Review

Hydration and Cognition: A Critical Review andRecommendations for Future Research

Harris R. Lieberman, PhD

US Army Research Institute of Environmental Medicine, Natick, Massachusetts

Key words: cognitive performance, mood, fatigue, dehydration, environment, heat, exercise

The limited literature on the effects of dehydration on human cognitive function is contradictory andinconsistent. Although it has been suggested that decrements in cognitive performance are present in the rangeof a 2 to 3% reduction in body weight, several dose-response studies indicate dehydration levels of 1% mayadversely affect cognitive performance. When a 2% or more reduction in body weight is induced by heat andexercise exposure, decrements in visual-motor tracking, short-term memory and attention are reported, but notall studies find behavioral effects in this range. Future research should be conducted using dose-response designsand state-of-the-art behavioral methods to determine the lowest levels of dehydration that produce substantiveeffects on cognitive performance and mood. Confounding factors, such as caffeine intake and the methods usedto produce dehydration, need to be considered in the design and conduct of such studies. Inclusion of a positivecontrol condition, such as alcohol intake, a hypnotic drug, or other treatments known to produce adverse changesin cognitive performance should be included in such studies. To the extent possible, efforts to blind bothvolunteers and investigators should be an important consideration in study design.

Key teaching points:

• Dehydration impairs cognitive performance and mood.• Adverse effects of dehydration on cognitive function are present at a 2% or more reduction in hydration and may be present at a

1% level of dehydration.• Most studies of the effects of dehydration on cognitive function have used heat and exercise to induce rapid dehydration.• Studies of the effect of dehydration on cognitive function should employ dose-response designs.• The specific behavioral functions most likely to be affected by dehydration have not been established; additional studies should

be conducted using state-of-the-art performance tests and mood questionnaires.• Double-blind procedures, or surrogates for these procedures, should be employed whenever possible in studies of cognitive

function and dehydration.

INTRODUCTION

Adequate hydration is essential for human homeostasis andsurvival, including maintaining brain function. In a relativelybrief period of time, failure to consume sufficient water willlead to deteriorating cognitive and neurologic function, organfailure, and death. However, the effects of dehydration oncognitive performance and brain function have not been thor-oughly investigated. There are a variety of reasons for the lackresearch on the effects of dehydration on human brain function.

Dehydration is difficult to assess accurately (see Armstrong[1]). Its study requires considerable specialized physiologicexpertise since producing reliable changes in hydration is com-plex and demanding. Attaining a precise level of dehydration isdifficult to achieve, and even the best studies do not preciselyreach the desired end point. Furthermore, there are many con-founders associated with such studies, including the nature andduration of the stressors used to produce dehydration, typicallyheat and sustained aerobic exercise. In addition, since there arephysiologically distinct categories of dehydration, such as

Address reprint requests to: Harris R. Lieberman, PhD, US Army Research Institute of Environmental Medicine, Natick, MA 01760. E-mail: harris.lieberman@us.army.mil

Presented at the ILSI North America 2006 Conference on Hydration and Health Promotion, November 29–30, 2006 in Washington, DC.

Conflict of Interest Disclosure: There are no conflicts of interest to declare in connection with this work.

Journal of the American College of Nutrition, Vol. 26, No. 5, 555S–561S (2007)Published by the American College of Nutrition

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hypotonic and isotonic dehydration, it can be difficult to gen-eralize results of specific studies.

Assessing human cognitive function is also difficult andcomplex. There are hundreds of tests of cognitive perfor-mance and little agreement among psychologists as to whichtests to use to assess specific functions, such as attention,learning, memory, and reasoning [2,3]. When examining anarea as unique as the effects of dehydration on cognitivefunction, with limited published literature available, thereare few established principals that can readily be applied byinvestigators working in the area. Perhaps the most closelyrelated areas of investigation are psychopharmacology andnutritional neuroscience, for which many methodologicalissues have been resolved [3].

Since severe dehydration inevitably leads to delirium,coma, and death, there is no question that it will producedecrements in cognitive function before such severe conse-quences are present (Fig. 1) [4,5]. However, there is insuf-ficient information available to determine the extent andduration of dehydration that will produce cognitive decre-ments and the cognitive functions that will be most im-paired. Dehydration is a unique stressor with unique physi-ological effects that may or may not be similar to the effectsof other stressors on cognitive performance. For example,dehydration, unlike other stressors, may produce shifts inelectrolytes that could substantially alter brain neurotrans-mission since neuronal function is highly dependent on thephysiological milieu of the central nervous system (CNS).For a discussion of possible physiological mechanisms thatmay be responsible for the adverse effects of dehydration onbrain function, see Wilson and Morley [5] and Maughan [6].Using the limited available literature in the area, this paperwill attempt to provide some guidance as to the effects ofdehydration on cognitive function, note some of the majormethodological issues in the area, and suggest future re-search agendas.

METHODOLOGICAL ISSUES

Inducing Dehydration

Inducing dehydration in a controlled, consistent manner isdifficult. Most studies examining the effect of dehydration oncognitive function have used a combination of exposure to heatand controlled exercise (for a comprehensive review, seeSawka [7]). Unfortunately, there are inherent limitations to thisprocedure. Exposure to heat and exercise alone will invariablyalter cognitive performance and mood state [8–10]. Investiga-tors will usually attempt to control for these potentially con-founding effects by using a fully-hydrated control conditionwith equivalent exposure to heat and exercise as the dehydrat-ing test condition. However, this assumes that the effects ofheat and exercise exposure will be identical regardless of theeffects of hydration state. In fact, a combination of stressorsmay have complex, non-linear effects on cognitive perfor-mance. The sum of the effects of combining heat, exercise anddehydration may not be additive relative to heat exposure andexercise alone. This could lead to either over- or underestima-tion of the effects of dehydration per se, depending on theextent and direction of any non-linear interactions of the com-bination of stressors. For example, if dehydration amplifies theeffects of heat and exercise then it will appear that dehydrationhas a greater effect on cognitive performance than would beobserved if the only treatment condition is dehydration. Studiesof heat- and exercise-induced dehydration should therefore notbe considered as studies of “pure” dehydration.

Another method to induce dehydration is withholding fluidsand foods with high water content. Unfortunately, this tech-nique has only apparently been employed in two recent studies,and they had significant limitations as discussed below [11,12].

Advantages of the Dose-Response Design

A standard, widely-employed design used in pharmacologyresearch is to administer the compound of interest in a gradedrange of doses. Like the dosage of a drug, dehydration is acontinuous variable and can be induced in a range of levels,within the bounds of volunteer safety, and has the capability toaccurately assess hydration state. There are various advantagesof dose-response studies, including a robust form of internalcontrol. In addition to yielding information on a particulartreatment’s parametric effects, if a study’s results are system-atically dose-related, this provides internal replication of itsfindings.

Using a dose-response design to study the effects ofdehydration on cognitive performance is a particularly ap-propriate strategy given the inherent complexity associatedwith the assessment of cognitive function. Since there are somany types of cognitive function, each governed by differ-ent brain regions and neurotransmitter systems, it is likelythese will be differentially affected by dehydration. TheFig. 1. Decline in function resulting from dehydration.

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internal, within-study replication provided by a dose-re-sponse study allows for much greater confidence that thestudy’s results are correct. Dehydration is particularly well-suited for study using a dose-response design since it ap-pears to produce graded, monotonic changes in cognitivefunction, as indicated by several studies [13,14]. Thesestudies suggest that, as the extent of dehydration increases,adverse effects on behavior increase. Pharmacologic andnutritional treatments do not always produce monotonicdose-response functions. With drugs and some dietary con-stituents, as the dose administered increases beyond optimallevels, beneficial effects will typically be reduced and ad-verse changes will occur. For example, low and moderatedoses of caffeine have positive effects on cognitive per-formance and mood, but high doses have adverse effects[15,16].

STUDIES OF DEHYDRATIONINDUCED BY HEAT AND/OREXERCISE

Most studies examining effects of dehydration on cogni-tive performance have employed heat, exercise, or the com-bination of these factors to produce hypohydration. For suchstudies to be interpretable, it is essential they include a fullyhydrated control condition where equivalent exposure toheat and/or exercise is studied [7]. One of the most compre-hensive studies conducted using this design assessed theeffects of dehydration levels ranging from approximately 1to 4% in 1% increments in 11 healthy males aged 20 to 25years [14]. The volunteers were acclimated to heat prior tostarting the study and each was tested in all five experimen-tal conditions. Volunteers performed continuous moderatework by stepping up and down on a 38 cm platform at a rateof 15 steps/min, stopping when the desired level of dehy-dration was reached. The chamber where they were dehy-drated was maintained at 45°C with 30% relative humidity(RH). In addition to a fully hydrated control condition withheat and exercise exposure, the investigators also included athermoneutral, no exercise control condition.

The behavioral tests employed were a serial addition test,word recognition test and trail-making test. Serial addition is anauditory test that assesses short-term memory and reasoning.Sixty random single digit numbers are grouped into 12 sets offive numbers; numbers from each set are read to the volunteerat a rate of one number every two seconds. The volunteer mustadd the five consecutive numbers and write down the sum. Thescore is percent correct answers. Word recognition is an audi-tory test in which 15 words from a 52 word list are read at a rateof one every two seconds. After hearing the 15 words, thevolunteer is given the written list and required to mark as manyof the 15 words as can be recalled. Score is percent of correct

markings. Trail-making assesses short-term memory, reason-ing, and motor control. The volunteer is presented with a testsheet with 48 scattered symbols—half are consecutive numbersand half are consecutive English-language letters. Volunteersmust trace a trail for 60 seconds, alternating between a numberand letter, as rapidly as possible, without lifting pencil frompaper [14].

Fig. 2a presents the results from the serial addition test. Theintended level of dehydration is plotted on the y-axis, anddirectly below it, in parentheses, the measured mean level ofdehydration achieved is presented. In all cases the achievedlevel of dehydration exceeded the planned level. Dehydration atthe planned 1, 2, 3 and 4% levels produced significant decre-ments in correct responses on this task. Furthermore, there wasa consistent, dose-related decline in performance across alllevels of dehydration. A similar pattern was present for thetrail-making test, with speed to complete significantly increas-ing with increasing dehydration at the 2, 3 and 4% levels (Fig.2b). The third test, word recognition, showed the same pattern,with percent correct words recalled decreasing with increasing

Fig. 2. a) Percent of correct responses on the serial addition test.Results are reported at thermoneutral, 0, 1, 2, 3 and 4% dehydrationconditions. The actual levels of dehydration are shown in parenthesesbelow the desired target levels (*p � 0.05; ***p � 0.001). Datare-plotted from Gopinathan et al. [14]. b) The mean speed (sec) on thetrail-making test at thermoneutral, 0, 1, 2, 3 and 4% dehydrationconditions. Actual levels of dehydration reached are shown in paren-theses below the desired target levels (***p � 0.001). Data re-plottedfrom Gopinathan et al. [14].

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dehydration. The effects were statistically significant at the 1,2, 3 and 4% levels [14].

The results of this study are particularly robust because allthree administered tests detected consistent dose-related dete-rioration in cognitive performance. In addition, the inclusion ofa second control condition, in which volunteers were exposedto thermoneutral conditions without exercise during the periodwhen dehydration would have been induced, provided an ad-ditional baseline test session for comparison. For each cogni-tive task, performance during this condition was nearly equal tothe other control condition (Fig. 2) providing additional supportfor the validity and accuracy of the findings.

A similar study was conducted by Sharma et al. [13] at thesame laboratory as the previously discussed study. This studyassessed the effects of dehydration at levels from 1 to 3% ineight males, 21 to 24 years old. In this study, moderate exercisein the heat was the principal method used to induce dehydrationand behavioral testing was conducted in three different condi-tions: thermoneutral (37°C, 50% RH), hot dry (45°C, 30% RH)and hot humid (39°C, 60% RH). Three levels of dehydrationwere induced approximating 1, 2 or 3%. The behavioral testsemployed were a symbol substitution test, concentration test,and an eye-hand coordination test. In the symbol substitutiontest, a volunteer is presented a sample code where 10 letters areassociated each with one of 10 designs. The volunteer is thenpresented with several horizontal rows of the geometric designsin random order. The volunteer has three minutes to write thecorrect letter from the code under as many symbols as possible.The concentration test assessed working memory. Ten series ofnumbers were read aloud at a rate of one per second. At randomintervals, the series was stopped and the volunteer attempted towrite down the last five numbers read in reverse order [17]. Theeye-hand coordination test used a metal plate with a 0.5 cmwide groove cut into an eight-cornered star pattern. The vol-unteer was required to move a stylus in the groove withouttouching the edge. Score is based on time to complete andnumber of errors [13].

Significant dose-related effects of dehydration were notedin two of three tests administered. In one of the tests, symbolsubstitution, significant effects of dehydration were only ob-served at the 3% level of dehydration (p � .05) (Fig. 3a), butfor another, eye-hand coordination, significant effects wereobserved at all three levels of dehydration in at least one of theenvironmental conditions tested (p � .01) (Fig. 3b). The resultsof this study are robust and internally consistent since testingwas conducted at three levels of hydration and in three differentenvironmental conditions. Across all environmental parame-ters, graded dose-related changes in cognitive performancewere observed (Fig. 3) [13].

Although these two studies provide insight into the effectsof dehydration on cognitive performance, they do not providean indication as to which behavioral functions are most affectedby dehydration. The tests employed did not adequately differ-entiate the general effects of dehydration from effects specific

to particular cognitive functions, such as learning, memory, orvigilance [13,14].

STUDIES OF DEHYDRATIONINDUCED BY WATER DEPRIVATIONALONE

Few studies have examined the effects of dehydration oncognitive performance induced by water deprivation alone. Onerecent study of 16 healthy men and women aged 20 to 34assessed the effects of 28 h of self-monitored water deprivationresulting at a mean dehydration level of 2.6% [12]. Eachvolunteer participated in a control and dehydration testingsession in random order. On the morning of day one, baselinedata were collected after volunteers abstained from all caffein-ated beverages that morning. All fluids were withdrawn during

Fig. 3. a) Results from the symbol substitution test at 0, 1, 2 and 3%dehydration levels at three environmental conditions; thermoneutral,hot-dry, and hot-humid. Mean levels of actual dehydration for onlyhot-dry and hot-humid conditions are shown in parentheses below thetarget levels. Actual dehydration levels for the thermoneutral conditionwere not reported. (*p � .05; this value has been computed from theoriginal data as the authors did not test significance below the p � 0.01level) Data re-plotted from Sharma et al. [13]. b) Results from theeye-hand coordination test at 0, 1, 2 and 3% dehydration levels at threeenvironmental conditions -thermoneutral, hot-dry, and hot-humid.Mean levels of actual dehydration for hot dry and hot humid conditionsare shown in parentheses below the target levels. Levels for thethermoneutral condition were not reported. (**p � 0.01; ***p � 0.001)Data re-plotted from Sharma et al. [13].

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the dehydration treatment session and volunteers selectedmeals from foods containing less than 75% water by weight forthe 28 h of the study. During the control session, volunteerscould consume any nonalcoholic beverage, except coffee.

The behavioral tests employed were a choice reaction timetask, a manual tracking task, paced auditory serial addition task,and the Stroop word-color conflict test. Choice reaction time isa five-minute test where a random sequence of colored lights ispresented and the volunteer responds by pressing correspond-ing buttons. In the manual tracking task, volunteers were re-quired to follow, with a pointer, a white circle moving on acomputer monitor for three minutes. The paced auditory serialaddition task is similar to the serial addition task used byGopinathan et al. [14], however this was a computerized ver-sion where numbers were presented at 2.5 s intervals andvolunteers added the last two digits as rapidly as possible andresponded verbally. The Stroop word-color conflict test as-sessed response time to conflicting stimuli. Color words werepresented on screen but written in a different color (for exam-ple, “green” appeared on screen in a red font). Volunteers wererequired to name the font color in one portion of the task andname the color as written in another. Questionnaires assessedself-reported tiredness and alertness, as well as the perceivedeffort and concentration required to complete the behavioraltests [12].

No significant effects of 2.6% dehydration on cognitiveperformance were detected. However, there were effects ofdehydration on self-reported levels of tiredness, alertness, ef-fort, and concentration (p � 0.05). There were some flaws withthe conduct of this study. First, volunteers lost weight and weretherefore somewhat dehydrated during the control (0.75%), aswell as the dehydrated condition (2.56%). Also, caffeine wasnot adequately controlled. Only coffee consumption was re-stricted, but it appears that other caffeinated beverages wereaccessible to the free-living volunteers during both conditions.Since withdrawal from caffeine produces adverse effects oncognitive performance and mood [18], the dehydrated condi-tion may have induced caffeine withdrawal not present in thecontrol condition.

Another study using water deprivation alone to producedehydration did not evaluate cognitive performance, but didassess mood at several time periods during a 37-hour period ofself-monitored water deprivation resulting in a 2.7% mean levelof dehydration [11]. Fifteen volunteers with a mean age of 30participated in two conditions, fluid restriction and euhydra-tion. For both conditions, volunteers arrived at the laboratoryafter refraining from alcohol consumption and strenuous exer-cise for 24 hours and having fasted for four hours. Afterbaseline measurements were taken, the volunteers were al-lowed to leave the laboratory. During the euhydration condi-tion, volunteers were instructed to consume their normal dietand fluid intake. During fluid restriction, volunteers were askedto refrain from all fluid intake and to select foods with a lowwater content from a provided list. Volunteers reported back to

the laboratory at 24-h and 37-h for measurements identical tothose taken at baseline [11].

At each assessment session, an eight-item self-report ques-tionnaire was administered using 100 mm visual analog scales.The scales assessed thirst, hunger, dry mouth, mouth taste, headsoreness, concentration, tiredness, and alertness. Ratings ofthirst and dry mouth were significantly greater during fluidrestriction condition than in euhydration (p � 0.05). Feelings ofhead soreness were significantly greater in the fluid restrictionstate versus euhydration at 24-h (p � 0.005) and 37-h (p �

0.05) (Fig. 4a), 1.8% and 2.6% dehydration, respectively (Fig4c). There was also a significant reduction in concentration(Fig. 4b) and alertness at 13-h (1% dehydration), 24-h (1.8%dehydration) and 37 h (2.6% dehydration) in the fluid restric-tion state compared to euhydration (p � 0.05), and fluid re-stricted volunteers reported feeling more tired at 24-h (p �

0.001) [11]. It does not appear that caffeine intake was con-trolled in this study, a significant weakness since the adverseeffects of dehydration may have been the result of caffeinewithdrawal. Caffeine withdrawal can result in headache insusceptible individuals, increase tiredness and decrease alert-ness, symptoms that were all observed in this study [18].

CONCLUSIONS ANDRECOMMENDATIONS FORFUTURE RESEARCH

Due to the lack of data, definitive conclusions regarding theeffects of fluid restriction on cognitive performance are notpossible. However, it is clear that dehydration induced by heatexposure, exercise, and fluid restriction impairs cognitive per-formance and mood. Adverse effects are present with levels ofdehydration at 1.3% body weight loss and may be present whenless dehydration is present. Since the few published dose-response studies did not employ state-of-the-art behavioral testsknown to be sensitive to the effects of experimental treatmentson cognitive performance, they may underestimate the adverseeffects of hypohydration. For discussions of the importance oftest sensitivity in behavioral studies of nutritional factors orsleep deprivation, see Balkin et al. [19] and Lieberman [3]. Inaddition, the tests used did not permit determination of thespecific behavioral functions affected by dehydration.

Future studies should be conducted using both heat/exer-cise-induced dehydration and fluid restriction alone. A batteryof behavioral tests known to be sensitive to various nutritionaland environmental factors, and that can assess specific behav-ioral functions, should be employed in such studies [3]. Asdiscussed above, the optimal design for such studies wouldassess dehydration at a graded range of levels. In addition, a“positive” control should be included in such studies, such asalcohol ingestion or treatment with a drug with hypnotic prop-erties, such as diphenhydramine or a benzodiazepine [20–22].A positive control will provide a metric for relating any effects

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of hypohydration to a factor with a known ability to impaircognitive performance. In addition, all studies should attemptto disguise the treatment condition from the volunteers and theinvestigators. Inclusion of a positive control condition, andmatched placebo treatment for that treatment, will help todisguise the dehydration condition as placeboes could be ad-ministered in all arms of the study. Alternatively, an intrave-nous line (IV) with the flow rate hidden from the volunteer andthe investigators responsible for conducting behavioral testing

could be used to maintain hydration. In the dehydrating con-dition, no fluid would be provided through the line [23].

ACKNOWLEDGEMENTS

This work was supported by the U.S. Army Medical Re-search and Materiel Command (USAMRMC). The views,opinions, and/or findings in this report are those of the authors,and should not be construed as an official Department of theArmy position, policy, or decision, unless so designated byother official documentation. Citation of commercial organiza-tion and trade names in this report do not constitute an officialDepartment of the Army endorsement or approval of the prod-ucts or services of these organizations.

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Received July 16, 2007

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