NUTRITION

Hydration Recommendations for Sport 2008Scott J. Montain

Military Nutriiion Division, U.S. Army Research Institute of Environmental Medicine, Natick, MA

MONTAIN, S.J. Hydration recommendations for sport 2008. Curr. Sports Med. Rep., Vol. 7, No. 4, pp. 187-192, 2008. Fluidreplacement remains an important sírateí,^ for preserving exercise- performance as dehydration in excess of 2% of body weight consistentlyimpairs aerobic exercise perfonmnce. Too much of a good thing, however, can have negative heakh comequences as persistent drinkingin excess of sweating rate can induce symptomatic exercise associated hypontremia. This short review highlights new position standsandlor policy statements regarding fluid replacement for sport, evidence that laboratory findings tramlate to team sport performance,and current hydration practices of athletes, h is culminated with tyractical strategies for drinking appropriately during physical activity'.

INTRODUCTION

It is broadly accepted that fluid replacement is animportant strategy for sustaining exercise performance.The basis for encouraging drinking is the consistentobservation that dehydration in excess of ~2%-3% bodymass (-1.5-2 liters of water for -70 kg individual) impairsaerobic exercise performance. To facilitate replacement,tluids are made readily available to athletes during mostsporting events.

Too mucb of a good thing, however, can produce adverseconsequences. Persistent drinking iti excess of sweating rateis associated with the development of exercise-associatedhyponatremia (EAH) a potentially lethal condition (1-3).While EAH ca.ses have primarily been limited to partic-ipants in marathon running races and ultramarathonendurance races (4). the condition can afflict atiy athletewith easy access to water or other electrolyte-poor drinkswhen they have relatively low sweating rates and a desire todrink copious amounts of fluid (5). It was the apparentincrease in EAH incidence, particularly in events such astbe 42 km marathon where E A H is an unexpected medicalevent that prompted critics (6,7) to charge professionalorganizations such as the American College of SportsMedicine (ACSM) with ovenealous promotion of fluidreplacement. In response, several professional organizationsrevised their Huid replacement advice.

Aiidrc;.i;.s fin anrespoijJtn«; Scott J. Montaiti, Ph.D., FACSM, Military NiitriiUinIlivjsmn, USARIEM. BMg 42, Kansas St.. Natick, MA 01746 (E-mail;-nitt.moniiim®iis.amiy.mii).

lS37-89ÜX/0704/m7 192(..uTTeiif Spcr« Medicine Reporií*.;opyrij;ht © 2008 hy the American College <if Sports Medicine

Despite years of active investigation into the physiolog-ical consequences of water deficits, much of what we knowregarding the consequences (if dehydration is from labora-tory experiments or field experiments investigating perfor-mance outcomes using relatively simple and quantifiabletests. However it is not known if the outcomes from tbeexisting scientific literature are transferable to tbe complexnature of team sports. Moreover, most of the experimentshave been limited to temperate and relatively warmenvironments. It is also not known if the same level ofwater deficit will compromise performance in coolertemperatures or if greater water deficit can be accruedbefore performance suffers. The interaction between tbelevel of dehydration and environmental temperatures is justbeginning to be characterized.

Tbe purpose of this article is to higbligbt some of tbe newpublications dealing with bydration and their contribuiiotisto our current understanding regarding fluid imbalance andfluid replacement recommendations for sport. Tbe impact ofexercise-induced fluid imbalances on performance/healthhave been the topic of a number oí recent review papers(8-11 ), and the reader is referred to those reviews for greaterdetail regarding tbe physiological consequences, risk factors,and medical management of water imbalance.

NEW FLUID REPLACEMENT POLICYAND POSITION STANDS

Over the past few years, several new and/or updated fluidreplacement policy or position stands have been publishedhy professional organizations involved in sport or sport-relevant research. The International Olympic Committee(IOC) in 2003 held a consensus conference on tbe topic ofnutrition for sports that considered fluid replacement. The

187

products of this meeting include a published consensusstatement on Sports Nutrition (12), a review paper withrecommendations for fluid and electrolyte replacement (13),and a practical guide written for athletes (H)- TheInternational Amateur Athletic Federation (lAAF) pub-lished a policy statement on fluid replacement (15) as wellas practical guidance geared to the track and field athlete(16). In 2001, ACSM released an updated Exercise andFluid Replacement Position Stand (9). A common theme ofeach document is that some dehydration is tolerable; thegoal of fluid replacement should be to prevent dehydratiun(water loss) in excess of 2% body mass from accming duringactivity. Moreover, each document includes language thatdrinking in excess of sweating rate should be avoided.

The 2007 ACSM Position Stand (9) includes a summary ofcurrent knowledge regarding exercise and fluid replacementas well as the impact of flu id-electrolyte imbalances onexercise performance and health. A new feature is theinclusion of Strength of Recommendation Taxonomy(SORT) to weigh the strength of evidence for eachconclusion and recommendation. The document considersthe viirioas factors that contribute to and/or modify fluid andelectrolyte requirements, describes practical techniques foras.sessing hydration state, and includes drinkiníí strategies forbefore, during, and after exercise. Tlie new Position Standstates that "the goal of drinking during exercise is to preventexcessive (>2% hsxly weight toss from water deficit) dehy-dration and excessive changes in electrolyte balance to avertcompromised perfomiance." Because of considerable varia-tion between subject differences in sweating rate and sweatcomposition, the Position Stand recommends tbe athletestake personal responsibility for sustaining their bydration anddeveltip CLLStomized fluid replacement programs.

THE IMPACT OF DEHYDRATION ON EXERCISEPERFORMANCE DEBATED

As part of the Contrasting Perspectives series in Medicine& Science in Sports & Exercisi.',«, Michael N. Sawka, Ph.D.,FACSM, U.S. Army Research Institute of EnvironmentalMedicine and Timothy 0. Noakes, M.D., FACSM, Uni-versity of Cape Town, recently (17) provided opposingperspectives to the question "Does dehydration impairexercise performance?" Dr. Sawka presented the prevailingview that performance can be expected to degrade whenwater deficits exceed - 2% body mass, whereas Or. Noakespresented the challenging view, and conceded that dehy-dration degrades aerobic exercise performance. However,Or. Noakes questioned the practical relevance of the"classically cited studies" to the real world of competitivesport and recreational exercise. This spirited debateprovides an excellent summary of the evidence andillustrates why organizations such as ACSM, IAAF, andothers have policy statements endorsing fluid replacementto sustain exercise performance. It also provides the(ipportunity for the reader to consider the merits of Dr.Noakes' provocative alternative hypothesis that it is notthe consequences of dehydration (e.g., increased osmolalityand hypovolemia) that lead to compromised performance.

hut instead it is the development of thirst. Accordingly, herecommends that athletes dritik according to the dictatesof their thirst.

NEW INSIGHTS INTO THE NEUROBIOLOGYOF THIRST AND SALT APPETITE

Our ability to sense thirst and develop a salt appetite ismade possible through an extensive neural network and theintegration of information from multiple input pathways.Allen Kim Johnson, Ph.D., recently provided an extensivereview that delineated the limits of our current under-standing of thirst and salt appetite (18). This active area ofresearch has included new insights into the contribution ofmechanoreceptors in botb stomach and proximal smallintestine to modulate thirst (19), as well as the neuro-anatomic pathways that link osmotic signals arising fromthe lamina terminalis cells to those brain regions responsiblefor changes in arousal and affect (20).

LABORATORY RESULTS TRANSLATE TOTEAM SPORTS

Laboratory studies investigatinfi the consequences ofdehydration on performance have consistently found thatdehydration has little oí no effect on muscle strength orballistic power (8,21) hut impairs the ability to performaerobic exercise (8,17). Until recently, there was a paucityof studies investigating the threshold and functiitnal impactof dehydration on team sport perfonnance and/or individualsports demanding high levels of skill/precision such astennis, stKcer, and boxing. Over the past several years,more information regardin}j the consequences of dehydra-tion on specific aspects oí team sport performance has begunto etnerge.

To test the effect of water deficit on sticcer performance,McCregor et al (22) examined the impact of drinking versusnot drinking on abiliry to complete a 90 min variableintensity shuttle run with timed-embedded 20 meter all-outsprints, followed hy a soccer dribbling test. The semi-professional soccer players who participated dehydrated hy-2.5% of initial body mass when fluids were restricted. Fluidrestriction was accompanied by greater perception of effortlate in the shuttle run protocol as well as degraded ability tosprint late in the .shuttle run. Time to complete the soccerdribbling task was reduced 5% when no fluid was consumed,hut maintained when fluids were consumed. A subsequentstudy by Edwards et al, (23) reported that soccer playersaccruing modest dehydration during; exercise and matchplay (fluid deflcit - 2% of initial body mass) perceivedexercise and match play as more difficult compared to whenfluids were consumed during hreaks and finished with ahigher internal btxly temperature. Consistent with McGregoret al, performance time was worse on the variable intensityrunning test designed to mimic the running pattern of a.soccer match when fluids were restricted. A novel aspect ofthe Edwards et ai. study was the inclusion of repeated mouthrinsing to attenuate subjective perception of thirst. While

188 Current Sports Medicine Reports www.acsm-csmr.org

mouth rinsing was somewhat effective at suppressinfj; thirst, itwas ineffective tor pre^ervin i performance during; match play.The importance of this ohservation is that it sLiggests that theskiwing of running pace was attrihutahle to the physiologicalchallenge(,s) imposed by excessive water deficit rather thandevelopment of thirst. Together, the two studies suggest thatsoccer players who accrue water deficits >2% htxJy mass willhave impaired ability to challenge for loose balls or drihhieb;ill with the same velocity as when euhydrated. The effectson shot accuracy, passing, and most importantly on soccerteam performance have not been examined.

Like soccer, ha.sketball playin^ ability appears sensitive towater deficits in excess of ~2%-3% body mass. To examineimpact of water deficit on haskethall skill perfonnanceDoufiherty et ai. (24), and Baker et al. (25), had volunteersinitially exercise in a hot room with and without fluidreplacement followed by a simulated basketball game thatcontained emhedded basketball skill tests. Dougherty et al.(24), reported that I2'to 15 yr-old players with water deficitoí -2% body mass made fewer basketball shots and wereslower at sprinting and lateral movement tests. Baker et ai.,(25) manipulated drinking during the exercise-heat stresspreceding a simulated basketball game so that the highly-skilled haskethall players who participated performed aprescrihed set oí basketball drills embedded in the simulatedbasketball game with 1%, 2%, 3%, and 4% body mass lossattrihutahle to dehydration. Consistent with Doughertyet £ii., time to complete a series of running drills was slowerwhen dehydrated greater than or equal to 2% body mass andthe magnitude of slowing increased with water deficit.While stationary shooting ability was remarkably consistentup to 4% body mass loss, the players attempted fewer shotsKnd were less able to make shots linked with movement(e.g., lay-up) when dehydration had accrued to 3% andshooting was further impaired when performed when 4%dehydrated. Together, these studies support that perfor-mance degrades after body water deficits equal to --2%-3%body mass. Given that sweating rates during basketball playcan be expected to be between 1-2 L/h (8) and producedehydration >2%-3% if fluids are not consumed, periodicdrinking during games should he encouraged so as tominimize the negative consequences of dehydration onbaskethal! ability. The mechanisms by which dehydrationcompnimises basketball shooting accuracy remain poorlyunderstood.

Devlin et al. (26) reported that cricket bowlers are lessaccurate when dehydration exceeds ~3% hody mass loss.Thus, the loss of accuracy is not unique to basketballshooting. It has been shown that hody sway is morepronounced after exercise and this effect is magnified wbendehydration is present (27,28). Explanations for thisdeterioration in balance control include changes in vestib-ular function and/or vestibular afferent sensitivity (28,29).As deterioration in posture control would be anticipated tumake it more difficult to reproduce complex motor move-ments, it may explain at least partially the decrease inshooting and throwing (fast bowling) accuracy accompany-ing dehydration.

The relationship between dehydration and performancemay be affected hy weather, Cheuvront et al. (30), recentlyrep(.)rted that performance decrements observed in temper-ate laboratory conditions when 3% dehydrated were nutapparent when the experiment was performed in coolertemperatures; suggesting that greater water deficits may betolerable during exercise in cold weather. The effect,however, appears modest as water deficits equal to 4%normal hody mass have been reported to compromiseendurance performance despite cool temperatures (31 ).Therefore, cool weather does not negate the need toconsider fluid replacement as a strategy for optimizing raceperformance, but may modestly increase the water deficitneeded to impair exercise perfiirmance.

ACCEPTABILITY OF CURRENT FLUIDREPLACEMENT PRACTICES

The cunent drinking behavior of athletes during horhtraining and competition has received recent attention andis summarized in the Tahle. Overall, it would appear thatmost athletes are choosing appropriate drinking practices, astheir behaviors driven hy thirst or by cognitive choice, arepreventing excessive dehydration. A recent survey (32) ofcollege athletes revealed that: 90% of athletes were awarethat dehydration can compromise exercise performance;80% were aware that drinking hefore competition is a goodpractice so as to start exercise well-hydrated; and 70% ofthose same athletes practiced pre-exercise drinking. Thus,cognitive choice could he a contributor to success of theirhydration practices. Somewhat troubling, however, is that

TABLE. Current drinking behaviors of athletes p:irticipatiny in various spcirts.

Source

(45)

(46)

(47)

(48)

(49)

(W)

Sport

Soccer

Soccer

Soccer

Tennis

Football

Football

Type of Training

Training

Training

Matcb play

Match play

Training

Training

Ambient Temp., °C

24-29

32

6-8

30-31

28-31

Z5 WBGT

Sweat U)ss, L

2.0(1.4-2.4)

2.2 (1.7-3.1)

1.5

4.2

V4

Fluid intake, L

1.0 (0.3-1.7)

1.0(0.2-1.7)

1.0 (0.1-2.2)

l.I

2.6

I.I

% Ri\]y Mtis.s Loss

1,4 (0.5-2.6)

1.6

0.9 (0-1.8)

0.9

1,7

1.0

Data arc mean (range), WBGT = wet bulb globe temperature.

Volume 7 • Number 4 • July/August 2008 Hydration Recommendations 189

while 66% of the collej e athletes surveyed recognized thatbody mass change is an effective method to evaluate theacceptability of drinking practices, only 15% actually checktheir body mass. As some college sports would not beexpected to produce excessive dehydration, either becausethe exercise intensities (and sweating rates) are too low and/or event duration is too short (e.g., the longest durationcollege track or cross country race is 10,000 meters), thismay partially explain why many athletes do not activelyassess the acceptability oí their fluid replacement practices.

PROBLEMS ASSOCIATED WITH OVERDRINKING -EXERCISE ASSOCIATED HYPONATREMIA

Symptomatic EAH most commonly develops in individ-uals participating in marathon and ultramarathon endur-ance activities. Slower, smaller participants appear to be atmost risk (3^,34). EAH arises primarily from persistentoverdrinking of fluids relative to sweating rate and theinability to excrete the relative fluid excess either during orin the initial recovery peritxl (4,5,10,35). However, there isboth theoretical (2) and observational (36,37) evidencethat during ultramarathon activities, fluid replacement withwater or other electrolyte-poor beverages can lead todilution of plasma sodium below 130 mEq/L withoutoverdrinking relative to sweating rate. As such, preventivestrategies must consider both drinking rate and electrolytereplacement.

Several notions exist regarding the etiology of EAH thatcause confusion hut are not supported by scientific evidence.Two such notions are that EAH is due to third spacing oíelectrolyte-free water (10) and/or movement i)f osmotically-active sodium into nori-ostnotic form/pool(s) (4). First,neither of these explanations are needed to explain thedevelopment of EAH, as the magnitude of sodium dilutionis predictable hased only on mass balance of water, sodium.

150

145

140

130

120

25 mEq/L

and potassium (2,38). Second, experimental induction ofhyponatremia leads to release of sodium from non-osmoticpools (e.g., bone) not vice versa (39). Therefore, it is veryunlikely that either third spacing or movement of sodiuminto non-osmotic pools contribute to the etiology of EAH.

Media attention to the negative consequences of exces-sive tluid intake has helped educate endurance sportparticipants that there is no advantage to drinking in excessof sweating rate and persistent overdrinking can in fact beharmful to not only performance but to health. Unfortu-nately, the death of a runner participating in the 2007London Marathon (3), is evidence that continuing educa-tion is necessary to eradicate this condition.

PUTTING HYDRATION SCIENCE INTO PRACTICEI

Prior to activities that prtxîuce vigorous and sustainedsweating, it is a good practice to drink 1 to 3 cups of wateror other fluid to increase the likelihood of starting theactivity well hydriited. During exercise, enough fluid shouldbe consumed to prevent excessive water loss (>2% of bodymass), but not so much that body mass increases. The easiestway to track acute hydration changes is to measure nudebody mass hefore and after exercise. If body mass falls morethan 2%, the individual is drinking too little; if body massincreases, the individual is drinking too much. If thecombination of environment, exercise intensity, and dura-tion result in fluid losses <2% of body mass withoutdrinking, then tluid intake can be ignored during exercise,but the athlete should réhydrate properly before their nextexercise bout. While sweating rate can be predictedreasonably accurately based on body mass chaiiges, it shouldhe recogni:ed that body mass is lost during exerciseindependently of sweat losses, due to the exchange ofoxygen and carbon dioxide, and respiratory water loss (40).When estimates of sweat rate are made during exercise

150

145

140

135

130

125

0 2 4 6 8Time, h

10 12120

50 mEq/L 150

145

140

135

^•^ü

0 2 4 6 8Time, h

10 12120

75 mEq/L

0 2 4 6 8Time, h

10 12

Figure. Predicted body mass loss (due to water deficit; left panel) for two 70-kg people of different body composition, running at 8.5 km h ' intemperate weather (18°C}, and drinking water at three rates (400 mL-h' ' (solid line). 600 mLh" ' (broken line). 800 mLh" ' (broken dotted line)). Thesolid shaded areas indicate when water loss would be sufficient to modestly degrade performance (upper), and when water loss would substantiallydegrade performance (lower). M marks the finishing time for a 42 km marathon, whereas IT marks the approximate finishing time for an ironman-distance triathlon, based on literature values for participants running the marathon portion at 8.5 kmh V Also predicted are plasma sodiumconcentrations for three rates sweat sodium loss (25. 50 and 75 mEg/L), Two lines sharing the same line style are the predicted outcomes for people oftwo different body compositions; with total body water accounting for 50% and 63% (leaner) of body mass. The hatched shaded areas denote thepresence of hyponatremia (plasma sodium concentration <130 mEg-L '). lAdapted from Montain, SJ. and S.N, Cheuvront, Fluid electrolyte andcarbohydrate reguirements for exercise. In Physiological Bases of Human Performance during Work and Exercise. 2008:563 573. Copyright fe' 2008Elsevier Limited. Used with permission,]

190 Current Sports Medicine Reports www.acsm-csmr.org

durations of —60 min, errors in prediction are fairly small.However, since metabolic gas exchanges and respiratorywater loss account for 5% to 15% of mass loss during physicalactivity, sweat tosses will be overestimated during multiplehour events if these contributors to mass loss are ignored.

During exercise lasting over 4 hours, the consumption ofelectrolytes can reduce the risk of developing hyponatremia(2,41). As illustrated in the Figure, EAH can be expectednot only when an individual persistently drinks in excess ofswearing rate (absolute overdrinking) hut also when fluidintake of water or electrolyte-poor drinks exceeds electrolytelosses (relative overdrinking). The actual need for electro-lyte replacement will, however, depend on the myriad offactors that determine sweat electrolyte losses (9,42). Bothexperimental studies (43) and theoretical models (2) havedemonstrated that electrolyte ingestion (in concentrationscomparable to commercial sports beverages, -20 mEq/L) canattenuate the development of EAH consequent to multiplehours of sustained sweating. If large sweat and electrolytelosses are anticipated during an athletic event, the electro-lyte deficits can be minimized hy ingesting salt-containingfood and beverages.

Acknowledgments

The atithor gratefully acknowledges the editorial assistance of MichaelN. Sawka, Ph.D., FACSM, Samuel N. Cheuvront, Ph.D., FACSM. anj.Andrew J. Young, Ph.L")., FACSM. The view, opinions, and/or findingscontained in this report are rhose of the authors and shtmld not he cunsttued;is an official Department of the Atmy position or decision, unless sodesignated by other official documentation. Approved for puhlic release;tiistrihution unlimited.

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