Sports Science Exchange 97

VOLUME 18 (2005) Number 2

Hydration Assessment ofAthletes

Samuel N. Cheuvront, Ph.D.
Michael N. Sawka, Ph.D. FACSM
Thermal and Mountain Medicine Division
U.S. Army Research Institute of Environmental Medicine
Natick, MA

KEY POINTS

• Although there is no scientificconsensus for 1) how best to assess the hydration status of athletes, 2) whatcriteria to use as acceptable outcome measurements, or 3) the best time toapply practical assessment methods, there are methods that can be used toprovide athletes with useful feedback about their hydration status
• Hydration assessment techniquesinclude 1) total body water measured by isotope dilution or estimated bybioelectrical impedance analysis, 2) plasma markers, such as osmolality,sodium, hematocrit and hemoglobin changes, or the concentrations of hormonesthat help regulate body fluids, 3) urine markers, such as osmolality, specificgravity, or color, 4) changes in body mass, and 5) other variables, such assalivary flow or gross, physical signs and symptoms of clinical dehydration.
• In most athletic settings, the useof body mass measurements in combination with some measure of urineconcentration at the first urination of the morning allows ample sensitivityfor detecting daily deviations from normal hydration (euydration). The methodsare simple, inexpensive, accurately distinguish euhydration from dehydration,and can therefore be used as a sole source for assessment.
• When more precision of acutehydration changes is desired, plasma osmolality, isotope dilution, and bodymass changes, used in appropriate context, provide for the accurate gradationsin measurement often required in research.
• Plasma markers (other thanosmolality), bioelectrical impedance analysis, saliva measures, and grossphysical signs and symptoms of dehydration are often confounded or tooinaccurate to reliably assess hydration of athletes.

INTRODUCTION

Body waterbalance represents the net difference between fluid intake and loss. Normalbody water turnover in a sedentary adult is from 1 to 3 L/day, the rangeaccountable primarily to differences in insensible water loss, or theevaporation of moisture from the skin (Sawka et al., 2005). Large variationsin fluid intake are controlled by the kidneys, which can produce more or lessurine, depending on changes in body fluid volumes. Water loss in air exhaledfrom the lungs is often ignored with respect to water balance because it isusually offset by water production occurring during aerobic metabolism (Sawkaet al., 2005). Over the course of a day, humans usually regulate daily bodywater balance remarkably well as a result of thirst and hunger drives coupledwith free access to food and beverage. This is accomplished by physiologicalresponses to changes in body water volume and to changes in concentrations of dissolvedsubstances in body fluids, as well as by non-regulatory social-behavioralfactors, such as drinking fluids at meetings and parties (Sawka et al., 2005).

Althoughminor perturbations in daily body water balance are easily restored tonormalcy, the imposition of exercise and environmental stress onto dailyactivity can seriously threaten fluid balance homeostasis, performance, andhealth (Panel on DRI., 2005). Abating these consequences is the underlying andunifying basis for developing guidelines for fluid intake before, during, andafter exercise (Casa et al., 2000; Convertino et al., 1996), but hydrationassessment remains a key component for ensuring full rehydration in athletesperforming frequent and intense exercise in hot weather.

Theselection of an appropriate hydration assessment method is a controversialaspect of fluid balance science (Oppliger & Bartok, 2002). All hydrationassessment techniques vary greatly in their applicability due to methodologicallimitations such as the necessary circumstances for measurement (reliability),ease and cost of application (simplicity), sensitivity for detecting small, butmeaningful changes in hydration status (accuracy), and the type of dehydrationanticipated (Oppliger & Bartok, 2002; Sawka et al., 2005).

Mostcircumstances involving strenuous physical exercise require the formation andvaporization of sweat as a principal means of heat removal. When sweat lossesproduce a body water deficit, the reduced volume of body fluids contains agreater than normal concentration of dissolved substances such as sodium andpotassium; this is known as hypertonic hypovolemia, the norm for dehydratedathletes (Sawka & Coyle, 1999). Clinicalhydration assessment techniques for detecting changes in hydration status relyheavily on this alteration in body fluid chemistry.

RESEARCH REVIEW

Objectives and Definitions

Thepurposes of this paper are to: 1) evaluate several common methods for assessinghydration status, 2) provide acceptable outcome criteria for the most accurateand reliable methods, and 3) offer application guidance for athletes andcoaches. Because considerable latitude is given when using terms common tohydration research, we define two here for clarity. "Euhydration" is a dynamicprocess rather than a static set-point (Greenleaf, 1992). It is mostaccurately defined as a normal total body water that fluctuates narrowly. Although dehydration and hypohydration have unique definitions, they are oftenused interchangeably as their differences are subtle. For this review, themore common term "dehydration" will be used in reference to a body waterdeficit.

Assessment Techniques

Population estimates offluid needs are based on qualitative and quantitative data (Sawka et al., 2005). Fluid intake surveys provide qualitative data, whereas water balance studiesand biochemical assessments offer quantitative support for the adequacy ofreported intakes. The combination of total body water and plasma osmolalityprovides the "gold standard" for hydration assessment.

TotalBody Water. The process of measuring waterbalance by collecting input and output data has been modernized by estimatingtotal body water (TBW), which entails measuring the dilution of trace amountsof an isotope (usually deuterium oxide, ²H₂O). Thedetails, assumptions, and limitations behind isotope dilution have beendiscussed elsewhere, but the accuracy of this method closely approximatesvalues measured by desiccation, i.e., the slow heating of cadaver tissue untilall water is removed (Ritz, 1998). In brief, a known volume and concentrationof isotope is taken into the body, and a new concentration of the isotope islater determined in a sample of body fluid (blood, saliva, etc.) after thetracer has become distributed equally throughout the body fluids. The unknownvolume (TBW) is then calculated, knowing that a low concentration of theisotope in the sample means that the body fluid volume must be relatively largeand vice versa. Like other quantitative techniques, isotope dilution does notallow determination of an adequate baseline due to the wide variability in bodycomposition and associated variability in normal total body water (Panel DRI2005). However, the total error of measuringTBW with tracer dilution is as low as 1% (Ritz, 1998), thus allowingmeasurement of small changes in body fluids.

PlasmaOsmolality. Plasma osmolalityis controlled around a euhydration set-point of ~285 mOsm/kg (Panel DRI 2005). Exercise sweat losses, if not replaced, reduce body water volume. Plasmavolume and extracellular water decrease because they provide the fluid forsweat, and plasma osmolality increases because sweat is hypotonic relative toplasma. In other words, sweat removes relatively more water from body fluidsthan solutes like sodium and chloride, and these osmotically active solutesbuild up in the blood plasma. The increase in plasma osmotic pressure isproportional to the decrease in total body water (Panel DRI 2005). Popowski etal. (2001) demonstrated under well-controlled conditions that plasma osmolalityincreases by ~5 mOsm/kg for every ~ 2% loss of body mass by sweating. Importantly, they also showed that plasma osmolality returns toward normalvalues during rehydration. Although field studies sometimes do not demonstratethis relationship, the discrepancy can be explained by environmentalconfounders such as altitude (Francesconi et al., 1987) or by small changes inhydration status (< 2% body mass) (Armstrong et al., 1994; Bergeron et al.,1995; Grandjean et al., 2003) that may fall within the normal fluctuating rangefor euhydration (Greenleaf, 1992).

These "goldstandards" of hydration assessment are good for sports science, medicine, orfor establishing reference criteria, but because they require considerablemethodological control, expense, and analytical expertise, they are not ofpractical use for monitoring day-to-day hydration status during training orcompetition. Table 1 should be consulted when choosing a complex hydrationmarker.

Simple Markers

UrineConcentration. Urinalysis is afrequently used clinical measure to distinguish between normal and pathologicalconditions. Urinary markers for dehydrationinclude a reduced urine volume, a high urine specific gravity (USG), a high urineosmolality (U_Osm), and a dark urine color (U_Col). Urineis a solution of water and various other substances, and the concentration ofthose substances increases with a reduction in urine volume, which isassociated with dehydration. Urine output is roughly 1 to 2 liters per day butcan be increased 10-fold when consuming large volumes of fluid (Sawka et al.,2005). This large capacity to vary urine output represents the primary avenueto regulate net body water balance across a broad range of fluid intake volumesand fluid losses from other avenues. Althoughit is impractical to measure urine volume on a daily basis, the quantitative(USG, U_Osm) or qualitative (U_Col) assessment of itsconcentration is far simpler. As a screening tool to differentiate euhydrationfrom dehydration, urine concentration as indicated by USG, U_Osm, or U_Colis a reliable assessment technique (Armstrong et al., 1994; Bartok et al.,2004; Shirreffs & Maughan, 1998) with reasonably definable thresholds.

Incontrast, urine measures often correlate poorly with "gold standards" likeplasma osmolality and fail to reliably track documented changes in body masscorresponding to acute dehydration and rehydration (Kovacs et al., 1999;Popowski et al., 2001). It appears that changes in plasma osmolality thatstimulate endocrine regulation of the reabsorption of renal water andelectrolytes are delayed at the kidney when acute changes in body water occur(Popowski et al., 2001). It is also likely that drink composition influencesthis response. Shirreffs and Maughan (1996) demonstrated that drinking largevolumes of dilute (hypotonic) fluids results in copious urine production longbefore euhydration is achieved. Urine concentration measurements can also beconfounded by diet, which may explain large cross-cultural differences in urineosmolality (Manz & Wentz, 2003). However, use of a sample from the first void(urination) of the morning following an overnight fast minimizes confoundinginfluences and maximizes measurement reliability (Armstrong et al., 1994; Fischbach,1992; Shirreffs & Maughan, 1998). Analysis of urinary specific gravity,osmolality, and color can therefore be used to assess and distinguisheuhydration from dehydration so long as the first void in the morning is used.

Body Mass. Body mass is often used to assess the rapid changes ofathlete hydration in both laboratory and field environments. Acute changes inhydration are calculated as the difference between pre- and post-exercise bodymass. The level of dehydration is best expressed as a percentage of startingbody mass rather than as a percentage of TBW because the latter varies widely (Sawkaet al., 2005). Use of this technique implies that 1 g of lost mass isequivalent to 1 ml of lost water. So long as total body water loss is ofinterest, failure to account for carbon exchange in metabolism represents theonly small error in this assumption (Cheuvront et al., 2002). Indeed, acute changesin body mass (water) are frequently the standard against which the resolutionof other hydration assessment markers is compared in the laboratory. In fact,if proper controls are made, body mass changes can provide a more sensitiveestimate of acute changes in total body water than repeated measurements bydilution methods (Gudivaka et al., 1999).

There isalso evidence that body mass may be a sufficiently stable physiological markerfor monitoring daily fluid balance, even over longer periods (1-2-wks) thatinclude hard exercise and acute fluid changes (Cheuvront et al., 2004; Leiperet al., 2001). Young, healthy men undergoing daily exercise and heat stressmaintain a stable body mass when measured first thing in the morning as long asthey make a conscious effort to replace sweat lost during exercise (Cheuvrontet al., 2004). Similarly, voluntary intakes of food and fluid compensate for sweatlosses incurred with regular exercise, resulting in a stable daily body mass(Leiper et al., 2001). Over longer periods, changes in body composition (fatand lean mass) that occur with chronic energy imbalance are also reflectedgrossly as changes in body mass, thus limiting this technique for assessment ofhydration. Clearly, if long-term hydration status is of interest and stabilityof body mass measured after awakening in the morning is used to monitor changesin hydration, body mass measurements should be used in combination with anotherhydration assessment technique (urine concentration) to dissociate gross tissuelosses from water losses.

Simplemarkers of hydration status afford athletes or coaches the ability to monitordaily fluid balance. Relatively inexpensive and easy-to-use commercialinstruments are available for assessing urine specific gravity and conductivity(an osmolality equivalent) (Bartok et al., 2004, Shirreffs & Maughan,1998). A urine color chart is also available (Armstrong et al., 1994). Solong as nude body mass is measured, almost any scale is suitable forself-monitoring of body mass, although a kilogram balance or medical-gradescale manufactured in accordance with international weighing standards ispreferred. Table 1 summarizes the strengths and weaknesses of using simplehydration markers.

Other Markers

Otherhydration markers have also been investigated. The limitations of thesemethods are outlined in Table 1. The following is a brief discussion of theirpotential.

OtherBlood Markers. Blood-borne markersof hydration other than osmolality include plasma volume, plasma sodium, and concentrationsof fluid regulatory hormones in plasma. Under controlled conditions (exercise,temperature, posture), most plasma markers reliably measure changes inhydration. Plasma volume decreases proportionally with the level ofdehydration, but this magnitude of change is markedly less in heat-acclimatizedathletes (Sawka & Coyle, 1999). Plasma volume changes can be estimatedfrom hemoglobin and hematocrit, but accurate measurement of these variablesrequires considerable controls for posture, arm position, skin temperature, andother factors (Sawka & Coyle, 1999). Plasma sodium provides an alternativeto measuring osmolality because osmolality changes are primarily a reflectionof sodium changes (Costill, 1977), but the relationship between hydration andplasma sodium is more variable than that between hydration and osmolality(Bartok et al., 2004; Senay, 1979). Fluid regulatory hormones, such as arginine-vasopressinand aldosterone, generally respond predictably to changes in body fluid volumeand osmolality, but the hormones are easily altered by exercise and heatacclimation (Francesconi et al., 1983; Montain et al., 1997) and require moreexpensive and complicated analysis techniques. Although all plasma markers forhydration assessment involve blood sampling with varying degrees of subsequentanalytical difficulty, plasma osmolality is the simplest, most accurate andreliable plasma marker for tracking hydration changes.

Bio-Impedance. Bioelectrical impedance analysis (BIA) is anoninvasive technique that can be used to estimate TBW. It uses low amperagecurrent (single or multiple frequency) passed between skin electrodes with theassumption that current resistance (impedance) varies inversely with tissuewater and electrolyte content. BIA is well correlated with TBW measures madeusing isotope dilution (O'Brien et al., 2002) under controlled laboratoryconditions in euhydrated subjects. Although BIA is sensitive for detectinghypertonic hypovolemia, it significantly underestimates the level of absolutefluid losses and is independently altered by changes in body fluid volume andtonicity (O'Brien et al., 2002). Shifts of body fluids between intracellularand extracellular compartments during exercise, sweating, rehydration, andother variables common to athletic situations also confound its accuracy andmake BIA unacceptable to monitor changes in hydration status (Panel DRI 2005).

Salivaand Symptoms. Salivais not as widely studied as other body fluids for potential monitoring ofhydration, but salivary osmolality appears to track changes in hydrationbrought on by sweating. However, individual responses of saliva osmolality tochanges in hydration are somewhat more variable than those for urine and muchmore variable than those for plasma (Walsh et al., 2004). Large variability insalivary flow has also been observed (Walsh, 2004), and salivary flow, likemany other measures, also offers no clear trend at low levels of dehydration(Ship & Fisher, 1999). Salivary specific gravity increases withdehydration, but the variability is too great for quantitative analyses (PanelDRI 2005). Importantly, the influence of common food and beverage intake andoral hygiene practices on saliva indices has not been investigated.

Clinicalsigns and symptoms of dehydration, such as dizziness, headache, tachycardia,and others are far too generalized to be of predictive use, while more severesymptoms, such as delirium or deafness, occur at dehydration levels outside thefunctional range for training athletes. Although genuine thirst develops onlyafter dehydration is present and is alleviated before euhydration is achieved (PanelDRI 2005), thirst is a useful symptom that draws attention to the need for morestructured drinking before, during, or after exercise. Table 1 reviews thecircumstantial limitations of choosing other markers to assess athletehydration.

Although plasmaosmolality and total body water measurements are currently the best hydrationassessment measures for large-scale assessment surveys of fluid needs (Sawka etal., 2005), there is presently no consensus for using any one approach overanother in an athletic setting. In most circumstances, the use of body mass measuredupon awakening in the morning combined with some measure of urine concentration(USG, U_Osm, U_Col) in a sample collected during the firstvoid of the morning offers a simple assessment method and allows ample sensitivityfor detecting meaningful deviations in fluid balance (> 2% body mass) fortraining and competing athletes. When more precision of acute hydrationchanges is desired, such as in the laboratory, plasma osmolality, isotope dilution,and acute changes in body mass allow gradations in measurement so long asproper techniques are used. Table 2 provides definable thresholds for thecomplex and simple markers of hydration recommended in this review for guidancein distinguishing euhydration from dehydration (Armstrong et al., 1994; Bartoket al., 2004; Casa et al., 2000; Cheuvront et al., 2004; Popowski et al., 2001;Ritz, 1998; Panel DRI 2005; Senay, 1979; Shirreffs and Maughan, 1998). Fluidbalance should be considered adequate when any two assessment outcomes areconsistent with euhydration thresholds.

Based uponthis review of the literature, an even simpler approach for self-monitoring ofday-to-day hydration changes is proposed for athletes. This approach isrepresented using a Venn Diagram decision tool (Figure 1). It combines threeof the simplest markers of hydration, including weight, urine, and thirst(WUT). No marker by itself provides enough evidence of dehydration, but thecombination of any two simple self-assessment markers means dehydration is likely. The presence of all three makes dehydration very likely. The detailsfor using this diagram are provided in the accompanying Sports ScienceExchange Supplement.

Acknowledgement. The opinions orassertions contained herein are the private views of the authors and should notto be construed as official or reflecting the views of the Army or theDepartment of Defense. Approved for public release: distribution is unlimited.

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Sports Science Exchange 97

VOLUME 18 (2005) NUMBER 2

SUPPLEMENT

Hydration Assessment ofAthletes

"WUT" IS the Answer?

"WUT" is amemory device designed to simplify athlete self-monitoring of day-to-dayhydration status. The concept for "WUT" is based on sound scientificprinciples of hydration assessment, but purposely requires nothing more than a body-weightscale. If adherence to fluid intake recommendations does not remedy suspecteddehydration using "WUT," or more objective measurement outcomes, such as plasmaosmolality or urine osmolality, should be used to confirm dehydration.

W stands for "weight." Athletes should maintain a day-to-day stablebody weight when measured first thing in the morning so long as they have freeaccess to food and beverage and replace sweat lost during exercise inaccordance with recommended fluid intake recommendations. Day-to-day bodyweight losses in excess of 1% may be an indication of dehydration. This is a day-to-dayloss of 1 lb (0.45 kg) for an athlete who weighs 100 lb (45.5 kg), 2 lbs (0.91kg) for an athlete weighing 200 lb (91 kg), or 3 lbs (1.4 kg) for an athleteweighing 300 lb (136.4 kg). Combine body weight information with thirst orchanges in urine (see Venn Diagram) to be more certain.

U stands for "urine". It is normalto produce more urine when body water is high and less urine when body water islow. Therefore, urine volume is generally more related to body water orhydration level than to drinking pattern. So if sweat losses are high, lessurine may be produced despite normal or even increased fluid intakes. Lowurine production can cause it to be more concentrated and a darker color. Areduced daily urine frequency and darkening of urine color in a sample takenduring the first urination of the morning may be an indication of dehydration. Combine urine information with information on thirst or body weight (see VennDiagram) to be more certain.

T stands for "thirst". The absenceof thirst does NOT indicate the absence of dehydration. However, the presenceof thirst IS an indication of dehydration and the need to drink. Therefore, ifthirst is present, combine that with urine or body weight information (see VennDiagram) to be more certain.

SIMPLE TESTS TO DETERMINE IF YOU ARE DEHYDRATED

There arethree simple questions you can ask yourself to determine if you are dehydrated:

• Am I thirsty?
• Is my morningurine dark yellow?
• Is my bodyweight this morning noticeably lower when compared to yesterday morning?

If theanswer to any one of these questions is "Yes," you may be dehydrated. If theanswer to any two of these questions is "Yes," it is likely that you aredehydrated. If the answer to all three of these questions is "Yes," it is verylikely that you are dehydrated.

Drinkingtoo little or too much during exercise can be dangerous to your health and canworsen your performance. Here are some tips to help you stay in fluid balance.

• To determine howmuch fluid you lose or gain during training or competition, use a chart likethe one below to record your nude body weight to the nearest pound before andafter your workouts.
• If you lost morethan 1% of your body weight, you drank too little during exercise; if yougained weight, you drank too much.
• If you regularlylose more than 1% of your body weight, try to drink more during and after exerciseto keep your body weight stable.
• Remember, it canbe dangerous to gain weight during exercise by drinking too much.