.5g (Q2), 3.3g (Q3), and 4.6g (Q4).
  • The reported median intake amounts for insoluble fiber in each quartile were 6.5g (Q1), 9.1g (Q2), 11.6g (Q3), and 15.9g (Q4).
• The inverse association of total fiber intake with disabling dementia was confined to dementia without a history of stroke.
• Systolic and diastolic blood pressure measurements were also inversely associated with total fiber intake.
• Survey results indicated that overall, men consumed less total fiber than did women.

This study adds to the evidence supporting the potential of lifestyle-based interventions to promote and sustain cognitive health. Our understanding of the gut-brain axis interplay indicates that consumption of dietary fiber can feed healthy gut bacteria, possibly benefitting brain function and cognition via the action of metabolites such as short chain fatty acids.^6,7 To underscore the potential importance of these fermentation products, a recent systematic review comprised of a majority of preclinical animal and cell model studies with a limited number of human studies suggested altered bacterial metabolite levels in subjects with various brain diseases, including Alzheimer’s disease, the most common form of dementia.⁸

Supporting a balanced gut microbial landscape through a nutritious diet with ample fibers, prebiotic foods, and phytochemicals promotes system-wide health benefits,⁹ including the potential reduction of inflammation and oxidative stress throughout the body, including the brain.^10-12 Studies continue to indicate the role of inflammation in the pathogenesis of Alzheimer’s disease and dementia impairments.¹³ However, proactive lifestyle practices such as an anti-inflammatory, nutrient-dense, fiber-rich diet is a promising approach to optimal cognitive functioning throughout the life cycle.

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References

1. GBD 2019 Dementia Forecasting Collaborators. Estimation of the global prevalence of dementia in 2019 and forecasted prevalence in 2050: an analysis for the Global Burden of Disease Study 2019. Lancet Public Health. 2022;7(2):e105-e125. doi:10.1016/S2468-2667(21)00249-8
2. Coelho-Júnior HJ, Trichopoulou A, Panza F. Cross-sectional and longitudinal associations between adherence to Mediterranean diet with physical performance and cognitive function in older adults: a systematic review and meta-analysis. Ageing Res Rev. 2021;70:101395. doi:10.1016/j.arr.2021.101395
3. Cao L, Tan L, Wang HF, et al. Dietary patterns and risk of dementia: a systematic review and meta-analysis of cohort studies. Mol Neurobiol. 2016;53(9):6144-6154. doi:10.1007/s12035-015-9516-4
4. Kaplan A, Zelicha H, Meir AY, et al. The effect of a high-polyphenol Mediterranean diet (GREEN-MED) combined with physical activity on age-related brain atrophy: the DIRECT PLUS randomized controlled trial. Am J Clin Nutr. Published online January 11, 2022. doi:10.1093/ajcn/nqac001
5. Yamagishi K, Maruyama K, Ikeda A, et al. Dietary fiber intake and risk of incident disabling dementia: the Circulatory Risk in Communities Study. Nutr Neurosci. Published online February 6, 2022. doi:10.1080/1028415X.2022.2027592
6. Dalile B, Van Oudenhove L, Vervliet B, Verbeke K. The role of short-chain fatty acids in microbiota-gut-brain communication. Nat Rev Gastroenterol Hepatol. 2019;16(8):461-478. doi:10.1038/s41575-019-0157-3
7. Cheng Y, Liu J, Ling Z. Short-chain fatty acids-producing probiotics: a novel source of psychobiotics. Crit Rev Food Sci Nutr. Published online May 6, 2021. doi:10.1080/10408398.2021.1920884
8. Tran SM, Mohajeri MH. The role of gut bacterial metabolites in brain development, aging and disease. Nutrients. 2021;13(3):732. doi:10.3390/nu13030732
9. Wagenaar CA, van de Put M, Bisschops M, et al. The effect of dietary interventions on chronic inflammatory diseases in relation to the microbiome: a systematic review. Nutrients. 2021;13(9):3208. doi:10.3390/nu13093208
10.  Yeh TS, Yuan C, Ascherio A, Rosner BA, Willett WC, Blacker D. Long-term dietary flavonoid intake and subjective cognitive decline in US men and women. Neurology. 2021;97(10):e1041-e1056. doi:1212/WNL.0000000000012454
11.  Poulose SM, Miller MG, Scott T, Shukitt-Hale B. Nutritional factors affecting adult neurogenesis and cognitive function. Adv Nutr. 2017;8(6):804-811. doi:10.3945/an.117.016261
12.  Milesi G, Rangan A, Grafenauer S. Whole grain consumption and inflammatory markers: a systematic literature review of randomized control trials. Nutrients. 2022;14(2):374. doi:10.3390/nu14020374
13.  Anuradha U, Kumar A, Singh RK. The clinical correlation of proinflammatory and anti-inflammatory biomarkers with Alzheimer disease: a meta-analysis. Neurol Sci. 2022;43(1):285-298. doi:10.1007/s10072-021-05343-7