By: Umm E Hani Waheed, dietetic student-volunteer, reviewed by Natalie Walsh, RD, and the JM Nutrition Team
How does hydration affect brain function? Let’s explore.
We often think about hydration in relation to physical health, exercise, and preventing dehydration. However, adequate hydration may also be important for many other reasons. These include how we think, concentrate, remember information, and feel (National Academies of Sciences, Engineering, and Medicine, 2005).
Water plays an essential role in maintaining normal physiological processes. This no doubt includes circulation, temperature regulation, and cellular function. The brain is sensitive to changes in the body’s fluid balance, and dehydration can be accompanied by symptoms such as thirst, fatigue, headache, as well as difficulty concentrating.
Research that examines the relationship between hydration and brain function has grown considerably. That said, however, the findings are not always consistent (Goodman et al., 2019).
Some studies report changes in attention, memory, fatigue, and mood during dehydration. At the same time, systematic reviews have found that the effects on overall cognitive performance can be relatively small or inconsistent (Wittbrodt & Millard-Stafford, 2018; Goodman et al., 2019; Katz et al., 2021).
Understanding this relationship is helpful to distinguish evidence-based hydration advice from the common claim that simply drinking more water will automatically make us smarter or more productive.
Why Does Hydration Matter to the Brain?
Hydration certainly affects brain function.
As per the National Academies of Sciences, Engineering, and Medicine, water is essential for maintaining the body’s fluid balance and supporting normal physiological functions.
The brain requires a stable internal environment to support normal neuronal and cellular activity. When the body loses more fluid than it replaces, dehydration occurs.
Fluid can be lost through processes such as sweating, urination, breathing and digestion.
Consequently, the body responds to changes in hydration through mechanisms including thirst and hormonal regulation, which help maintain fluid balance.
When dehydration becomes more pronounced, symptoms such as thirst, fatigue, headache, and difficulty concentrating may occur.
These physiological changes have led researchers to investigate whether hydration status can also influence cognitive performance.
Hydration and Cognition
Cognition refers to the mental processes involved in activities such as attention, memory, learning, reasoning, processing information, and problem-solving (Katz et al., 2021).
Researchers have investigated whether dehydration affects these different aspects of cognition. The results, however, have not been completely consistent.
A 2018 meta-analysis examining 33 studies found that dehydration was associated with a small but statistically significant reduction in overall cognitive performance (Wittbrodt & Millard-Stafford, 2018). The same meta-analysis found that attention and executive-function tasks appeared to be more sensitive to dehydration than some other cognitive domains.
However, another systematic review and meta-analysis found no significant overall impairment in cognitive performance following active hypohydration (Goodman et al., 2019). That review also found no significant impairment in complex attention, executive function, learning, or memory following hypohydration.
A 2021 systematic review examining hydration status and executive function similarly found inconsistent results across studies (Katz et al., 2021).
Differences between studies may be related to a number of factors. These include how dehydration was induced, how much fluid was lost, the characteristics of participants, and the type of cognitive test used (Goodman et al., 2019; Katz et al., 2021).
For example, dehydration may occur through exercise, heat exposure, fluid restriction, or a combination of these factors. It’s important to note these conditions may not affect the brain in exactly the same way.
Therefore, current research does not support the idea that dehydration always causes a decline in every aspect of cognition.
Instead, the evidence suggests that certain cognitive functions may be more sensitive to hydration status than others. This is particularly true under conditions that involve greater fluid loss (Wittbrodt & Millard-Stafford, 2018).
Hydration and Attention
Attention allows us to focus on relevant information and maintain concentration during a task. It may be one of the cognitive functions most sensitive to dehydration (Wittbrodt & Millard-Stafford, 2018).
In a controlled study of young men, approximately 1.6% loss of body mass through exercise-induced dehydration was associated with more errors during a visual vigilance task (Ganio et al., 2011). The same study found slower responses during a visual working-memory task when participants were dehydrated. Participants also reported greater fatigue and tension when they were dehydrated.
In addition, a meta-analysis found that dehydration had a greater effect on attention than on some other measures of cognitive performance (Wittbrodt & Millard-Stafford, 2018).
However, not every study has found impaired attention following dehydration (Goodman et al., 2019). This suggests that the relationship between hydration and attention may depend on factors. These include the severity of dehydration, environmental conditions, physical activity, and the individual being studied (Wittbrodt & Millard-Stafford, 2018; Goodman et al., 2019).
According to National Academies of Sciences, Engineering, and Medicine, for people exercising for prolonged periods or spending time in hot environments, maintain it’s particularly important to maintain adequate hydration. This is because fluid losses can increase dramatically under these conditions.
Hydration and Memory
Memory is a complex cognitive process that includes several different functions. This includes working memory, short-term memory, and longer-term memory (Katz et al., 2021). Working memory allows us to temporarily hold and manipulate information while completing a task.
Some experimental studies have found that dehydration can influence specific aspects of memory performance (Zhang et al., 2021).
For example, a randomized controlled trial involving young adults found that 24 hours of water restriction was associated with poorer performance on a portrait-memory task. The same study found that water supplementation improved performance on some measures of working memory and processing speed after water restriction.
However, these findings do not mean that dehydration inevitably causes memory loss.
A systematic review and meta-analysis found no significant overall impairment in learning or memory following active hypohydration (Goodman et al., 2019).
A later systematic review also described the evidence regarding hydration and working memory as inconsistent (Katz et al., 2021).
This suggests that dehydration may affect certain memory tasks under specific conditions, but there is not enough evidence to conclude that drinking additional water beyond adequate hydration consistently improves memory (Goodman et al., 2019; Katz et al., 2021).
Hydration, Mood and Mental Fatigue
Hydration may influence not only cognitive performance, but also how people feel.
Fatigue is one of the symptoms that has been reported in experimental studies of dehydration (Ganio et al., 2011).
In a controlled study of young men, mild dehydration increased feelings of fatigue and tension compared with the hydrated condition.
Another randomized controlled trial found that 24 hours of water restriction increased fatigue and total mood disturbance among young adults (Zhang et al., 2021). The same study found that water supplementation improved some measures of mood and cognitive performance following water restriction.
A separate randomized controlled trial also found that water supplementation following 12 hours of water restriction improved certain measures of mood and cognitive performance (Zhang et al., 2020).
These findings suggest that hydration status may influence subjective experiences such as fatigue, tension, and mood, particularly when people are experiencing fluid restriction or dehydration.
However, hydration should not be considered a treatment for depression, anxiety, or other mental-health conditions. Instead, adequate hydration should be viewed as one component of supporting overall physical and mental well-being.
Does Drinking More Water Improve Brain Function?
The relationship between hydration and brain function does not mean that drinking more water is always better. This is an important point to keep in mind.
In people who are already adequately hydrated, evidence that additional water consistently improves cognitive performance is limited (Goodman et al., 2019; McCartney et al., 2017).
Some studies have found improvements in specific cognitive measures after water consumption. This is particularly true when participants were previously dehydrated or had undergone water restriction (Zhang et al., 2020; Zhang et al., 2021).
However, systematic-review evidence suggests that the cognitive benefits of rehydration are less consistent than some of the benefits observed for physical performance (McCartney et al., 2017).
This distinction is important because the goal of hydration is not to consume as much water as possible.
Instead, the goal is to maintain adequate hydration and replace fluids when losses increase.
How Much Water Do We Need?
There is no single amount of water that is appropriate for every person. Fluid requirements vary according to factors. These include age, body size, physical activity, climate, diet, and health status (National Academies of Sciences, Engineering, and Medicine, 2005).
Total water intake includes water from drinking water, other beverages and foods that naturally contain water.
The National Academies established an adequate intake of approximately 2.7 litres of total water per day for adult women and 3.7 litres for adult men, with these values including water from both food and beverages. These values are population-level reference values rather than individual requirements.
Similarly, Health Canada recommends making water the beverage of choice and notes that fluid requirements can increase with physical activity and exposure to hot temperatures (Health Canada, 2025).
Water can also be obtained from foods such as fruits, vegetables, and soups, which contribute to total daily water intake.
For most healthy adults, drinking regularly throughout the day and responding to increased fluid needs during exercise or hot weather can help support adequate hydration. Our sports nutrition dietitians underscore this fact.
People with medical conditions that affect fluid balance may have individualized fluid requirements and should follow guidance from their healthcare professional.
Simple Ways to Stay Hydrated
Keep water readily available because it can make it easier to drink regularly throughout the day. If you choose water as the primary beverage, you can help support hydration without adding calories from sugar-sweetened beverages. Water-rich foods, including fruits and vegetables, can also contribute to total water intake.
Fluid needs may increase during prolonged physical activity because sweating increases fluid losses (National Academies of Sciences, Engineering, and Medicine, 2005). Likewise, hot and humid environments can also increase fluid losses and therefore increase hydration requirements.
Rather than focusing on a single daily number, hydration needs should be considered in the context of individual circumstances, activity levels, and environmental conditions.
How Hydration Affects Brain Function: Final Thoughts
Hydration is essential for normal physiological function, and the brain depends on the body’s ability to maintain an appropriate fluid balance.
Research suggests that dehydration can influence certain aspects of cognitive performance, particularly attention and executive function. That said, findings across studies are inconsistent (Wittbrodt & Millard-Stafford, 2018; Goodman et al., 2019; Katz et al., 2021).
Some studies have also reported changes in working memory and other memory-related tasks during dehydration. However, systematic reviews have not found consistent impairment across all memory measures (Goodman et al., 2019; Katz et al., 2021).
Dehydration has also been associated with increased fatigue, tension, and changes in mood in several controlled studies (Ganio et al., 2011; Zhang et al., 2020; Zhang et al., 2021).
At the same time, the evidence does not suggest that drinking excessive amounts of water will automatically improve cognition in someone who is already adequately hydrated (Goodman et al., 2019; McCartney et al., 2017).
The most evidence-based approach is therefore to maintain adequate hydration and replace fluids when losses increase, rather than deliberately consuming excessive amounts of water.
Hydration may be a simple habit, but it is one part of the broader foundation that supports both physical health and normal brain function.
Conclusion
We hope the information on how hydration affects brain function has been helpful. We also hope that it shed some light on its effects on cognition, attention, memory and mood.
Should you feel you require personalized sessions with a dietitian to help you manage this concern or any other related matter, book a free consultation or contact us for an appointment. As always if you have comments or questions, we encourage you to let us know.
References
Ganio, M. S., Armstrong, L. E., Casa, D. J., McDermott, B. P., Lee, E. C., Yamamoto, L. M., Marzano, S., Lopez, R. M., Jimenez, L., Le Bellego, L., Cheuvront, S. N., & Lieberman, H. R. (2011). Mild dehydration impairs cognitive performance and mood of men. British Journal of Nutrition, 106(10), 1535–1543.
Goodman, S. P. J., Moreland, A. T., & Marino, F. E. (2019). The effect of active hypohydration on cognitive function: A systematic review and meta-analysis. Physiology & Behavior, 204, 297–308.
Health Canada. (2025). Canada’s dietary guidelines: Make water your drink of choice. Government of Canada.
Katz, B., Airaghi, A., Davy, K. P., & others. (2021). Does hydration status influence executive function? A systematic review. Journal of the Academy of Nutrition and Dietetics, 121(7), 1284–1305.
McCartney, D., Desbrow, B., & Irwin, C. (2017). The effect of fluid intake following dehydration on subsequent athletic and cognitive performance: A systematic review and meta-analysis. Sports Medicine, 47, 137–150.
National Academies of Sciences, Engineering, and Medicine. (2005). Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate. National Academies Press.
Wittbrodt, M. T., & Millard-Stafford, M. (2018). Dehydration impairs cognitive performance: A meta-analysis. Medicine & Science in Sports & Exercise, 50(11), 2360–2368.
Zhang, J., Zhang, N., He, H., Du, S., & Ma, G. (2020). Different amounts of water supplementation improved cognitive performance and mood among young adults after 12 h water restriction in Baoding, China: A randomized controlled trial. International Journal of Environmental Research and Public Health, 17(21), 7792.
Zhang, N., Du, S. M., Zhang, J. F., & Ma, G. S. (2021). Effects of water restriction and supplementation on cognitive performances and mood among young adults in Baoding, China: A randomized controlled trial. Nutrients, 13(10).
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About Author:
Natalie is a Toronto-based dietitian who who focuses her practice on weight loss support, dietaary management of menopause, digestive health, heart health and more. Natalie is registered with the College of Dietitians of Ontario.
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