Hydration Assessment Techniques Lawrence E. Armstrong, PhD, FACSM

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1 June 2005: (II)S40 S54 Hydration Assessment Techniques Lawrence E. Armstrong, PhD, FACSM Water in the human body is essential for metabolism, temperature regulation, and numerous other physiological processes that are consistent with good health. Accurate, precise, and reliable methods to assess body fluid compartments are needed. This review describes the hydration assessment techniques of isotope dilution, neutron activation analysis, bioelectrical impedance, body mass change, thirst, tracer appearance, hematologic indices, and urinary markers. It also provides guidance for selecting techniques that are appropriate for use with unique individuals and situations. Key words: total body water, extracellular fluid, intracellular fluid, osmolality 2005 International Life Sciences Institute doi: /nr.2005.jun.S40 S54 THE IMPORTANCE OF HYDRATION ASSESSMENT Water serves as the essential solvent for cellular biochemical reactions and facilitates the thermal equilibrium of cells. It comprises about 63% of the entire body mass 1 and 80% to 84% of kidney, lung, and skeletal muscle tissues. 2 Water must be consumed by humans because the amount lost in metabolism exceeds the amount synthesized by the body. These facts and the following observations emphasize the importance of accurate, precise, and reliable methodologies with which to evaluate hydration: 1. The human body constantly loses water from the lungs, skin, and kidneys. 2. Even when normal hydration exists, fluid moves into and out of cells and the circulation. 3,4 Dr. Armstrong is with the Human Performance Laboratory, Departments of Kinesiology and Nutritional Sciences, University of Connecticut, Storrs, Connecticut. Address for correspondence: Dr. Lawrence Armstrong, Human Performance Laboratory, Departments of Kinesiology and Nutritional Sciences, University of Connecticut, Unit 1110, 2095 Hillside Road, Storrs, CT ; Phone: ; Fax: ; lawrence.armstrong@uconn.edu. 3. Strenuous labor, exercise, or environmental heat stress may increase sweat losses to the point that the daily water requirement increases 2 to 6 times above daily living in a mild environment (i.e., up to 15 L/d) Prior to athletic competition in the sport of wrestling, some athletes purposefully dehydrate to qualify for a weight class and experience increased hemoconcentration that would otherwise be classified as pathological (i.e., mean plasma osmolality of 320 mosm/kg; n 12) Various illnesses disrupt whole-body fluid and electrolyte balance acutely and chronically. 6. Insufficient or excessive dietary intake of water changes cellular volume and affects a variety of cellular functions, including metabolism, excitation, transport, hormone release, cell proliferation, and cell death Some individuals consume too much fluid, which dilutes total body water abnormally, and experience the clinical condition known as water intoxication hyponatremia Total body water varies across the lifespan 9 from childhood 10 to adolescence 11 to adulthood 12 and to elder adulthood Increasing the dietary intake of sodium chloride or protein increases the obligatory water requirement for whole-body osmotic equilibrium Mild dehydration of 1% or 2% of body mass can reduce exercise performance, cognitive function, and alertness. 14,15 This review is designed to describe hydration assessment techniques, provide selection guidelines appropriate to unique individuals and situations, and to highlight recent publications by Shirreffs and Maughan, 16 Kavouras, 17 Opplinger and Bartok, 18 and Manz and Wentz. 13 BODY FLUIDS: DEFINITIONS In this review paper, the following definitions 1,19,20 will be utilized. Total body water (TBW) is the fluid that occupies intracellular and extracellular spaces, comprising about 0.6 L/kg (63.3%) of body mass. Extracellular S40 Nutrition Reviews, Vol. 63, No. 6

2 volume (ECV) is all fluid outside of cells, including the interstitial fluid and plasma water, and comprises about 0.2 L/kg (24.9%) of body mass. Intracellular volume (ICV) is the fluid within tissue cells, comprising about 0.4 L/kg (38.4%) of body mass. Plasma volume (PV) is the liquid portion of the blood, which makes up about 5% of body mass. Interstitial fluid (ISF) is the fluid located in the spaces between tissue cells with a chemical composition similar to that of lymph; it is usually calculated as ECV PV, and makes up about 21% of body mass. An electrolyte is any compound that, in solution, conducts electricity and is decomposed (electrolyzed) by it; i.e., an ionizable substance in solution. Osmolality is the concentration of a solution expressed in milliosmoles of solute particles per kilogram of water. HYDRATION ASSESSMENT TECHNIQUES Dilution Techniques: Fluid Compartment Size Figure 1. The time required for deuterium oxide ( 2 H 2 O) to reach all accessible human body water spaces is 3 to 4 hours. This representative graph depicts the increase in plasma 2 H 2 O. Oral or intravenous administration of a tracer substance, before and after sampling a body fluid or expired air, enables measurement of human fluid compartment size. If the amount of substance is known, and if the baseline and equilibration concentrations are measured, the volume into which the tracer has been diluted can be calculated. Measuring tracer concentration before it has equilibrated throughout the fluid space (i.e., less than 3 or 4 hours) is the primary threat to reliability and accuracy. Considering all of the methods in subsequent sections, dilution techniques are considered to be the gold standard for measurements of body fluid spaces. TBW is assessed with stable (i.e., non-radioactive) isotopes of hydrogen or oxygen 21 that distribute in 3 to 4 hours throughout virtually all body fluid compartments (Figure 1). Deuterium ( 2 H), deuterium oxide ( 2 H 2 O, D 2 O), and oxygen-18 ( 18 O, which is a constituent of heavy water, H 2 18 O) are most commonly used. The radioactive isotope tritium ( 3 H) or tritiated water ( 3 H 2 O) are utilized less frequently in TBW studies. Using these naturally occurring isotopes, the smallest detectable change in TBW is about 0.8 L (L.E. Armstrong, unpublished observations). TBW is regulated tightly by neurohumoral mechanisms, so that it varies only 0.9% to 1.0 % from one day to the next. 22 ECV measurements historically involved thiocyanate and isotopes of sodium and chloride (i.e., 22 Na, 24 Na, 36 Cl), but in recent years bromide has been utilized widely. 23,24,25 A corrected bromide space can be calculated from the serum concentration after administration of a known amount of bromide, which is an excellent approximation of ECV. Oral bromide is completely absorbed from the intestine, has a biological half-life of about 12 days, and is lost in urine and feces in insignificant amounts for 2 to 4 hours after an oral dose. 24 Thus, ECV can be determined from bromide that is administered either orally or intravenously within a 2- to 4-hour window. 25 ICV is calculated as the difference between TBW and ECV. 1 Whole-body counters (i.e., neutron activation analysis) providing alternative methods of ECV assessment are described below. During blood volume measurements, a small quantity of radioactive tracer is applied to erythrocytes or a plasma component, which are injected intravenously after a baseline blood sample is drawn. The coefficient of variation of these techniques for repeated measures is 3.0%. 26 After allowing ample circulatory mixing time, an equilibrium blood sample is drawn, the radioactivity is counted, and the blood volume is calculated. 27 The tracers 51 Cr and 125 I have been widely used to label erythrocytes and human serum albumin, respectively. Using these techniques, Sawka et al. 28 developed algorithms to successfully predict volumes of blood and plasma in healthy young men; lean body mass was the anthropometric characteristic that was most closely correlated with vascular fluid volumes. Closely regulated by osmotic and oncotic forces, PV varies only 0.7% from day to day in resting individuals. 29 Although human subject review boards at some institutions may voice concerns for subject safety, one dilution technique allows the simultaneous assessment of multiple fluid spaces. 19 This method incorporates simultaneous dilution of the following radionuclides via intravenous injections: Na 51 Cr (to assess erythrocyte volume; 3.4% of body mass), human fibrinogen labeled with 125 I (to assess PV; 4.6% of body mass), Na 82 Br (to assess ECV; 25.7% of body mass), and 3 H 2 O (to assess TBW; 65.3% of body mass). Corrected for the presence of protein, urine loss, and migration of isotopes out of the vascular space, calculations 30 also allow determination Nutrition Reviews, Vol. 63, No. 6 S41

3 of ICV (38.8% of body mass), ISF volume (21.1% of body mass), and blood volume (7.9% of body mass). 19 Neutron Activation Analysis: Fluid Compartment Size Neutron activation analysis identifies and quantifies about 70% of all known elements. A specimen is irradiated in a nuclear reactor, producing specific radionuclides that emit characteristic gamma rays during decay. These emissions allow scientists to identify and measure, via radiation detectors, the amount of a substance that is present. This technique has proven to be useful in forensic investigations, geology, archaeology, and biochemistry. Although this method is considered to be the reference standard for all element identification (with a precision to parts per billion), it requires a nuclear reactor and technical expertise that exist in few laboratories. Total body neutron activation analysis originally was applied to the study of calcium and nitrogen metabolism. Subsequently, analyses of total body chloride, potassium, and sodium were used to calculate ECV, ICV, and total exchangeable extracellular sodium, respectively. 31 TBW was calculated as the sum of ECV plus ICV. One whole-body scan thus provided a non-invasive measurement of fluid compartment sizes, with the assurance of small error and elemental equilibrium. Bioelectrical Impedance: Fluid Compartment Size Electrical current that flows through the human body (i.e., from hands to feet) is resisted by body tissues and water. Measurements of bioelectrical impedance utilize this property to provide estimates of body composition, including body water. This technique assumes that the human body is a conductor of homogeneous composition, with a fixed cross-sectional area throughout and uniform current density. Clearly, the body meets none of these assumptions perfectly. 31 Impedance at One to Four Frequencies: TBW Prior to 1990, single-frequency impedance (i.e., at 50 or 100 khz) was used almost exclusively for estimates of fat mass and fat free mass, as validated by hydrostatic weighing. 32,33 Subsequently, single-frequency impedance has been used to estimate TBW in children, 10 older adults, 35,36 and patients with varied disease states. 35 Validation studies (i.e., utilizing dilution procedures, as described above) verified that this method is reliable and valid, 10,35,36,37 with a coefficient of variation for repeated measurements of 1.5% to 3.4% 10,34,38 in a 70-kg adult. However, it is widely recognized that several environmental and host factors may reduce the reliability and accuracy of this technique. These factors include electrode site placement, 39 skin temperature, 3 skin blood flow, 38,40 posture, 41 recent fluid ingestion, 38 composition of ingested fluids, 38 exercise, and changes in plasma osmolality 38,42 or plasma sodium concentration. 41,42 Bioimpedance Spectroscopy: TBW, ECV, ICV During the 1990s, theoretical principles of physics and technological advancements allowed impedance to be scanned at numerous frequencies (e.g., 50) in rapid succession. 43,44 Also, because today s instruments cannot measure impedance reliably at very low ( 1 khz) or very high ( 500 khz) frequencies, statistical modeling programs have been developed for computers. Bioimpedance spectroscopy (BIS) is a statistical technique that extrapolates values for resistances at very high and very low frequencies from resistance values in the frequency range that is reliable (i.e., khz). 45 After TBW and ECV have been determined, ICV is calculated as TBW minus ECV. In studies of body fluid volumes, this method has caused single-frequency impedance analyzers to be replaced, because spectroscopy offers the potential of measuring TBW, ECV, and ICV separately. 46,47 BIS has been validated by using dilution techniques (see above) as the reference standard in elderly individuals, 48 women, 3,43,49 and healthy males, 3,43,50 with statistical correlation (r 2 ) ranging from 0.87 to Figure 2 presents comparisons of BIS and dilution measurements in resting subjects. 1 As is the case with single-frequency methods, the reliability and accuracy of BIS decrease if measurement protocols are not standardized. 38 For example, investigations on the effects of fluid compartments have shown that postural changes (i.e., raising the arm from the hips to an overhead position), 32,41 dehydration, and ingestion of fluid with difference tonicities 38 invalidate BIS estimates of TBW and ECV. When such factors are controlled, the TBW prediction error (as a percentage of cellular volume) ranges from 3.5% to 6.9% 1,37,38,43,49,51 for adults with a body mass of approximately 70 kg. Although this variance likely makes BIS inappropriate for measuring the small changes ( 1 L) of TBW, ECV, or ICV that occur during fluid intake or sweating, BIS is a useful clinical and research tool when the above factors are controlled. 1,22 Hematocrit and Hemoglobin: Extracellular Fluid Shifts Harrison s classic review of the effects of exercise and thermal stress on PV 4 describes the interactions of S42 Nutrition Reviews, Vol. 63, No. 6

4 tion and volume of ingested fluid and sweat; (c) adaptations to physical training; and (d) acclimatization to hot or high-altitude environments. 5,29 These factors partly explain why PV responses vary within subjects from one experiment to another. 4 The first evidence that environmental heat exposure resulted in hemoconcentration was published in When immersed for 30 to 45 min in 38 C water, then wrapped in dry wool for 2 hours, most subjects exhibited an increase in hemoglobin concentration. Approximately 100 years later, awareness of the importance of PV during exercise-heat exposure prompted attempts to quantify hemoconcentration and hemodilution. The first attempt 52 utilized hematocrit measurements to calculate changes in PV, despite acknowledging that the ratio between red cell volume and blood volume (i.e., hematocrit) was not linearly proportional to PV. Two years later, a second publication 53 calculated PV changes from hematocrit and hemoglobin concentration. Hemoglobin was included because it resides inside erythrocytes and does not leave the circulation, and because changes in its concentration represent changes in PV. This publication also acknowledged that changes of plasma osmolality affect erythrocyte volume and, in turn, hematocrit. Due to its simplicity and relevance, this technique is widely used today. Between 1994 and 2004, for example, this paper was cited in more than 1300 peer-reviewed scientific publications (Science Citation Index, Thomson Learning, Inc., Florence, KY). Tracer Appearance: Fluid Absorption and Equilibration Figure 2. Linear regression comparisons of total body water (TBW, panel A), extracellular volume (ECV, panel B), and intracellular volume (ICV, panel C) as estimated with bioimpedance spectroscopy (BIS). Deuterium oxide dilution (see Figure 1) and bromide dilution (see text) techniques were used as reference standards. Intracellular volume (ICV) was calculated as TBW ECV. (Reprinted from Armstrong et al. 1 with permission.) numerous factors that cause extracellular water to move into or out of the circulation. These include: (a) changes of posture, exercise mode, exercise intensity, sodium chloride balance, and plasma osmolality; 29 (b) composi- The rate at which water and nutrients appear in blood is a function of gastric emptying and intestinal absorption. However, the addition of a tracer (e.g., 2 H 2 O) to an ingested fluid and collection of timed blood samples provides a tool to assess the integrated effects of gastric emptying and intestinal absorption. 54 Because net water uptake in the stomach is negligible, tracking the rate at which deuterium accumulates in blood allows comparisons of different mixtures and fluid properties such as temperature and osmolality. 55 Objections have been raised regarding the use of 2 H 2 O because water movement in the intestine is a bidirectional process. 54 Nevertheless, the experimental findings from deuterium tracer studies agree with the known gastric emptying and intestinal absorption characteristics of the solutions involved 55 and are quantitatively similar to the triple lumen perfusion technique, which measures net water flux in the proximal intestine. 56 Further, Lambert and Maughan 57 also reported that the use of 2 H 2 O was reliable in repeated measures performed on different days. Nutrition Reviews, Vol. 63, No. 6 S43

5 Heavy water (H 18 2 O) also has been used as a tracer to assess fluid movements during exercise in a hot environment. Analyses utilized an isotope-ratio mass spectrometer and the H 2 O/CO 2 equilibration method. In one investigation, 58 healthy male subjects drank 100 ml of H 18 2 O (15.3% 18 O; 84.7% 16 O) during 6 hours of heat exposure with intermittent cycling exercise (42.7 C air temperature). Samples of saliva, blood, and urine were collected, with the result that the isotopic enrichment in saliva and urine (collected 3 6 hours post-dosing) was similar to that of plasma. These findings indicated that water transport could be assessed with H 18 2 O. A second investigation 59 involved the same exercise and tracer protocol as above, but in 37.1 C air temperature. The tracer H 18 2 O appeared in body fluids rapidly (8 18 min) after consumption. The peak enrichment (post-dose; 18 O/ 16 O ratio) occurred at 21 to 28 minutes in plasma and 21 to 45 minutes in sweat, then declined slowly for the remainder of the 6-hour experiment. Interestingly, neither the plasma nor the sweat 18 O enrichment plots (i.e., 18 O/ 16 O ratio versus time) was altered by exercise intensity or duration. Plasma or Serum Osmolality: Index of Hydration Status Although hematocrit and hemoglobin assess fluid movements into and out of the circulation (see above), plasma or serum osmolality is the most widely used hematological index of hydration status, because extracellular fluid osmolality stimulates important fluid-regulatory mechanisms. Indeed, some investigators consider it to be the only valid index of hydration status. 60 For example, an osmolality increase of only 1% initiates the sensation of thirst (i.e., increasing fluid consumption) and an increase of arginine vasopressin that exceeds basal values by 100% (i.e., reabsorbing water at renal distal tubules). 61,62 The neuroendocrine regulation of osmolality is such that normal values rarely deviate by more than 1% to 2% from a basal mean value of 287 mmol/kg in healthy, well-hydrated individuals. 63 Direct laboratory measurements of plasma osmolality are performed with either a freezing point or vapor pressure depression osmometer. A coefficient of variation of less than 0.3% to 0.4 % is desirable and attainable by competent technicians. 63 It is important to note that osmolality must be measured directly after a blood sample has been collected and centrifuged, because as storage time in mild or cool laboratories increases (i.e., h), plasma osmolality decreases. This decrease may be due to changes in ph, dissolved CO 2, lactic acid concentration, or the binding of electrolytes to protein. 63 Body Mass Difference: Hydration Status The measurement of body mass change represents a commonly used, safe technique to assess hydration status, especially during dehydration that occurs over a period of 1 to 4 hours, with or without exercise. 17 When an individual is in caloric balance, her/his body mass loss essentially equals water loss (i.e., when corrected for the mass of fluid and food intake, urine and fecal losses, sweat evaporation, and sweat absorbed by clothing), because no other body constituent is lost at a similar rate. 64 When body mass measurements are made with an interval of more than 4 hours, water exchange due to substrate oxidation and respiratory water loss become large enough that the body mass difference should be corrected for by these factors. 64,65 When using body mass to represent water loss, the following three factors should be considered: 1. In clinical and athletic settings, a baseline body mass is required but often not available From one day to the next, body mass fluctuates kg (mean SD), with a group coefficient of variation for repeated days of %. This means that three consecutive measurements provide an accurate assessment of daily body mass variability in active men who replace 100% of sweat lost during exercise If body mass measurements are performed over several weeks or months, this technique cannot be interpreted, because the gain or loss of adipose tissue is unknown unless precise whole-body scans are available to interpret changes in fat mass (i.e., dual x-ray absorptiometry). Urinary Indices of Hydration Status As noted above, the body mass of active men fluctuates only 0.66% from day to day. 66 Unless sweating causes a loss of water that exceeds 3% of body weight, the regulation of whole-body fluid-electrolyte balance generally is adequate. Osmoregulation is affected by alterations of renal water and electrolyte excretion under the influence of arginine vasopressin. TBW volume is associated with water and sodium balance and is controlled by fluid regulatory hormones. Given normal renal function, urine is concentrated and scanty when the body is dehydrated and conserving water, or is dilute and plentiful when a temporary excess of body water exists. An obvious question then arises: how accurate are urine variables in assessing hydration? Urine Osmolality Urine osmolality, a measure of total urine solute content, is affected by all dissolved particles in a known S44 Nutrition Reviews, Vol. 63, No. 6

6 volume (i.e., mass) of fluid. Analyses require an osmometer (as described above for plasma/serum) and a trained laboratory technician and are time-consuming. Because osmolality is the most accurate measurement of total solute concentration, it provides the best measurement of the kidney s concentrating ability. 61 However, because urine properties are regulated by several interactive mechanisms, and because water turnover is constantly changing, no universally accepted technique exists to determine whether humans are well hydrated, euhydrated, or hypohydrated. 67 For example, urine osmolality may not accurately reflect hydration status when used immediately after exercise. 68 Further complicating matters, large intercultural differences exist, as evidenced by mean 24-hour values from Germany (860 mosm/kg) and Poland (392 mosm/kg). 13 Regarding the day-to-day use of urine osmolality within a group, one investigation 16 evaluated the hydration status of athletes who trained in a hot environment, and found that hydration status during competition or training in a warm environment can be effectively monitored by measuring urine tonicity. Urine Specific Gravity The specific gravity of urine refers to the density (mass per volume) of a sample in comparison to pure water. Any fluid that is denser than water has a specific gravity greater than Normal urine specimens usually range from to in healthy adults. 69,70 During dehydration or hypohydration, urine specific gravity exceeds When excess water exists, values from to are typically seen. 69,70 Specific gravity can be measured quickly and accurately with a handheld refractometer. A few drops of a urine specimen are placed on the stage of the refractometer and it is pointed toward a light source, which passes through the specimen. Although this device can be used indoors or outdoors, purchasing and using a refractometer may intimidate the average, non-technical adult. One investigation 70 involving 34 healthy males demonstrated that urine specific gravity (as measured with a refractometer) and urine osmolality (as measured with an osmometer) may be used interchangeably. The correlation (r 2 ) of these measurements was Other techniques are used to measure specific gravity less frequently (i.e., dipsticks); 71 their validity and reliability require further validation. Twenty-Four-Hour Urine Volume Urine volume may be used to assess hydration status as compared with normal adults of similar body mass. A healthy woman produces L (mean SD) of urine per day, whereas a healthy male produces L/d. 72 This means that women and men should produce at least 0.29 and 0.48 L/d of urine, respectively, to avoid being two standard deviations below the mean. Children 10 to 14 years of age will produce proportionately less urine each day (girls, L/d; boys, L/d), as will elderly adults over the age of 90 years ( L/d). 72 Urine Color Attempting to simplify urinalysis, our research group conducted a series of experiments involving the color of urine. We reasoned that virtually anyone could determine when they need to rehydrate if urine color were directly proportional to the level of hydration. Our initial study involved developing a numbered scale that includes colors ranging from very pale yellow (number 1) to brownish green (number 8). 69 Those individuals who maintained a pale yellow urine color always were within 1% of their baseline euhydrated body mass. This project demonstrated that urine color did not offer the same precision and accuracy as urine specific gravity or osmolality, but it likely would be effective in athletic and industrial settings that do not require high precision. Our second laboratory investigation evaluated the effects of heavy physical training and large water turnover ( 4% of body weight) on urine color. 70 Nine subjects performed strenuous exercise in a hot environment (36.7 C), then undertook a 21-hour period of oral rehydration. The change in body mass was the reference standard by which all hydration indices were evaluated, because it represented body water fluctuations. Figure 3 illustrates changes of body mass, urine color, urine specific gravity, urine osmolality, total plasma protein concentration, plasma sodium concentration, and plasma osmolality of highly trained cyclists across the five phases of this study. These graphs demonstrate that urine color, specific gravity, and osmolality followed a pattern similar to that of fluid loss. The minor exceptions to this pattern, in urine specific gravity and urine osmolality at phase D, indicate that urine color mimics body water loss as effectively or more effectively than urine specific gravity, urine osmolality, plasma osmolality, plasma total protein concentration, or plasma sodium concentration. Our third investigation observed women as they undertook 6 weeks of physical training and heat acclimation. 73 Measurements of urine specific gravity and urine color once per week indicated that these variables can be used interchangeably. The weekly statistical correlation (r 2 ) ranged from 0.77 to The above statements should not be interpreted to mean that urinary (or any) hydration indices are foolproof. For example, when a large bolus of pure water or hypotonic Nutrition Reviews, Vol. 63, No. 6 S45

7 Figure 3. Changes of body mass, plasma, and urinary indices of hydration status during a 41-hour dehydration and rehydration protocol involving highly trained cyclists. B Baseline state before testing; D dehydration ( 4% body mass); E after cycling exercise to exhaustion; 4H 4 hours of ad libitum rehydration; and 21H 21 hours of ad libitum rehydration. (Reprinted from Armstrong et al. 70 with permission.) fluid is consumed during a brief period (e.g., 1.5 L/h), the ingested fluid rapidly dilutes the blood and the kidneys excrete dilute urine over a range of hydration states. 70 This occurs even if dehydration exists, 68 because the urine variables mirror the volume of fluid consumed rather than the amount of water retained in the body. 18 Time Interval Between Measurements Considering the short-term dynamics illustrated in Figures 3 and 4, it would be useful to know if indices of hydration status are valid across days or weeks. Seeking to answer this question, two distinct studies compared hydration assessment techniques prior to deployment on day 1 compared with the end of long-term residence (day 14) in sub-arctic conditions 74 and in a field environment (day 44). 67 Both research teams concluded that urinary and hematologic techniques could not be used to assess hydration status, in part due to unknown changes in body composition. PERCEPTUAL RATING OF THIRST When instrumentation or technical expertise is unavailable, or when an approximation of hydration state is acceptable, the sensation of thirst can be used to an- S46 Nutrition Reviews, Vol. 63, No. 6

8 the negative effects of mild dehydration on health and human performance. At only 1% or 2% of body mass, exercise performance capacity, cognitive function, and alertness decline 14,15 and physiological strain (i.e., heart rate, tissue heat storage) increases. 76 Thirst can be measured with a simple numerical rating scale ranging from 1 (not at all thirsty) to 9 (very, very thirsty), which was developed by Young et al. 77 Between a score of 3 (a little thirsty) and 5 (moderately thirsty), an individual can safely assume in most situations that he/she is mildly dehydrated. As a caveat, it is important to acknowledge that numerous host factors may alter the perception of thirst. These include fluid palatability, time allowed for fluid consumption, gastric distention, older age, gender, and heat acclimation status. 62,73,75 Thus, this approach to hydration assessment is at best an approximation, but may serve as a reminder to individuals who anticipate undertaking dehydrating exercise, labor, or dietary restriction. INVESTIGATIONAL DEVICES From 1998 to 2004, research teams evaluated the efficacy of two instruments as hydration monitors. Although still under investigation, these bioengineering devices represent the ongoing quest to find accurate, precise, and reliable yet practical methods to assess hydration status. Urine Conductivity Analyzer Shirreffs and Maughan 16 investigated day-to-day hydration status in athletes who trained in a hot environment and lost approximately 2% of body mass via sweating. Urine samples from 60 subjects were analyzed with a conductivity meter that was designed for field use by athletes. A laboratory osmometer was used as the reference standard technique. This handheld device provided values that were in good agreement with laboratory osmometer data, was portable, and provided rapid feedback to individuals who trained in a warm environment. As with other urinary hydration indices, urine conductivity may not accurately reflect hydration status when used within a few minutes of exercise. 68 Figure 4. Plasma osmolality, urine osmolality, and urine specific gravity as a function of hydration status. (Reprinted from Popowski et al. 78 with permission.) nounce the threshold of hypohydration that affects physiological responses and health. This occurs when the TBW loss reaches 1% or 2% of body mass. 62,75 Recognizing that one has attained this level of dehydration is meaningful, because recent publications have reported Arm Radio Frequency Absorption This device consisted of a power source, two antennae, a power sensor, and an open exposure chamber that held the wrist of a test subject. Theoretically, electromagnetic waves interact with tissue water and solutes, but the mode of interaction varies according to the wave frequency. This technique utilized a comparison of attenuation of radio waves by tissue at frequencies ranging Nutrition Reviews, Vol. 63, No. 6 S47

9 from 450 to 2120 MHz in healthy male subjects who experienced a 1% to 2.5% loss of body mass by exercise and water restriction. 60 A statistical correlation (r 2 ) of 0.73 was determined between body mass loss and change in radio frequency absorption pattern. The authors recommended further study and development of this technique. SELECTING AN APPROPRIATE ASSESSMENT TECHNIQUE No infallible method for assessing hydration exists. 17 Table 1 compares the techniques described above on the basis of commonly used criteria. The ratings of each characteristic have been designed to make a low score preferable to a high score. Laboratory Assessment The process of selecting an appropriate technique for laboratory use is quite different from selecting one for use during daily activities. Precision, accuracy, and reliability are the hallmarks of sound laboratory practices. With the appropriate technical expertise, time, and financial resources, these three criteria are met by isotope dilution techniques, neutron activation analysis, osmometry, and measurements of hematocrit and hemoglobin. Bioelectrical impedance and BIS show promise as techniques, but are not sufficiently accurate or reliable to measure TBW when posture or hydration state (i.e., sweat loss or fluid ingestion) fluctuate. 37 If environmental and test subject factors are not controlled carefully, measurements are not valid. Assessment during Daily Activities Exercise enthusiasts, laborers, and military personnel may experience a large water turnover on consecutive days, which eventually may lead to significant hypohydration. To monitor proper hydration, they should utilize an approach that requires little technical expertise or sophisticated instrumentation. Ideally, this method also should be accurate, safe, and inexpensive. The ratings in Table 1 show that urine specific gravity, urine Table 1. Comparison of Techniques to Assess Hydration Status Technique Isotope dilution, isotope appearance Neutron activation analysis Plasma and urine osmolality Hematocrit, hemoglobin Bioelectrical impedance and BIS Urine specific gravity Urine conductivity meter Urine color Body mass change Rating of thirst Purpose Instrument Cost Time Required Per Analysis Technical Expertise Required Accuracy Portability Risk to Individual Health Fluid volume ,3* Fluid volume, whole body K Fluid concentration urine, 1 plasma, 2 % plasma volume change Fluid volume Body water change Body water change Body water change Body water change Body water change Key to ratings: 1 small 1 small 1 little 1 high 1 portable 1 low 2 moderate 2 moderate 2 intermediate 2 moderate 2 moderate 2 moderate 3 great 3 great 3 much 3 low 3 not portable 3 high *Depending on the type of isotope involved (i.e., radioactive, stable, non-radioactive) Bioimpedance spectroscopy Portable, hand-held meters are available 16 Using a floor scale S48 Nutrition Reviews, Vol. 63, No. 6

10 color, and body mass are the techniques that best meet these requirements (i.e., they have the lowest sum of ratings). Two previous publications 13,64 support this rating system, in that urinary indices (i.e., osmolality and specific gravity) were considered to be superior to other hydration assessment techniques. However, disagreement exists regarding the relative efficacy of plasma osmolality versus other hydration indices. One laboratory study 70 showed that urinary indices represented body water loss as well as, or better than, plasma osmolality (Figure 3). A subsequent laboratory investigation 78 found that plasma osmolality responded to acute dehydration faster than urine indices, purportedly because sweat originates from ISF, which rapidly equilibrates with the circulation (Figure 4). These minor differences of interpretation may be due to the unique dehydration protocols of each study, as well as the timing of blood samples, and may not be significant in field settings. During daily activities, it is likely that only body mass measurements will detect small fluctuations in hydration ( 0.5 L) when compared within and across days. Such comparisons require that an individual know her/his baseline body mass. A recent investigation 66 determined that, considering daily variability ( kg), a valid baseline value can be determined by measuring body mass on three consecutive days. Finally, the consensus statements of professional sports medicine organizations and regulations of athletic governing bodies represent state-of-the-art applications of the scientific literature. The National Collegiate Athletic Association is a prime example. When three university wrestlers died during the 1998 season, the NCAA prohibited the use of intentional dehydration to make weight, and included body mass and urine specific gravity measurements as safeguards against harmful weight-reduction practices. 77 Similarly, the National Athletic Trainers Association position statement regarding fluid replacement 80 recommends that three techniques be used to assess the hydration status of athletes: body weight change, urine specific gravity, and urine color. This supports the ratings in Table 1 and demonstrates that these three techniques can be used successfully in clinical settings. ACKNOWLEDGEMENTS This research was funded by the Nestlé Research Center, Lausanne, Switzerland. PANEL DISCUSSION Irwin Rosenberg: Can one be confident that we have a gold standard against which to validate hydration assessment techniques, and if so, what would you use as the gold standard? Larry Armstrong: The accuracy that I refer to in Table 1 represents whether I believe the technique was validated or not. And with the body water measurements, typically deuterium oxide or some isotope dilution technique was used to validate these. That is how I derived the numbers 1, 2, and 3, which represent high, moderate, and low accuracy. In this case, you will notice that I considered field-adaptable techniques to be less accurate. For body water, isotope dilution techniques are considered to be the gold standard today. For the elemental content of the body, neutron activation analysis is considered to be the gold standard. Friedrich Manz: An athlete will look at his urine color just before competing to assess his hydration status. At what time during the day should an ordinary subject, who has no specific event or physical exercise to prepare for, look at the urine color to determine if the hydration status is adequate? Larry Armstrong: Debate exists in the scientific literature about the best time of day. Some have recommended using the first morning void. Others have said, and I have published this based upon our observations, that the first morning void may not be valid in terms of the total 24-hour losses. Rather than taking one specific time of the day as a gold standard, perhaps a person ought to do this often, just as a matter of habit. My recommendation would be that, if they must do it one time a day, to do it daily at that same time. To date, no evidence verifies that one time is better than another. Antonio Dal Canton: Your experience with urine color is really interesting simple and useful. I would add just a suggestion to this. High grades of color, for example, 7 or 8, in rare circumstances can be misunderstood as high urine concentration when they are actually gross hematuria or bilirubin excretion because of jaundice. So, I would advise: drink water but if the color persists after 12 hours, go to a physician. In some cases, there may be the chance that a change in color is not due to high urine osmolality but to a disease. Larry Armstrong: Yes, indeed that is true and I mentioned the dietary conditions that may affect urine color, although we found in three studies that few of the dietary issues arose, even though these were active people who often took vitamins. I would agree that the average person probably needs the advice to look at their urine. But I don t know that this panel can address exceptions such as hematuria or renal disease. Do you have any suggestions? Antonio Dal Canton: I expect that if urine color has increased because of high urine concentration and then you drink, after some hours the urine will become diluted, and the color will change. However, if the darker color persists, my advice would be to go to a physician. Denis Barclay: If you were advocating the use of a Nutrition Reviews, Vol. 63, No. 6 S49

11 urine color chart for the general public, apart from those specified by Dr. Dal Canton, what other qualifiers would you want to include so that such a chart would not be misused or misinterpreted? Is it possible to have a light urine color and still be dehydrated? Are there false negatives and false positives that we would need to think about if we wanted to use such a color chart for the general public? Larry Armstrong: Yes. A large bolus of water at one time, say 800 to 1000 ml, dietary intake of vitamins, and other compounds can all alter urine color. There are also compounds, even in vegetables, that alter the urine color. Patrick Ritz: There are also some medications that affect urine color. Denis Barclay: So are you saying to monitor urine color over time and look for changes that are taking you to level 7 and above, that this would be a valid way of advocating their use? Larry Armstrong: I think that the key to using it, and again it is not a laboratory technique but a field expedient, would be to train oneself to look at urine color with each urination. I ve done this with athletes time and again, and they are initially amazed at how dark their urine is. But to understand the concept that drinking more results in a pale urine color, as simple as that may seem, they must intellectually connect fluid intake with urine output. With training, people seem to be able to adapt quite readily. Patrick Ritz: And do you have any data showing that those athletes who increased their water consumption in order to ensure that their urine became pale had improved performance? Larry Armstrong: I think the answer to that lies in the large data sets that Dr. Sawka and Dr. Shirreffs showed earlier demonstrating the effects of hydration on performance. Michael Sawka: For urine osmolality and specific gravity, are these effective field measures in terms of screening, using a threshold, or are they quantitative for individual athletes? Larry Armstrong: In terms of quantitation for field use, these techniques should require little technical expertise, no sophisticated instrumentation, and should be accurate, safe, and inexpensive. That would eliminate osmolality, unless you utilize a handheld unit. Michael Sawka: So can urine specific gravity be used quantitatively for dehydration, or is it a tool to use in the field for screening? For example, the American College of Sports Medicine in a consensus statement and others have recommended setting a threshold above or below which you probably are euhydrated or dehydrated, and if you are in between you really don t know. That s what they re saying? Larry Armstrong: Does this mean that you do not accept the common clinical perspective that urine specific gravity is valid? Michael Sawka: Yes, absolutely. I don t think it s quantitated. You can go back to the original work of Adolph that shows individual variability, you can go to Popowski and others, who show no significant correlation with plasma osmolality. I know also that there are a number of things in a normal individual that affect urine flow. For example, physical exercise or just heat stress. There are a number of things that a person can do that will concentrate the urine, so my feeling is and this has been stated by the American College Sports Medicine in a consensus statement that it certainly is an effective screen; that if the urine specific gravity falls below 1.02 or above 1.03 you can tell when a person is dehydrated or euhydrated. In between, it s difficult to quantitate. Larry Armstrong: I think the question is also complicated by the fact that the change in body mass (Figure 3) over this short period of time represents water loss from the body. We find that these curves are very similar and, at least under these laboratory conditions, urine specific gravity increases when body mass increases. Michael Sawka: I think most people would not argue with that. However, I think the question is, if you are actually going to use it as a measure for hydration status, i.e. if you get a urine specific gravity value of X, is it indicative of dehydration or some level of hydration? Susan Shirreffs: I would use osmolality or specific gravity. If I see a value of urine osmolality of near 900 or 1000, I would say there s a good chance this person is dehydrated, such that we could quantify it to around 2% body mass loss. For values at 700, we couldn t say that they were dehydrated by a specific amount less, and if it s 1100, then a specific amount more, I don t think I could use it in that way. When we also look at large numbers of individuals at pre-training, we do see an indication that if you measure their osmolality at that point, that there s a reasonable correlation between that pre-training osmolality and how much they choose to drink voluntarily during training. So that if it s higher, suggesting perhaps they are slightly dehydrated, they do voluntarily drink more during training. I think it s an indicator, rather than a quantification. Michael Sawka: I would agree with that. Now, I was really interested in your impedance work, Dr. Armstrong, particularly the multi-frequency, because the single impedance has always been shown to be a good rough estimate of TBW. But as you know, when you dehydrate, there are problems that arise in different types of dehydration and a variety of confounding factors, but you seem to think that the multi-frequency with the Cole correction works pretty well. I know that Bartok and Schoeller have recently looked at that, and they found S50 Nutrition Reviews, Vol. 63, No. 6

12 that not necessarily TBW but a change in TBW really was unsatisfactory for individual values. What are your thoughts? Larry Armstrong: Both single-frequency and bioimpedance spectroscopy with multiple frequencies, I think are not valid when dehydration or fluid shifts occur. Also, changes in skin-blood-flow, skin temperature, or posture might affect these values. There are many things that have to be controlled very carefully. In the studies done by my group, by Long, and by Bartok, at the beginning of the measurements the subjects were well hydrated, stationary, and the conditions of the environment had been controlled carefully. That is very important and I would agree with you. Across time, when there are perturbations in the fluid state, I would not trust impedance values. Michael Sawka: So you think impedance has some value as a rough estimate of TBW, but when you start getting into hydration changes, you don t think there is a lot of value there? Larry Armstrong: If you are comparing point A to point B. Patrick Ritz: I m not actually sure that isotopic measurements would provide you with sufficient precision to be able to see a small change. Larry Armstrong: I believe that you are correct. When I was an investigator at USARIEM, in Natick, MA, I observed that the precision of bioimpedance spectroscopy is about 800 ml. Patrick Ritz: And for isotopic techniques the precision is about 1 L, so any change below 2Lisdifficult to see with any of these techniques. Michael Sawka: Usually in such studies, they measure TBW; they will be very standardized and then they start using weight changes. Antonio Dal Canton: In my experience, urine specific gravity is a very good surrogate for urine osmolality, because the molecules that contribute to urine osmolality are pretty small (i.e. urea and electrolytes), and their weight and their number rise together with some exceptions. For example, in proteinuria you have very large and heavy molecules and specific gravity will increase, but osmolality of urine will not increase. It may be that you have some problems in measuring specific gravity with a refractometer, because to the layman, it might be quite difficult to use that instrument. But maybe if you use a floating densitometer it is much easier to read and maybe even more reliable. I don t know whether using a refractometer may be misleading or may induce mistakes. Michael Sawka: Yes, I can t really comment on the instrumentation approach. All I can say is that the studies I have seen that have actually compared urine osmolality and urine specific gravity show a good relationship, and that s always fairly constant. But what they can t show is the relationship between that and change in plasma osmolality. When you go back and do careful calculation of the body weight changes, as Adolph did in the desert, being very precise, this follows the general trend, but there is a wide scatter. So I certainly would agree with Dr. Armstrong that there are good indices for screening, such as urine color, specific gravity, and osmolality. But I am not so sure how good it is just to take one measure and say this is your hydration level. Larry Armstrong: I think that Dr. Sawka s view as a scientist is exactly right. But the question then becomes, what is the need for research in the laboratory versus field use? Those uses are different. Michael Sawka: Exactly. When we give advice to soldiers, we don t use the urine color charts, but we tell them to look at their urine volume and the color; that if they are dark and they are not urinating enough, and if it persists, they are probably dehydrated, and certainly we are not going to argue that. It s just translating that into a value. Irwin Rosenberg: Dr. Sawka just mentioned urine volume as well as color. When we talked a little earlier about some of the special considerations with the elderly, and Dr. Dal Canton reminded us that kidney function and concentration capacity are declining with age to some extent, what could we say about the use of color alone, without volume, as a measure of hydration status in a 75-year-old, let s say, who might be excreting greater volume in order to be able to excrete his or her solute load? How would we apply that information, which you derived largely from young athletes, to an assessment of an older population? Larry Armstrong: I don t recall anyone utilizing the urine color chart in an elderly population. However, during a heat stroke study that I did some years ago, we evaluated a military officer who was about 45 years old and nearing retirement. We tested him three times over the course of 11 months to see if he was heat tolerant. He had experienced hypo-filtration after a heat stroke, probably because of renal damage due to hyperthermia. We observed that he always had a urine specific gravity of to 1.004, representing hypo-filtration. If we lose the ability to filter with age and if it decreases 60% or 70% by the age of 80, as we have heard, then there is likely to be a difference in the use of the urine color chart; it may have a different meaning. Irwin Rosenberg: Obviously, that would require some tests to see what the applicability across populations might be. Larry Armstrong: Yes, and for children as well. Monique Ferry: In elderly people, the main problem is that many are incontinent and it is difficult to obtain urine. Urine volume is quite impossible to obtain, Nutrition Reviews, Vol. 63, No. 6 S51