Interestingly, women also reported an increased frequency of headaches. However, while mood was affected, the same study did not show differences in cognitive test performance Armstrong et al. These two studies, carried out in the same conditions and using the same methods, suggest differences between men and women regarding the impacts of mild dehydration.

While mild dehydration appears to affect mainly cognitive performance in men, in this study, women demonstrated little impact on cognitive functions and greater effects on mood. In most studies carried out on adults, mood appears to be affected by exercise-induced mild dehydration, while evidence regarding the impacts on cognitive performance is not consistent and varies between studies.

This may be due to the fact that exercise in itself has cognitive impacts, and thus may confound or mask any effect of hydration. More carefully-controlled studies would be required to tease out the differential effects of mild dehydration from exercise on cognitive function. Little has been done to evaluate the mechanisms by which dehydration may impact cognition, and this topic is addressed in part V.

A study performed on adolescents provides some insight into potential physical changes in the brain as a result of mild dehydration. Kempton et al. Using brain imaging techniques, they measured neuronal activity while the subjects performed a cognitive task.

While they observed no differences in task performance, they did observe increased brain activity in areas mediating executive functions Kempton et al. The authors speculated that in the dehydrated condition, subjects may have had to increase the cognitive resources needed to complete the task, thereby suggesting that tasks may become more demanding when mildly dehydrated.

Over the past few years, to avoid the possible confounding effect of exercise, water deprivation alone has been used to induce mild dehydration on healthy young subjects.

As it is a new area of interest, only a few studies are available to date. Results vary between studies, probably due to differences in methods used to assess cognitive functions. In a study carried out by Pross et al.

on young women, authors found that a 24h fluid deprivation resulted in impaired mood, with several parameters affected, including fatigue and vigor, alertness, confusion, calmness and contentedness, tension and emotional state Pross et al.

In a study by Shirreffs et al. Subjects self-reported even greater difficulty to concentrate and to stay alert after 24 and 37 hours Shirreffs et al.

However, on 10 young men mean age 25 , Petri et al. found no effects of a 24h fluid deprivation on mood parameters Petri et al. A plausible explanation to these differences in results could be the sex of subjects involved. Indeed, it appears that men and women may not be affected the same way by mild dehydration Armstrong et al.

This hypothesis is supported by a study from Szinnai et al. who found a significant gender effect on several cognitive tasks Szinnai et al. Without any induced dehydration, some biomarkers can underline a suboptimal hydration. High urine osmolality can for instance occur when fluid intake is insufficient to adequately compensate water losses, leading to the conservation of body water through antidiuresis.

This phenomenon is commonly called voluntary dehydration and has mostly been reported in children and elderly. In children this is mainly explained by the lack of available water in schools, while in elderly it may be due to decreased thirst sensation, and to incontinence Bar-David et al.

Consequences of voluntary dehydration on cognition have not been thoroughly investigated. In children, Bar-David et al. found that voluntary dehydration affects immediate memory: children who had a morning urine osmolality above mOsm dehydrated group had lower scores at auditory number span test than hydrated children, defined as children whose urine osmolality in the morning was bellow mOsm Bar-David et al.

Some interest has been given to voluntary dehydration in the elderly, but in these studies the topic of cognition has been largely overlooked. Suhr et al. found correlations between hydration status and a psychomotor processing speed, attention and memory performance in healthy older aldults, b declarative and working memory in postmenopausal women Suhr et al.

In adults, Kenefick et al. reported an increased rate of industrial accidents during summer months, suggesting that voluntary dehydration concurrent with high temperatures could affect cognitive performance and decision-making Kenefick and Sawka There also is growing evidence that cognitive functions might be impaired.

Figure 3. Commonly reported impacts of dehydration on mood state and cognitive function. Disparities in methods make it complex to compare results between studies and to conclude on the global effects of mild dehydration on cognition.

Recommendations for future research include controls for exercise, for water intake and for other fluids consumed, as well as accurate measurement of hydration status using hydration biomarkers More research is required to make further recommendations regarding cognitive tests sensitive to hydration and nutritional interventions Lieberman Overall, most of these studies found that mild dehydration altered several mood parameters.

Conclusions are still unclear regarding cognitive performance for which results vary depending on the methods used, the parameters studied and for which there appears to be a gender effect Lieberman ; Masento et al. As subjects often report increased difficulty to concentrate and to complete the cognitive tasks, a common hypothesis is that cognitive compensating mechanisms are involved see V Szinnai et al.

Dehydration has deleterious impacts on cognition. Uncompensated water losses can thus lead to decreased cognitive functions. Cognitive implications of water supplementation and immediate effects of water intake were mostly studied in children because they are known to be at particular risk of water deficit, and because it is ethically difficult to restrict water intake in children Masento et al.

In children, water intakes of to mL have been shown to immediately reduce thirst and to increase subjective happiness, memory, motor skills, visual attention and visual search Benton and Burgess ; Booth et al.

Over the course of one class day, Fadda et al. asked children to increase their fluid intake with 1. As compared to children who did not drink additional water, children who drank reported higher vigor and performed better at short-term memory tasks Fadda et al. More recently, in their cohort study led on undergraduates, Pawson et al.

found that students who brought water to the exam performed better although they did not actually measure the water volumes consumed Pawson et al. When water intake occurs without previous fluid deprivation, water still appears to enhance alertness and visual attention Edmonds et al.

However, several studies found that the beneficial effect of water intake on state of arousal depends on whether or not subjects were thirsty before they drank water Edmonds et al.

Headaches have been reported in subjects under induced dehydration Armstrong et al. On recurrent migraine patients, Spigt et al. found that an increase in water intake of at least 0. Immediate effects of water intake were repeatedly found on state of arousal, improving perceptions of vigor, as well as performance on task requiring attention and alertness.

In children, water also appears to enhance short-term memory. Figure 4. Commonly reported benefits of water intake on mood state and cognitive function. To our knowledge, only one study has looked at the effect of a change in water intake on mood.

In this study carried out by Pross et al. Groups were formed based on the average intakes of the French population: one third of subjects drinking less than 1. The 22 high drinkers subjects started from 2.

No differences in mood were observed at baseline between the two groups. After only three days of intervention, results showed increased thirst and decreased contentedness, calmness and positive emotions in high drinkers subjects who decreased their water intake.

In the low drinkers group, increasing water intake resulted in decrease in thirst and confusion Pross et al. Figure 5. Reported effects of a change in daily water intake on mood state Pross et al. Scientific evidence is scarce regarding the effects of a change in daily water intake on cognition.

Existing data suggest a change in mood after only 3 days: a decrease in water intake would alter mood, while an increase could decrease confusion thus enhancing state of arousal. It has been hypothesized that positive effects of water on cognition could be due to a psychological effect of expectancy.

This hypothesis was however recently rejected when Edmonds et al. enrolled 47 adults among which some received water without knowing it was part of the experiment subjects were given a drink during a conversation without notice. Some groups of subjects were informed that water could have an impact on their cognitive functions, and others were not.

The authors found positive effects of acute water intake whether or not they were in the expectancy group Edmonds et al. Rosinger said the findings suggest older adults may want to pay close attention to their hydration status, by both consuming enough liquids to avoid dehydration as well as ensuring adequate electrolyte balance to avoid overhydration.

Larry Kenney, Marie Underhill Noll Chair in Human Performance, and David M. Almeida, professor of human development and family studies, also participated in this work. Research Hydration may affect cognitive function in some older adults. December 16, By Katie Bohn Print.

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