How Diet May Help Slow Age-Related Cognitive Decline

By Mark Messina, PhD, MS 

There is a strong need to identify the role of various lifestyle factors, including diet and physical activity, in age-related cognitive decline. As U.S.¹ and European populations² age and as life expectancy in these regions increase,³ this need becomes even more urgent. According to the World Health Organization, the pace of population aging is accelerating. As of 2024, there are approximately 830 million people globally aged 65+ years, accounting for almost 10% of the world’s population.⁴ This segment has been growing at an increasing rate and is expected to be about a quarter of the global population by the year 2100.

Multiple cross-sectional studies have shown that there is an improvement in crystallized abilities (referring to knowledge, vocabulary, and skills gained through education, learning, and life experience) until approximately age 60 followed by a plateau until age 80. Further, there is steady decline in fluid abilities (defined as the capacity to reason, solve new problems, think abstractly, and process information quickly without relying on prior knowledge) from age 20 to age 80.⁵ Tests of general knowledge (e.g., reading comprehension, math, science), historical information, and vocabulary reflect crystallized abilities whereas tests of fluid abilities require the subject to attend to one’s environment and process new information quickly to solve problems.⁶  

As in the case of a wide variety of chronic diseases, research suggests dietary habits impact cognitive abilities and risk of developing cognitive impairment, including dementia, which is a general term for loss of memory, language, problem-solving, and other thinking abilities that are severe enough to interfere with daily life. Several medical conditions lead to major neurocognitive disorder; however, Alzheimer’s disease (AD) is the most common cause and accounts for about 70% of cases. According to the dominant theory, AD-associated changes in the central nervous system occur many years prior to the first symptoms of the disease appearing.⁷ Consequently, there is a possible therapeutic window to preclude or, at least, delay debilitating memory loss and other dementia symptoms. 

However, the relationship between diet and cognition among adults, and especially older adults, is much less understood than the relationship between diet and cardiovascular disease. Nevertheless, exciting insights into the diet and cognitive function relationship have been gained within the past few years. Notable in this regard are recent results from the UK Biobank, an ongoing, multi-center prospective cohort study involving over half a million participants, that provides a resource for investigating the determinants of disease in middle and older age. A recent analysis of incident all-cause dementia risk in 60,298 participants who were followed for an average 9.1 years found higher adherence to a Mediterranean diet was associated with an approximate 23% lower dementia risk.^8,9 Two different methods were used to determine adherence, but in general the Mediterranean diet emphasizes whole, plant-based foods like fruits, vegetables, legumes, nuts, and whole grains, along with moderate amounts of fish, poultry, and dairy. 

In alignment are the results of a randomized controlled trial that found after two years of complex intervention, cognition improved in the experimental group compared to the control group. Over 1,000 participants aged 60–77 were randomized into intervention or control groups. The intervention group included nutritional guidance, physical exercise, cognitive training, social activities, and managing vascular and metabolic risk factors. The control group received regular health advice. Adherence to healthy diet at baseline predicted improvement in global cognition, regardless of intervention allocation. Dietary improvement was associated with beneficial changes in executive function, especially in the intervention group. Dietary guidance adhered to Finland’s national recommendations, emphasizing fish, fruits, vegetables, and whole grains, aligning with the “healthy Nordic diet” for brain health.¹⁰ 

Protein intake may also influence cognition and may affect the progression of mild cognitive impairment to AD. A two-year interim analysis of a three-year longitudinal study found that plasma concentrations of all nine essential amino acids were lower in the AD-convert group with the three branched-chain amino acids (valine, leucine, and isoleucine) showing the most significant difference.¹¹ In agreement, an analysis of participants of the Nurses' Health Study and the Health Professionals Follow-up Study who were followed for over 20 years, indicated that in comparison with carbohydrates, consumption of protein is associated with lower chances of developing cognitive decline later in life.¹²  When substituting 5% energy from protein for the equivalent percentage of energy from total carbohydrates, the pooled multivariable-adjusted odds ratios (95% confidence intervals) were 0.89 (0.85, 0.94) for total protein, 0.89 (0.84, 0.94) for animal protein, and 0.74 (0.62, 0.88) for plant protein. When substituting 5% of energy from animal protein with plant protein, the odds ratio was 0.84 (95% confidence intervals: 0.72, 0.97). 

While soy foods are recognized as important sources of plant protein, it is the isoflavone component of soybeans that has attracted the attention of investigators studying cognitive function. Clinical research evaluating the impact of isoflavone intake, either through tablets or protein powders, on cognition has been underway for more than 20 years, although most work has involved postmenopausal women. The results can be summarized as encouraging but also inconsistent. For a comprehensive review of the evidence and emerging research in this area, see the lead article in this issue, “Feeding the Mind: Cognitive Effects of Soy”. This piece examines the biological mechanisms, clinical findings, and potential applications of soy isoflavones in supporting brain health across the lifespan. 

Importantly, a meta-analysis of 16 randomized controlled trials involving 1,386 participants with a mean age of 60 years found that soy isoflavones improved overall cognitive function; however, when looking at individual cognitive function domains (e.g., visuospatial reasoning, executive function, psychomotor speed, etc.), only memory was significantly improved.¹³ Of the total number of participants, 90% were postmenopausal women.  

In summary, several lines of evidence suggest that diet influences cognitive function. Adopting a healthy diet—especially earlier in life—may help reduce the risk of cognitive decline. The potential benefits of a higher-protein diet, particularly one rich in plant protein, combined with the findings from isoflavone research, suggest soy foods may play a role in maintaining cognitive function during the aging process.

REFERENCES

1. Ortman JM, Velkoff VA. An aging nation: The older population in the United States. Current population Reports. 2014;May. 

2. The Lancet Regional H-E. Securing the future of Europe's ageing population by 2050. Lancet Reg Health Eur. 2023;35:100807. 

3. Woolf SH, Schoomaker H. Life expectancy and mortality rates in the United States, 1959-2017. JAMA. 2019;322:1996-2016. 

4. UN, World Population Prospects (2024). https://ourworldindata.org/data-insights/by-2060-the-number-of-people-aged-65-and-older-will-be-more-than-four-times-what-it-was-in-2000.   

5. Salthouse TA. Selective review of cognitive aging. J Int Neuropsychol Soc. 2010;16:754-60. 

6. Murman DL. The impact of age on cognition. Semin Hear. 2015;36:111-21. 

7. Beason-Held LL, Goh JO, An Y, et al. Changes in brain function occur years before the onset of cognitive impairment. J Neurosci. 2013;33:18008-14. 

8. Shannon OM, Ranson JM, Gregory S, et al. Mediterranean diet adherence is associated with lower dementia risk, independent of genetic predisposition: findings from the UK Biobank prospective cohort study. BMC Med. 2023;21:81. 

9. Christodoulou CC, Pitsillides M, Hadjisavvas A, Zamba-Papanicolaou E. Dietary Intake, Mediterranean and Nordic Diet Adherence in Alzheimer's Disease and Dementia: A Systematic Review. Nutrients. 2025 Jan 17;17(2):336. doi: 10.3390/nu17020336. PMID: 39861466; PMCID: PMC11767999. https://pmc.ncbi.nlm.nih.gov/articles/PMC11767999/#:~:text=Eligible-studies%20meeting%20the%20inclusion,between%20diet%20and%20cognitive%20function 

10. Lehtisalo J, Levalahti E, Lindstrom J, et al. Dietary changes and cognition over 2 years within a multidomain intervention trial-The Finnish Geriatric Intervention Study to Prevent Cognitive Impairment and Disability (FINGER). Alzheimers Dement. 2019;15:410-7. 

11. Ikeuchi T, Kanda M, Kitamura H, et al. Decreased circulating branched-chain amino acids are associated with development of Alzheimer's disease in elderly individuals with mild cognitive impairment. Frontiers in nutrition. 2022;9:1040476. 

12. Yeh TS, Yuan C, Ascherio A, Rosner BA, Blacker D, Willett WC. Long-term dietary protein intake and subjective cognitive decline in US men and women. Am J Clin Nutr. 2022;115:199-210. 

13. Cui C, Birru RL, Snitz BE, et al. Effects of soy isoflavones on cognitive function: A systematic review and meta-analysis of randomized controlled trials. Nutr Rev. 2020;78:134-44.