Hydration for recreational sport and physical activity

Robert W Kenefick and Samuel N Cheuvront

This review presents recommendations for fluid needs and hydration assessment forrecreational activity. Fluid needs are based on sweat losses, dependent on intensityand duration of the activity, and will vary among individuals. Prolonged aerobicactivity is adversely influenced by dehydration, and heat exposure will magnify thiseffect. Fluid needs predicted for running 5–42 km at recreational paces show thatfluid losses are <2% body mass; thus, aggressive fluid replacement may not benecessary. Competitive paces result in greater fluid losses and greater fluidneeds. Fluid needs for recreational activity may be low; however, carbohydrateconsumption (sport drinks, gels, bars) can benefit high-intensity (�1 h) andless-intense, long-duration activity (�1 h). Spot measures of urine color orurine-specific gravity to assess hydration status have limitations. First morning urineconcentration and body mass with gross thirst perception can be simple ways toassess hydration status.© 2012 International Life Sciences Institute

INTRODUCTION

Recreational physical activity is broadly defined by the USDepartment of Health and Human Services as“any bodilymovement produced by the contraction of skeletalmuscle that increases energy expenditure above a basallevel.”1 This definition encompasses numerous activitiesfrom gardening and dancing to hiking, running, orcycling. Individuals who participate in these activities orexercise modalities may participate on an individual basisor may train and compete in a sport as a member of a clubor community-based team or league. Whatever the activ-ity, the primary purpose of recreational physical activityis usually participation, with related goals of improvedhealth, physical fitness, fun, and social involvement. Rec-reational sport/physical activity is typically perceived tobe less stressful, both physically and mentally, on the par-ticipants relative to competitive sport. However, in prac-tice, the division between the concepts of recreation andcompetition is often blurred and definitions of eitherpursuit are somewhat arbitrary because many individualstrain and compete in casual sporting activities in a highlycompetitive manner. Regardless of the degree of commit-ment to recreational sport or physical activity, the role of

hydration remains equally important. However, fluidneeds for individuals are highly variable depending uponthe amount and intensity of physical activity and envi-ronmental conditions,2 among other factors. Becausehydration plays an important role in the ability toperform prolonged athletic and recreational sport/physical activities, individuals must be able to monitortheir hydration status accordingly. Thus, the purpose ofthis review is to provide general recommendationsregarding fluid needs for recreational sport/physicalactivity and to present an overview of hydration assess-ment and the impact of hydration on performance.

ENVIRONMENTAL HEAT STRESS, DEHYDRATION,AND PERFORMANCE

Fluid needs are likely to be low for many recreationalsport/physical activities that are intermittent, of shortduration (<1 h), and of lower exercise intensity. However,because many individuals partake in recreational sport/physical activity in a competitive manner, performance isan important consideration and, accordingly, hydrationwill be a concern. Recreational sport/physical activities

Affiliation: RW Kenefick and SN Cheuvront are with the US Army Research Institute of Environmental Medicine, Natick, Massachusetts, USA.

Correspondence: RW Kenefick, US Army Research Institute of Environmental Medicine, Thermal and Mountain Medicine Division, KansasStreet, Natick, MA 01760, USA. E-mail: Robert.Kenefick@us.army.mil. Phone: +1-508-233-6344. Fax: +1-508-233-5298.

Key words: fluid needs, leisure activity, predicted sweat loss

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Supplement Article

doi:10.1111/j.1753-4887.2012.00523.xNutrition Reviews® Vol. 70(Suppl. 2):S137–S142 S137

include those that are predominantly aerobic or anaero-bic or some mix of the two. Activities that primarilyrequire aerobic metabolism and are prolonged will bemore adversely influenced by dehydration than tasksthat require anaerobic metabolism or muscular strengthand power.3 Unless the types of activities are repeatedbouts,4 the majority of literature in this area suggests it isunlikely that dehydration degrades muscular strengthand power5,6 or anaerobic performance7,8 in any meaning-ful way.

Although the specific mechanism of action is asubject of debate, there is consensus that body water defi-cits will often degrade aerobic exercise performance.9 Acomprehensive review10 of a number of studies investi-gating the impact of dehydration on physical exerciseperformance reported that in the majority of studies,exercise performance decreased with levels of dehydra-tion of 2% body mass loss, and this reduction in exercisecapacity was further accentuated when combined withheat stress.11 In addition, as the level of dehydrationincreases, aerobic exercise performance is degraded pro-portionally and the magnitude of performance decre-ment is likely related to environmental temperature andexercise task. A recent review by Sawka et al.12 highlightsthe impact of high skin temperature in combination withdehydration on the impairment of submaximal aerobicexercise performance. This review presents an overviewof work supporting the concept that dehydration impairssubmaximal aerobic performance and that the effect isaccentuated as skin temperature (i.e., ambient tempera-ture) increases (Figure 1).12–15 It is important to note thatby maintaining a well-hydrated state during exercise bydrinking small amounts of fluid, exercise performanceimpairment due to dehydration can be abated.16

WATER AND ELECTROLYTE NEEDS FOR RECREATIONALSPORT/PHYSICAL ACTIVITY

Physical activity results in increased water requirementsthat parallel sweat losses for evaporative heat exchange.17

Heat gain from metabolism is balanced by both dry andevaporative (sweating) heat loss, but very high metabolicrates coupled with warm weather result in a larger bio-physical requirement for evaporative cooling,18 leadingto greater sweat losses and consequently larger waterrequirements. Thus, the magnitude of sweat lossesincurred during recreational sport/physical activity isdependent primarily on metabolic rate, but also on exer-cise duration and environment.17 Usually hydration statusfluctuates narrowly within the course of a day,19 butphysical activity can increase water losses substantially.The adequate intake for total water per day is 3.7 L foryoung men and 2.7 for women. These values include the

fluids contained in foods, which contribute approxi-mately 20%.20 The daily water needs alone for sedentarymen are reported to be around 1.2 to 2.5 L and increaseto approximately 3.2 L if performing modest physicalactivity.21 Compared to sedentary adults, active adultswho live in a warm environment are reported to havedaily water needs of approximately 6 L,22 and highlyactive populations have been reported to have markedlyhigher values (>6 L).23 It is important to note thateven under standardized conditions, in fairly homoge-neous populations, inter-subject sweating variability issignificant.24–26 In addition to air temperature, other envi-ronmental factors such as relative humidity, air motion,solar load, and protective clothing will influence heatstrain and thus water needs. Other factors can alter sweatrates and ultimately fluid needs. For example, heat accli-matization results in higher and better sustained sweatingrates, whereas aerobic exercise training has a modesteffect on enhancing sweating rate responses.27

Recreational sport/physical activity comprises abroad range of activities. The individuals who partake inthese activities are of varying levels of fitness and bodysize (among other factors); thus, precise fluid recommen-dations are difficult to determine. To illustrate fluid needsfor a range of exercise durations and for individuals ofdiffering abilities and body masses, the recently developedsweat prediction equation of Gonzalez et al.2 was used topredict sweat losses for two hypothetical recreational

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Figure 1 Percentage decrement in submaximal aerobicperformance (versus well hydrated) as a function ofskin temperature (Tsk) when dehydrated by 3–4% ofbody mass. Data are means (error bars are 95% confidenceintervals) compiled from three studies13–15 employingsimilar experimental procedures and time trial (TT) perfor-mance tests. Closed circles (black) represent 15-min TT tests,open circles (white) represent 30-min TT tests, and the grayband is the performance coefficient of variation (5%). Repro-duced from Sawka et al.12 (public domain).

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runners over distances of 5 km to the marathon (42 km).Running was chosen as an example because it is acommon component of most sports and is a continuousactivity that can be modeled with greater accuracy.2

These predictions were made for temperate (22°C) andwarm (30°C) conditions common to mass-participationrunning events28 and using typical finishing times of lesscompetitive (Figure 2) and highly competitive recre-ational runners (Figure 3). These fluid losses were thenexpressed as the percent loss in body mass over the dura-tion of the competition. As previously mentioned, athreshold of 2% loss of body mass is considered animportant threshold, as losses >2% are associated withdecrements in aerobic exercise performance.29,30 Whatcan be observed is that for most recreational runners,typical finishing times result in fluid losses <2% of bodymass for distances up to 21 km and it is not until themarathon distance in hot conditions (30°C) that heavierindividuals (80 kg) lose >2% of body mass by the very endof the event. For more competitive recreational runners(Figure 3), fluid losses are more appreciable and bothsmaller and larger runners exceed 2% loss of body

mass in both warm and hot conditions during the mara-thon but are below 2% loss for the other distances(5–21 km).

These predictions are important because they illus-trate that fluid replacement becomes increasingly criticalfor recreationally competitive athletes as exercise dura-tion increases and weather warms. For individuals whoare less concerned with performance and are exercisingor performing activities at lower intensities, a fluidreplacement plan may not be as important because fluidlosses do not appear to approach 2% body mass loss. Aspreviously mentioned, sweat rates can be highly variable26

and the predicted group sweating rates2 and fluid lossespresented (Figures 2 and 3) may be greater or smaller fora particular individual. It is therefore recommended thatindividuals concerned with performance determine theirown sweat rate. This can be accomplished by measuringacute changes in body weight before and immediatelyafter exercise. Exercise is best done in the environmentalconditions and at the exercise intensity anticipated forcompetition. In the absence of drinking, change in bodyweight can be used as an approximation of the volume of

Moderate Pace, 22°C

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Figure 2 Percent loss in body mass predicted from sweat rate for recreational runners of average ability weighing60 kg (clear bars) and 80 kg (speckled bars) during 5 km (25 min), 10 km (60 min), 21 km (130 min), and 42 km(270 min) road races. Dotted line demarks 2% body mass loss. Losses assume no fluid intake.

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Figure 3 Percent loss in body mass predicted from sweat rate for competitive recreational runners weighing 60 kg(clear bars) and 80 kg (speckled bars) during 5 km (21 min), 10 km (43 min), 21 km (95 min), and 42 km (200 min)road races. Dotted line demarks 2% body mass loss. Losses assume no fluid intake.

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sweat lost (e.g., 1 kg = 1 L), although there may be somesmall sources of error in this assumption.31

CARBOHYDRATE AND ELECTROLYTE NEEDS

Absolute fluid needs may be relatively low for shorter-duration activities (<60–90 min), especially in coolerenvironments. For activities performed for longer dura-tions especially in warm/hot environments, intake ofelectrolytes and supplemental carbohydrate can beadvantageous for retention of fluid and exercise perfor-mance. The average American diet contains approxi-mately 3.5 g/day (~150 mEq) of sodium20 in comparisonto sweat sodium concentration, which averages approxi-mately 35 mEq/L (range, 10–70 mEq/L).32 Therefore,sodium supplementation is generally not necessaryexcept if performing very heavy activity during the firstseveral days of heat exposure, as normal dietary sodiumintake otherwise appears adequate to compensate forsweat sodium losses.30 Although the inclusion of electro-lytes in sport drinks is less important as a means ofreplacing those lost during recreational sport/physicalactivity, they do play important roles in retention of fluidand fluid acquisition.33

Carbohydrate consumption can be beneficial tosustain exercise intensity during high-intensity exerciseevents of >1 h, as well as less intense exercise events sus-tained for even longer periods.34 Sport drinks containinga carbohydrate-electrolyte solution between 5% and 10%,consumed at 1 L/h or 1 g/min, have been shown to helpmaintain blood glucose levels and exercise performance35

and enhance fluid retention by the kidneys. Sport drinksattempt to provide both adequate fluid replacement andcarbohydrate supplementation. However, if fluid needsare low (e.g., cooler conditions, slower paces; Figures 2and 3) but the need to provide carbohydrate exists(activity > 1 h), it may be difficult to deliver carbohydrateat a rate of 1 g/min. In such cases, supplementation with amore concentrated sport drink or nonfluid carbohydratesources such as gels, energy bars, and other foods can helpto sustain exercise performance.

Children and prepubescent youth are likely to takepart in recreational sport/physical activity to a greaterdegree than adults, but have been studied to a lesserextent. Children have been reported to have lower sweat-ing rates than adults and these lower sweating rates areprobably the result of smaller body mass and thus a lowermetabolic rate. In addition, children’s sweat electrolytecontent has been reported to be similar to or slightlylower than that of adults.36 Given their lower sweat rates,in addition to the fact that organized recreational sportpractice and competition for youth tends to be shorterduration, intermittent activity with numerous fluidand rest breaks, fluid replacement and carbohydrate/

electrolyte supplementation is less of a concern. However,recreational sport for pre- and post-pubertal youth can beof a highly competitive nature and can involve prolongedpractice sessions and intense competition multiple timesa day. In these cases a sport drink or other nonfluidsources of carbohydrate and electrolyte may be advisable.

HYDRATION IN THE WORKPLACE

Hydration in the workplace is a specific concern becausedehydration can affect productivity and safety. It shouldbe noted that a more comprehensive overview of thistopic than presented here is available.37 During physicalwork in the heat, sweat output can often exceed waterintake, which can lead to dehydration. Bishop et al.38

observed that, in simulated industrial work conditions,encapsulated protective clothing increased sweat rates upto 2.25 L/h. Given such high fluid losses, maintainingfluid balance can be difficult during the work day, espe-cially during shifts of greater than eight hours. Workersmay not only become dehydrated on the job but mayremain in this state until the start of the next work day.Few studies exist on the impact of dehydration on manuallabor productivity; however, one investigation reportedthat a >1% body mass loss resulted in a 12% reduction inwork output.39 Dehydration has also been shown toadversely influence decision making and cognitiveperformance,40–42 which may contribute to a decline inproductivity and could be associated with increased riskof work-related accidents.43 Informing individuals, espe-cially those who work in a hot environment, about signsand dangers of dehydration and strategies in maintaininghydration while working can reduce dehydration in theworkplace. In addition, consumption of meals can alsoplay an important role in helping to stimulate the intakeof additional fluids and restoration of fluid balance.33 Fur-thermore, many foods contain substantial amounts ofwater. It is also important to note that in both recreationalsport/physical activity and workplace settings, accuratehydration assessment is a key to fluid intake guidance.

HYDRATION ASSESSMENT

Presently there is no scientific consensus regarding thebest method to assess hydration status, particularlyoutside the laboratory.20 Recreational sport/physicalactivity most commonly results in hypotonic fluid losses(sweat), which increases the relative concentrations ofblood and urine. Field expedient measures to assesshydration status are available such as body mass, urine-specific gravity and color, and sensation of thirst.However, each has limitations that should be taken intoaccount to allow for accurate results. Changes in body

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mass are often used for rapid assessment of acute hydra-tion status in both laboratory and field settings and areoften the standard against which other methods of hydra-tion assessment are compared. Acute changes (overhours) require a valid baseline and control of confound-ing variables. When using changes in body mass, it isassumed that the acute loss of 1 g is equivalent to the lossof 1 mL of water. This method is most effective when thepre-exercise baseline body mass is measured when theindividual is well hydrated. First morning nude bodymass that deviates by >1% (1–2 lb or 0.5–1 kg) also sug-gests the presence of dehydration when combined withmarkers of urine and thirst.

As a screening tool to dichotomize hydration state(well hydrated versus dehydrated), measures of urineconcentration can be a reliable assessment technique44–46

with reasonably definable thresholds. Measures of urine-specific gravity and urine color are simple to measure in afield setting. Urine color can be compared against a urinecolor chart44 or assessed relative to the degree of darknesswith darker yellow/brown urine color indicative of agreater degree of dehydration. However, urine-specificgravity and urine color can be easily confounded whenproper controls are not employed, such as when they areobtained during periods of rehydration47 or after exercisewhen glomerular filtration rate has been reduced.48

However, use of the first morning void following an over-night fast minimizes confounding influences and maxi-mizes measurement reliability. A urine-specific gravityvalue that is above 1.02030,44 has been suggested to indi-cate a state of dehydration. However, given elevatedurine-specific gravity values that can occur as a result ofdiet,49 heavy daily exercise,50 high lean body mass, andhigh protein turnover, values below 1.025 may be moreindicative of a normal hydration state.45,50

Sensation of thirst is a qualitative tool that can beused for hydration assessment. While genuine thirstdevelops only after dehydration is present and is allevi-ated before complete rehydration is achieved,19 it is theonly reliable subjective sensation that can be reported inresponse to dehydration.51 Sensation of thirst works wellonly at rest,51 which limits its use during exercise becauseit is usually not perceived until 2% body mass loss hasalready occurred.52 Lastly, use of sensation of thirst toassess hydration status has limited application to childrenand elderly persons53 as this sensation may be alteredcompared to adults.

A simple Venn diagram decision tool does exist thatcombines markers of hydration, including weight, urine,and thirst (Figure 4).54 By themselves, no one marker pro-vides enough evidence of dehydration; however, the com-bination of any two simple self-assessment markersmeans dehydration is likely, and the presence of all threemakes dehydration very likely. In a field setting, in which

a scale may not be available for measures of body mass,the combination of first morning urine color and thirstmay provide a reasonable indication of the presence ofdehydration.

CONCLUSION

In conclusion, this review presents general recommenda-tions regarding fluid needs for recreational sport andphysical activity. In regard to the importance of hydra-tion, a 2% body mass loss is a recognized thresholdbeyond which exercise dehydration impairs aerobic exer-cise performance in temperate conditions and this isaccentuated by warm and hot environments. If fluidrequirements are low but the need for carbohydratesupplementation exists, more concentrated sport drinksor gels, energy bars, and other foods can help to sustainexercise performance. Dehydration in the workplace canaffect productivity, safety, and morale. Strategies to main-tain hydration while working can help reduce dehydra-tion. In either a workplace or field setting, thecombination of change in body mass, urine color, andthirst can be used to assess hydration and determine theadequacy of fluid intake practices.

Acknowledgments

The authors thank Dr. Michael N. Sawka for his editorialcontributions and Ms. Mary Pardee for her technical

Figure 4 Venn diagram showing the likelihood of dehy-dration related to certain conditions. From Cheuvrontand Sawka54 (public domain).Abbreviations: W, weight; U, urine; T, thirst.

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assistance. The opinions or assertions contained hereinare the private views of the authors and should not beconstrued as official or reflecting the views of the Armyand the Department of Defense.

Declaration of interest. The authors have no relevantinterests to declare.

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