Investigating the associations between hydration and exerciseperformance: methodology and limitations

Ronald J Maughan

Loss of body water, if sufficiently severe, impairs most physiological functions, butthe body water content fluctuates over the course of a normal day with noimplications for physical or mental performance. The point at which an effect ofdehydration becomes apparent has been the subject of much debate, in part, atleast, because of the different tests that have been applied, differences in themethodologies used to induce dehydration and also because of differences in thefitness and other physiological characteristics of the subjects studied. The act ofdrinking itself and the conscious denial of access to water will also have implicationsfor subjective responses to the exercise task. In many published studies, it is difficultto separate the effects of ingestion of water from those of carbohydrate, electrolytes,and other drink components. Nevertheless, there is good evidence that drinkingappropriate amounts of water, especially cold water, can enhance exerciseperformance in many situations.© 2012 International Life Sciences Institute

INTRODUCTION

Deprivation of water for more than a few days inevitablyleads to death, though survival for up to 10–14 days ispossible if food intake is also absent. In the periodimmediately prior to death, all physiological functionswill be impaired, even if normal food intake is main-tained. There can be no doubt, therefore, that dehydra-tion, if sufficiently severe, can impair both physical andmental performance. There is a strong body of experi-mental evidence to support the idea that a substantialreduction in the body water content (perhaps 5–10%)will reduce performance in a range of exercise tasksinvolving strength, power, endurance, or skilled move-ment.1 Equally, small fluctuations in body water contentnormally occur throughout the day with no perceptibleeffect on physical or mental performance. There is,however, considerable debate as to the effects of inter-mediate levels of body water loss, such as those that arelikely to be incurred in daily living for some individuals,on exercise performance.2–4

ASSOCIATIONS BETWEEN HYDRATION ANDEXERCISE PERFORMANCE

The most appropriate question, perhaps, is whether thelevels of water loss encountered in normal activities ofdaily living, including those of elite athletes, military per-sonnel, and others exposed to hard physical exercise andclimatic extremes, can cause a loss of exercise capacity ora loss of cognitive function. This may be very differentfrom the effects observed in the artificial environment ofthe laboratory. In reviewing the literature, it is also essen-tial to consider whether the study population is relevantto the target population and whether the exercise test hasappropriate sensitivity and validity. In any assessment ofthe literature, there is also a need to consider whether anyeffects on performance of a reduction in body watercontent are a consequence of the process (dehydration)or the state (hypohydration) and to separate the effects ofhypohydration from those of the methods used to induceit. These include fluid restriction, with or without foodrestriction; exercise, which may induce hyperthermia; and

Affiliations: RJ Maughan is with the Loughborough University, School of Sport, Exercise and Health Sciences, Loughborough, UK.

Correspondence: RJ Maughan, Loughborough University, School of Sport, Exercise and Health Sciences, Loughborough LE11 3TU, UK.E-mail: r.maughan@lboro.ac.uk.

Key words: dehydration, endurance capacity, fatigue, hypohydration, water

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Supplement Article

doi:10.1111/j.1753-4887.2012.00536.xNutrition Reviews® Vol. 70(Suppl. 2):S128–S131S128

diuretic use. These methods have variable effects on elec-trolyte losses and this leads to differences in the distribu-tion of water losses between the different body watercompartments. There is also a need to quantify the mag-nitude of any hypohydration induced. This is normallybased on change of body mass, with the assumption that1 kg of mass loss is equivalent to 1 L of water loss;however, this may seriously overestimate the fluid deficitincurred in some situations.5

VALIDITY OF EXERCISE TESTS

The validity, reliability, sensitivity, and ecological valid-ity of tests commonly used as a measure of exercise per-formance have generated considerable controversy inrecent years. It is clear that most laboratory and fieldmeasures used to assess exercise performance are tooinsensitive to detect small, but potentially meaningful,changes in performance.6 In elite sports, the marginbetween victory and defeat is often vanishingly small.Likewise, in the industrial environment, small errors ofjudgment may have serious and potentially fatal conse-quences in occupations such as air traffic control or inthe operation of heavy machinery. Conclusions based onlaboratory tests should take into account the limitationsof those tests.

Until more sophisticated ergometers were developedin the 1980s, both treadmill and cycle ergometer testsusually consisted of exercise at constant speed or powerthat was sustained until the subject decided to terminatethe test. Constant power tests to volitional exhaustion arestill employed to examine the influence of various inter-ventions on performance, but this method of testing isfrequently criticized for a lack of ecological validity andpoor test-retest reliability. The findings of Jeukendrupet al.,7 who found a large day-to-day variability (coeffi-cient of variation, 27%) in time-to-exhaustion tests, and amuch smaller variability in a time-trial protocol (<4%),are commonly cited to support this view. Although somecontinue to voice concerns over a lack of ecological valid-ity, data from our research group report more consistentperformance (coefficient of variation, 6%) in time-to-fatigue tests8 and recent reports have highlighted similarerrors of measurement when changes in performance arenormalized across tests.9 A key factor to consider whenselecting an appropriate exercise test is its sensitivityand the smallest worthwhile effect that can be reliabilitydetected.10 Amann et al. demonstrated that time-to-exhaustion and time-trial protocols display a similar sen-sitivity to the effects of hypoxia and hyperoxia onperformance, and suggested that this finding will extendto other factors influencing performance.11 This isbrought about by larger effects on performance inresponse to an intervention with constant power tests

than are typically observed in time-trial protocols. Thiscompensates for the larger test-retest variability, resultingin a very similar signal/noise ratio to that seen with time-trial protocols.10,11 In some research situations, theobvious limitation of time-trial-type testing is a difficultyin comparing the effect of an intervention on the physi-ological response to exercise, because at any given timeone volunteer’s relative power output may differ greatlyfrom that of other participants. This can be overcome bythe addition of a period of constant load exercise under-taken before the time trial, as described by Jeukendrupet al.7 However, the resulting test still does not resemblethe real world of competitive sport or of the workplace.The ecological validity of time trial tests is often cited asan advantage when the intention is to apply the results tosports competitions. In reality, though, races are very dif-ferent from time trials performed in isolation in thelaboratory. Race pace in open competition is usuallydetermined by the complex interaction of the leadingrunners rather than by an individual in isolation. Fewcompetitors in a race have the luxury of choosing theirown pace to achieve the best possible time. The one-hourrecord attempt in track cycling is perhaps the only com-petitive event that resembles the laboratory time trial, buteven then, it is usual to conceal from laboratory subjectsthe distance covered.

A further disadvantage of time-trial tests is the needfor extensive familiarization trials before experimentaltrials are undertaken. Even with time-to-fatigue tests,there is a need for familiarization, but adequate familiar-ization in these tests can usually be achieved with one ortwo familiarization trials in subjects who are habituallyactive and who are accustomed to performing strenuousexercise. Because of the need for subjects to learn effectivepacing strategies, time-trial tests require a much greaternumber of familiarization trials. In many publishedstudies, however, there is no mention of adequate famil-iarization trials having been performed, nor is there anymention of whether results were tested for the presence ofan effect of trial order.8 It should also be clear that thechoice of tests may be strongly influenced by the reasonsfor doing the studies. Studies with applications for indus-trial or sporting contexts should take into account theenvironment in which performance will occur, butattempts to better understand the underlying science maybe better achieved with the more controlled conditions ofthe constant power-to-fatigue test.

Tests of endurance exercise performance have beenmore extensively studied than have tests of strength orpower in high-intensity exercise. However, similar con-siderations apply to other exercise tests.6 A comprehen-sive review of the reproducibility, validity, advantages,and disadvantages of a wide range of tests of physicalperformance and fitness was undertaken by Saris et al.6

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It is clear from the available literature that a rangeof different exercise tests can be used to investigateresponses to hydration status. However, it is equally clearthat whatever the test selected, care must be taken tostandardize test conditions if valid and reproducibleresults are to be obtained. This includes the provision ofsufficient familiarization trials to reduce any learningeffects6 and an appropriate interval between tests to limitany changes in fitness or acclimation status over the timecourse of a study.12

PRE-EXERCISE HYDRATION AND DEHYDRATIONINDUCED DURING EXERCISE

Studies investigating the association between hydrationstatus and exercise performance fall into two main cat-egories: those in which hypohydration is induced prior toexercise and those in which hypohydration is allowed todevelop as exercise progresses. The latter clearly must beof sufficient duration and intensity and the environmentmust be such as to induce significant levels of sweat loss.In practice, this usually means exercise lasting at least30–60 min carried out in a warm environment. Theformer can include tests of strength and power as well asof endurance, and can use a range of different methods toinduce hypohydration. In most cases, hypohydration isinduced by a combination of exercise and heat stress,which raises the challenge of separating the effects ofhypohydration and hyperthermia. It is well establishedthat hyperthermia has a negative effect on the capacity toperform endurance exercise,13 but the effects on short-term high-intensity exercise are less clear.14 This variabil-ity might account for some of the uncertainty as to theeffects of pre-exercise hypohydration on short-durationhigh-intensity exercise.3,4

Failure to control or account for the interactionsbetween hydration status, body temperature, and theacute fatiguing effects of exercise may limit the interpre-tation of studies. Different experimental models may beequally valid, but as with exercise tests, the test parametersmust be understood. Experimental models can be chosento allow the effects of hypohydration to be separated fromthe acute effects of exercise and disturbances of bodytemperature. This can be done by inducing hypohydra-tion the day before the experimental assessment andallowing or preventing restoration of water and electro-lyte balance in the intervening period.15 A short period ofrehydration to restore water deficits may or may notrestore cognitive function and there are likely also differ-ential effects on components of exercise performance.16 Ifthe aim is to investigate the effects of an occupational orexercise stress that induces an acute loss of body water,then it may be desirable not to separate these effects. In

synthesizing the available evidence, however, it is impor-tant not to mix studies carried out using different experi-mental models.

It is clear that there is a need for standardization ofmethods if sensitivity of the experimental model is to bemaximized, and the following suggestions might bemade. First, repeat trials should be performed on thesame day of the week and should be preceded byadequate familiarization trials. Absence of a learningeffect should be verified by analyzing data for a trialorder effect. Second, trials should be adequatelypowered to give confidence in the findings. Thereshould be standardization of eating, drinking, and exer-cise patterns for 48 h before each trial. Third, alcoholshould be avoided and caffeine consumption standard-ized (and limited, if necessary, rather than avoided com-pletely, so as to minimize any confounding effects ofcaffeine withdrawal) for 48 h before trials. Finally, sub-jects should ingest a 500-mL bolus of water 2 h prior totesting to achieve euhydration.17

When interpreting data, authors and readers shouldbe aware of the potential for confounding effects intro-duced by preventing drinking when subjects wish to doso, requiring subjects to drink when they do not wish todo so, providing drinks that subjects like or do not like,providing familiar or unfamiliar drinks, and controllingeffects other than those of hydration itself (carbohydratecontent, temperature, etc.). It is also important to recog-nize that endurance performance will be affected byambient temperature18 and humidity,19 so the choice ofenvironment may influence whether or not an effect isseen.

CONCLUSION

In summary, the effect of hydration status on perfor-mance of various exercise tasks has been extensivelystudied, but uncertainties remain. This arises in large partfrom the limitations of the studies that have been per-formed and the inappropriate interpretation and applica-tion of some of the findings from these studies. In spite ofthe uncertainties and contradictions in the published lit-erature, it is clear that severe reductions in body watercontent will impair performance and it is equally clearthat athletes can benefit from intake of an appropriateamount of a well-formulated drink. The recognition,however, that each athlete is different and that generalguidelines are of little value – and indeed may be danger-ous – is not new,20 although repeated studies have sinceemphasized the interindividual variability in the sweatingresponse to exercise, even in an apparently homogeneouspopulation under the same exercise and environmentalconditions.21

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Acknowledgments

Funding. The author’s research program has been sup-ported for many years by grants from a number of dif-ferent companies with an interest in the manufacture andsale of beverages.

Declaration of interest. The author has no relevant inter-ests to declare.

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