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II. Dehydration and cognition

There is growing evidence that cognitive functions are impaired in case of uncompensated body water loss (EFSA 2011). More generally, many studies have focused on cognitive impairment due to levels of dehydration of more than 2% body mass loss. The question of the impacts of mild dehydration (≤2% body mass loss) on cognition was only addressed in the last decade (Grandjean 2007; Masento et al. 2014).

II.1. Dehydration above 2% body mass loss

Impacts of dehydration on cognitive performance were first studied in extreme conditions, on soldiers or athletes. Levels of dehydration were usually higher than 2% body mass loss, achieved with intense physical exercise and/or extreme heat, sometimes combined with fluid deprivation (Grandjean 2007; Lieberman 2007; Masento et al. 2014).


In 1988, Gopinathan et al. studied the impacts of four levels of dehydration on cognitive performance. Their study was conducted on 11 young soldiers (aged 20-25 years) and involved dehydration induced through a combination of water deprivation, heat and exercise. When levels of dehydration induced were above 2% body mass loss, results show reduced short term memory, arithmetic efficiency, as well as motor speed and attention (Gopinathan et al. 1988).


Negative impacts due to dehydration above 2% body mass loss were repeatedly found both on subjective feelings and on cognitive performance. At these levels of dehydration, cognitive abilities affected include short term memory, attention, concentration, information processing, executive functions, coordination functions and motor speed (Baker et al. 2007; Cian et al. 2000; Epstein et al. 1980; Sharma et al. 1986). Above 2% body mass loss dehydration mood and feelings were also altered, including fatigue and perceived exertion, tension, confusion, anger, emotional state and global mood (Cian et al. 2000; Ely et al. 2013; McGregor et al. 1999; Sharma et al. 1986). This still has to be interpreted cautiously, as other studies have found no impact of dehydration on functions such as short term memory, grammatical learning, information processing, attention or alertness (Ely et al. 2013; Grego et al. 2005; Tomporowski et al. 2007).


These disparities and contradictions in results from one study to another may be due to the wide variety of methods involved. Many tests and questionnaires exist to assess cognitive performance and subjective feelings. Levels of dehydration can range from 2 to 4 percent of body mass loss and methods involved to induce dehydration also vary between studies. Finally, exercise in itself, as well as concurrent hyperthermia both induce changes in cognitive performance and can be confounding factors (Tomporowski and Ellis 1986). Also, in the control condition, ensuring euhydration through water intake to compensate for sweat loss makes it difficult for the study to be blinded (Grandjean 2007; Lieberman 2007; Masento et al. 2014).

Dehydration of 2% or more body mass loss can increase perception of fatigue, induce impairment in cognitive functions such as memory and attention, and alter mood.

II.2. Dehydration below 2% body mass loss

In the last decade, scientists began investigating the implications of mild dehydration (below 2% body mass loss) on cognition. Because water balance changes throughout the day, mild dehydration may be experienced in daily life, which explains the growing interest regarding cognitive consequences. Two types of experimental design are usually involved: either a combination of fluid restriction and exercise-induced sweat loss, or water deprivation alone.

II.2.1. Exercise-induced dehydration

In 2009, D’Anci et al. induced dehydration through exercise on 31 young athletes (16 men, 15 women, mean age 20). The exercise consisted of 60 minutes of intense rowing and was followed by a cognitive-test battery and self-rated mood and thirst assessments. Dehydration achieved was 2.0% body mass loss in men and 1.65% in women, and resulted in a deterioration of all POMS (Profile Of Mood State) parameters. None of the mood parameters differed according to sex except for choice reaction time and visual vigilance (D’anci et al. 2009). Although the authors used validated tests, the study was not blinded and there was no control of body temperature. Inducing dehydration through exercise indeed implies some limitations: again, exercise, body hyperthermia, as well as water intake ensuring euhydration in the control condition, are known to be confounding factors (Grandjean 2007; Lieberman 2007; Masento et al. 2014).


More recently, Armstrong et al. and Ganio et al. published two well-controlled studies involving exerciseinduced mild dehydration. They both considered hyperthermia as a possible confounding factor for cognitive performance and controlled that dehydration was achieved without any raise in body temperature. The studies were blinded with a diuretic condition. They involved three conditions: exercise-induced dehydration plus a diuretic, exercise-induced dehydration plus placebo, and exercise while maintaining euhydration. The exercise consisted of 40 minutes treadmill walks in a mild environment (27.7°C). In both of these studies, dehydration induced was above 1% body mass loss and led to an increase in urine specific gravity (Armstrong et al. 2012; Ganio et al. 2011). In men, mild dehydration of 1.6% body mass loss led to reduced visual vigilance and working memory, and to increased perception of fatigue and anxiety (Ganio et al. 2011). In women, mild dehydration of 1.4% body mass loss led to deterioration in vigor, to increased fatigue, to a greater total mood disturbance and to increased difficulty to complete cognitive tasks and to concentrate. 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. 2012). 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. induced dehydration of 1-2% body mass loss through exercise. 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. 2011). 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.

II.2.2. Mild dehydration achieved through water deprivation

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 studies that have shown results, the effects of water deprivation appear to be on the perception of sleepwake feelings such as tiredness, fatigue or exertion are often reported (D’anci et al. 2009; Shirreffs et al. 2004; Szinnai et al. 2005). 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. 2013). In a study by Shirreffs et al., inducing a 37h fluid deprivation, the 9 men and 6 women enrolled reported decreased alertness and ability to concentrate as of 10h. Subjects self-reported even greater difficulty to concentrate and to stay alert after 24 and 37 hours (Shirreffs et al. 2004). 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. 2006). 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. 2012; Ganio et al. 2011). This hypothesis is supported by a study from Szinnai et al. who found a significant gender effect on several cognitive tasks (Szinnai et al. 2005)

II.2.3. Voluntary dehydration

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. 2005; Masento et al. 2014).


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 800mOsm (dehydrated group) had lower scores at auditory number span test than hydrated children, defined as children whose urine osmolality in the morning was bellow 800mOsm (Bar-David et al. 2005). 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. 2004; Suhr et al. 2010).


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 2007).


Even milder levels of dehydration (lower than 2% body mass loss) can increase fatigue and alter mood. There also is growing evidence that cognitive functions might be impaired.

< 2%
> 2%

Figure 3. Commonly reported impacts of dehydration on mood state and cognitive function.

II.2.4. Recommendations for future research

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 2007).


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 2007; Masento et al. 2014). 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. 2005).

Take home messages

Dehydration has deleterious impacts on cognition. Even below 2% body mass loss fatigue appears, mood is altered, and several cognitive functions might be impaired (memory, attention …). Uncompensated water losses can thus lead to decreased cognitive functions.

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