Cognition and dietary sugars

This content is authored by Registered Dietitian, Juliette Kellow.

Cognitive Function

Cognitive function refers to the mental processes involved in acquiring knowledge and processing information, including our ability to remember, think, learn, focus, understand language, make decisions and solve problems (1).

Many factors influence cognitive function. These include our genetics, and physical and mental health. Lifestyle factors such as engaging in regular physical activity, avoiding smoking, limiting alcohol intake, getting adequate sleep, and staying mentally active and socially connected also affect cognition (2, 3, 4, 5).

Our cognitive abilities change throughout life, with different cognitive functions often peaking at different ages, before gradually declining as part of normal ageing (6, 7). If this decline occurs more rapidly than would be expected for a person’s age, it may be classed as mild cognitive impairment (MCI), which can increase the risk of developing dementia. Around 20% of people with MCI go on to develop dementia each year, though this is not inevitable – cognitive function may actually improve for a similar proportion of people (8), particularly if lifestyle changes are made to protect against dementia risk factors. Lifestyle factors which may increase the risk of dementia include high blood pressure, high cholesterol, smoking, physical inactivity, and loneliness (9). For more information on dementia, click here.

The role of diet in cognitive function

The brain needs a steady supply of energy, nutrients and fluid to work effectively. Around 75% of the brain is made up of water, so staying hydrated is important. Water contributes to the maintenance of normal cognitive function (10) with a daily intake of at least 2 litres. Studies indicate that dehydration can affect some cognitive abilities, including attention, decision-making and coordination, when fluid losses exceed 2% of body weight (11).

Dehydration may result from excessive sweating, which is influenced by many factors, including prolonged physical activity, hot weather, humidity, or having a fever. Illnesses that cause diarrhoea, sickness or increased urination (such as uncontrolled diabetes), as well as medications with a diuretic effect (which increase urine production), can also raise fluid loss from the body and make dehydration more likely.

Many nutrients also play a role in supporting brain health. The brain relies on glucose – its preferred source of energy (12) – which comes from the breakdown of carbohydrates during digestion. Indeed, carbohydrates contribute to the maintenance of normal brain function (10). Iron, zinc and iodine also play a role in normal cognitive function, while vitamins B1, B3, B6, B7, B9, B12 and C, and magnesium contribute to normal psychological functioning (10) – the mental processes such as emotion, perception, thinking, and memory.

Diet and cognitive function

The role of diet in cognitive function has been the subject of much research. A recent consensus statement from an international expert panel concluded that nutrition plays an important role in brain health throughout life and may help to maintain cognitive function and reduce the risk of cognitive decline (13).

The experts noted that current research indicates overall dietary patterns are more effective than single-nutrient interventions for supporting cognitive health. They also highlighted that the Mediterranean and MIND (Mediterranean-DASH Intervention for Neurodegenerative Delay) diets have shown promise for supporting cognitive function, but emphasised that evidence linking specific dietary patterns to cognitive outcomes remains limited and more robust clinical trials are needed (14).

The World Health Organization (WHO) also recognises the importance of diet for cognitive function. In its guidelines on reducing the risk of cognitive decline and dementia, WHO recommends following a healthy, balanced diet and notes that a Mediterranean-style diet may be recommended for adults with normal cognition or mild cognitive impairment.

However, WHO does not recommend using supplements of B vitamins, vitamin E and polyunsaturated fatty acids or multi-complex supplements to reduce the risk of cognitive decline and/or dementia, as there is insufficient evidence to support their use.

WHO also advises offering support to manage risk factors such as excess body weight, high blood pressure, diabetes and dyslipidaemia (unhealthy levels of fat in the blood such as high cholesterol) to reduce the risk of cognitive decline and dementia – all risk factors that may be influenced by diet (15).

Do dietary sugars affect cognition?

It is sometimes suggested that normal fluctuations in blood glucose levels (small increases and decreases) may affect certain cognitive functions, such as memory or concentration. However, scientific evidence for this in healthy people is limited and inconsistent. This makes it difficult to draw firm conclusions about whether cognitive performance is affected by blood glucose levels that fluctuate within the normal physiological range in people without metabolic disease (e.g., type 2 diabetes) (16).

In contrast, in people with impaired glucose tolerance or diabetes, there is stronger evidence that blood glucose levels falling below or rising above the normal range may negatively affect some aspects of cognitive function (17, 18, 19).

Sugars and cognition

The specific impact of dietary sugars on cognitive function also remains unclear. While some observational studies suggest that higher long-term intakes of sugars may be associated with poorer cognitive outcomes, others report no association (20, 21). However, observational studies can only identify a link and cannot prove whether sugars directly influence cognitive function.

Furthermore, sugars are rarely consumed in isolation and are eaten as part of a wider mixed diet, which includes other nutrients that may affect brain health. This makes it difficult to determine whether any observed effects are due specifically to sugars or other aspects of the diet.

Experimental studies investigating the short-term effects of sugars on cognition also produce mixed findings (22). Some studies report improvements in memory, attention and processing speed following glucose consumption, while others find no effect (19). In many cases, these studies give glucose to participants who have been fasting before measuring cognitive outcomes. This makes it hard to determine whether any observed improvements are due to dietary sugars themselves, or simply due to raised blood glucose levels after fasting (19).

Drawing conclusions from research on dietary sugars and cognition is challenging due to differences in study design, participant characteristics, and methodological and analytical approaches. For example, studies vary widely in the amount of sugars provided, the form in which sugars are consumed, the age and health of participants, whether testing occurs in a fed or fasted state, and the time between administering glucose and assessing cognition. Cognitive outcomes also vary, with studies measuring different aspects of cognition and using a wide range of tests (19, 21, 23).

Summary

The lack of standardisation in research investigating the effects of dietary sugars on cognition makes it difficult to directly compare findings and draw firm conclusions. This helps to explain why results across studies are often inconsistent and highlights the need for further well-designed research.

Overall, current evidence suggests that a healthy, balanced diet and staying hydrated are likely to be more important for supporting normal cognitive function and overall brain health than focusing on individual foods and nutrients such as dietary sugars.

References

1. ScienceDirect. Cognitive Function. Available at: https://www.sciencedirect.com/topics/neuroscience/cognitive-function

2. Colita E, et al. Cognitive Decline in Ageing and Disease: Risk factors, Genetics and Treatments. Curr Health Sci J. 2024;50(2):170-180.

3. Iqbal S, et al. The association between mental health and cognitive ability: Evidence from the Understanding Society survey. PLoS One. 2025;20(10):e0318910.

4. Gómez-Soria I, at al. Cognitive stimulation and cognitive results in older adults: A systematic review and meta-analysis. Arch Gerontol Geriatr. 2023;104:104807.

5. Samtani S, et al. SHARED consortium for the Cohort Studies of Memory in an International Consortium (COSMIC). Associations between social connections and cognition: a global collaborative individual participant data meta-analysis. Lancet Healthy Longev. 2022;3(11):e740-e753.

6. Hartshorne JK, Germine LT. When does cognitive functioning peak? The asynchronous rise and fall of different cognitive abilities across the life span. Psychol Sci. 2015;26(4):433-43.

7. Deary IJ, et al. Age-associated cognitive decline. Br Med Bull. 2009;92:135-52.

8. Anand S, Schoo C. StatPearls [Internet]. Mild Cognitive Impairment. NIH National Library of Medicine. Last updated: 11 January 2024.

9. Avila-Villanueva M, Avila J. Reversion or compensation of mild cognitive impairment to normal cognition: strategies to prevent the development of Alzheimer’s disease continuum. Explor Neuroprot Ther. 2024;4:392–400.

10. European Commission. EU Register of Health Claims. Available at: https://food.ec.europa.eu/food-safety/labelling-and-nutrition/nutrition-and-health-claims/eu-register-health-claims_en

11. Liska, D et al. Narrative Review of Hydration and Selected Health Outcomes in the General Population. Nutrients. 2019;11(1):70.

12. Mergenthaler, P et al. Sugar for the brain: the role of glucose in physiological and pathological brain function. Trends Neurosci. 2013;36(10):587.

13. Johnstone AM, et al. Consensus statement on exploring the Nexus between nutrition, brain health and dementia prevention. Nutr Metab (Lond). 2025;22(1):82.

14. Johnstone AM, et al. Consensus statement on exploring the Nexus between nutrition, brain health and dementia prevention. Nutr Metab (Lond). 2025;22(1):82.

15. World Health Organization. Risk Reduction of Cognitive Decline and Dementia. WHO Guidelines. 2019. Available at: https://iris.who.int/server/api/core/bitstreams/8fc586d7-b864-42eb-9274-95462496b328/content

16. Kirvalidze M, et al. The role of glucose in cognition, risk of dementia, and related biomarkers in individuals without type 2 diabetes mellitus or the metabolic syndrome: A systematic review of observational studies. Neurosci Biobehav Rev. 2022;135:104551.

17. Chen M, et al. Association between Insulin Resistance and Cognitive Impairment. J Coll Physicians Surg Pak. 2022;32(2):202-207.

18. Ye M, et al. Effect of hypoglycemic events on cognitive function in individuals with type 2 diabetes mellitus: a dose-response meta-analysis. Front Neurol. 2024;15:1394499.

19. Lin Y, et al. Relationship between glycemic control and cognitive impairment: A systematic review and meta-analysis. Front Aging Neurosci. 2023;15:1126183.

20. Gillespie KM, et al. The Impact of Free and Added Sugars on Cognitive Function: A Systematic Review and Meta-Analysis. Nutrients. 2023;16(1):75.

21. Gillespie KM, et al. The Impact of Free Sugar on Human Health-A Narrative Review. Nutrients. 2023;15(4):889.

22. García CR, et al. Effect of glucose and sucrose on cognition in healthy humans: a systematic review and meta-analysis of interventional studies. Nutr Rev. 2021;79(2):171-187.

23. Riby LM. The Impact of Age and Task Domain on Cognitive Performance: A Meta-Analytic Review of the Glucose Facilitation Effect. Brain Impairment. 2004;5(2):145-165.