How does dehydration affect sustained attention?

Mild dehydration impairs sustained attention, producing increased mental fatigue, slower processing speed and more errors on prolonged focus tasks. Elevated serum osmolality is associated with poorer performance on continuous attention measures.

Which attention type is most sensitive to hydration?

Sustained attention is most sensitive to hydration changes; working memory, inhibition and cognitive flexibility show fewer consistent effects from mild dehydration in recent studies.

What is serum osmolality for hydration assessment?

Serum osmolality measures blood solute concentration; values above 300 mOsm/kg indicate dehydration and serve as the clinical reference biomarker for objective hydration assessment.

Who is at higher risk of dehydration-related attention decline?

Older adults, people with cognitive impairment, individuals in hot climates and institutionalized patients face higher risk due to reduced thirst, forgetfulness or increased fluid losses.

Can drinking water rapidly restore attention?

Acute water intake can improve attention in some mildly dehydrated individuals, especially during repetitive or prolonged tasks, but regular hydration is more reliable for sustained cognitive benefits.

What clinical measures prevent dehydration-related attention loss?

Implement routine hydration checks, consider serum osmolality in unexplained cognitive changes, promote scheduled fluid intake, facilitate access to drinks and avoid excessive diuretic beverages.

Impact of dehydration on cognitive functioning

This article summarizes the most relevant data from a recent longitudinal study that analyzes the direct impact of dehydration on cognitive functioning.

Introduction

This article takes as a starting point the longitudinal study recently published by Rosinger, John and Murdock (2024) in the American Journal of Human Biology, which analyzes how mild dehydration negatively influences sustained attention in middle-to-older aged adults. Based on these findings, an expanded review is developed on the relationship between hydration and attention, with special emphasis on clinical implications and in the context of neurorehabilitation.

Hydration is an essential component for life and overall health. Although commonly associated with functions such as thermal regulation or physical performance, more and more studies show that it also exerts a direct influence on cognitive functioning. Among all cognitive functions, attention, especially sustained attention, appears to be particularly sensitive to hydration status.

What do we mean by attention?

Types of attention

Attention is a complex executive function that allows selection of relevant stimuli from the environment, inhibition of distractions and maintenance of focus for prolonged periods.

It is classified into:

Selective attention: focusing on one stimulus while ignoring others.
Alternating attention: switching focus between tasks.
Divided attention: attending to multiple sources of information simultaneously.
Sustained attention: maintaining concentration for a prolonged period.

Why is sustained attention key in neurological patients?

In patients with neurological, neurodegenerative diseases or in rehabilitation, sustained attention becomes a key predictor of functionality. It is related to the ability to follow instructions, maintain motivation in therapy, and complete activities of daily living, since it is a basic function for the development and functioning of other more complex ones.

Therefore, any factor that may negatively influence this function must be identified and treated. One of these factors is dehydration.

The role of water in the brain

Water composition of the nervous system

The brain is composed of 73–75% water. This high proportion is not accidental: water facilitates synaptic transmission, nutrient exchange, toxin removal and cerebral thermal regulation.

A 1–2% decrease in water volume can alter neuronal metabolism and modulate neurotransmitters such as dopamine and glutamate, which are key for attention and other executive functions.

Effects of dehydration at the brain level

Mild dehydration can produce:

Decreased cerebral blood flow.
Increased mental fatigue.
Reduced processing speed.
Impairment of working memory and attention.

This is especially important in older adults, where the perception of thirst is reduced and the risk of chronic dehydration increases.

Assessment of hydration: beyond the volume of water consumed

What is serum osmolality?

Serum osmolality (Sosm) measures the concentration of solutes in blood. A value greater than 300 mOsm/kg indicates a physiological state of dehydration. It is considered the reference biomarker to assess hydration status at the clinical level.

Why is asking how much water someone drinks not enough?

Fluid intake may not reflect true hydration status. Factors such as sweating, ambient temperature, diet, use of diuretics or chronic diseases can alter water balance without the person perceiving it. Therefore, the use of biomarkers such as osmolality is essential for an objective assessment.

Scientific evidence: hydration and its effect on sustained attention

Design of the reference study

A longitudinal study conducted in adults aged 50 to 75 years (Rosinger et al., 2024) evaluated the relationship between hydration and cognitive functions, including attention, working memory, inhibition and cognitive flexibility. Hydration was measured by serum osmolality across three visits over three months.

Participants were classified as dehydrated if they had Sosm > 300 mOsm/kg. Standardized neuropsychological batteries were applied, including the Conners’ Continuous Performance Test II (CPT-II) to assess sustained attention.

Main results

Dehydration was associated exclusively with poorer performance in sustained attention.
No significant differences were found in inhibition, working memory or cognitive flexibility.
The negative effect was equivalent to a loss of 0.65 standard deviations in the dehydrated group.
This association persisted even after adjusting for age, sex, BMI and educational level.

Clinical interpretation

The finding highlights sustained attention as one of the cognitive functions most vulnerable to mild dehydration. In tasks that require continuous concentration, such as following a clinical conversation, performing therapeutic exercises or adhering to medication regimens, hydration status can make the difference between functional success or failure.

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Who is at risk?

Older adults

Aging involves physiological changes that reduce sensitivity to thirst. In addition, many older adults voluntarily restrict water intake for fear of incontinence, which increases the risk of subclinical chronic dehydration.

Patients with cognitive impairment

Patients with dementia, Parkinson’s disease or brain injuries may forget to drink water, have difficulty communicating or show less initiative, which makes them especially vulnerable.

Hot climates or institutionalization

Increased ambient temperature (with or without physical exertion) raises water losses. Care homes, hospitals or day centers should implement active hydration protocols, especially in summer.

Clinical recommendations based on evidence

Routine hydration assessment

Include questions about fluid intake, presence of nonspecific symptoms (headache, fatigue, dizziness) and physical signs.
Consider requesting Sosm in patients with cognitive alterations without an apparent cause.

Preventive interventions

Promote regular water intake, even in the absence of thirst.
Facilitate access to fluids (cups with a straw, visual reminders, structured schedules).
Avoid diuretic beverages such as coffee, black tea or excessive alcohol.
Monitor hydration on hot days or during febrile infections.

Therapeutic applications

In neurorehabilitation or cognitive therapy contexts, ensuring proper hydration prior to the session can enhance performance on sustained attention tasks, thereby improving therapeutic effectiveness.

Limitations of studies and future research lines

Methodological limitations of the study

It was conducted in a population predominantly of white people, which limits generalizability.
Exact amounts of water ingested were not directly evaluated.
Sustained attention was the only function clearly altered, which raises questions about the specificity of the effect.

Future studies needed

Replicate the study in more diverse populations.
Analyze the effect of overhydration.
Incorporate ecological attention tasks that simulate real situations (for example, prolonged reading, simulated driving, dual tasks).

Conclusions

Current evidence indicates that adequate hydration not only prevents physical disorders but also contributes to the maintenance of attention, especially in older adults. Sustained attention, as a key function in multiple clinical and everyday activities, can be significantly compromised even by mild dehydration.

In this context, water becomes a silent but powerful therapeutic tool. From the clinical consultation to neuropsychological rehabilitation, promoting hydration should be considered an essential intervention to optimize cognitive performance and preserve quality of life.

Bibliography

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Armstrong, L. E., et al. (2012). Mild dehydration affects mood in healthy young women. The Journal of Nutrition, 142(2), 382–388.
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.
Kenney, W. L., & Chiu, P. (2001). Influence of age on thirst and fluid intake. Medicine & Science in Sports & Exercise, 33, 1524–1532.
Popkin, B. M., D’Anci, K. E., & Rosenberg, I. H. (2010). Water, hydration, and health. Nutrition Reviews, 68, 439–458.
Rosinger, A. Y., John, J. D., & Murdock, K. W. (2024). Ad libitum dehydration is associated with poorer performance on a sustained attention task but not other measures of cognitive performance among middle-to-older aged community-dwelling adults: A short-term longitudinal study. American Journal of Human Biology, 36(6), e24051. https://doi.org/10.1002/ajhb.24051

Bahnfleth CL, Strupp BJ, Caudill MA, Canfield RL. Prenatal choline supplementation improves child sustained attention: A 7-year follow-up of a randomized controlled feeding trial. FASEB J. 2022;36:e22054. https://doi.org/10.1096/fj.202101217R
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Frequently asked questions about hydration and attention

1. How does dehydration affect attention?

Dehydration, even in mild degrees, can negatively affect sustained attention. Recent studies have shown that people with elevated serum osmolality levels (an indicator of dehydration) perform worse on tasks that require maintaining attention for prolonged periods.

2. Which type of attention is most sensitive to hydration status?

Sustained attention is the most affected by dehydration. This function allows maintaining continuous focus on a task for a long interval of time, as occurs in therapy sessions, reading clinical reports or prolonged driving. Other functions such as working memory or selective attention seem to be less sensitive to mild changes in hydration.

3. How much water should I drink to maintain good attention?

There is no one-size-fits-all amount, but generally a consumption of between 1.5 and 2 liters of water per day is recommended. Maintaining adequate hydration helps preserve cognitive performance, especially in tasks that require high attention. It is important to drink fluids throughout the day, even without feeling thirsty.

4. Is attention affected in older adults by poor hydration?

Yes. Older adults are especially vulnerable, as their sensation of thirst decreases with age. This can lead to chronic dehydration that negatively impacts attention, increasing the risk of errors, falls and reduced ability to follow treatments or participate in cognitive rehabilitation programs.

5. Does drinking water improve attention immediately?

In certain cases, yes. In slightly dehydrated individuals, drinking water can quickly improve attention, especially in repetitive or long tasks. However, the most effective approach is to prevent dehydration by maintaining regular water intake throughout the day.

6. How can I tell if my attention is affected by dehydration?

Some indirect signs include difficulty concentrating, increased errors, mental fatigue and a feeling of confusion. If these symptoms appear along with dry mouth, headache or low urine frequency, it is possible that your attention is being compromised by insufficient hydration.

7. What recommendations exist to improve attention through hydration?

Drink water regularly, not only when you feel thirsty.
Include fluids with each meal.
Monitor fluid intake in hot conditions or during infections.
Ensure good hydration before activities that require high attention, such as clinical sessions, driving or exams.