Reference Values for Hydration Biomarkers: Optimizing Athletic Performance and Recovery

Introduction

Deficits of total body water are relevant to all athletes because substantial fluid losses degrade competitive sport performance^1–4 and increase the risk of exertional heat illnesses, especially heat exhaustion.^5–9 Dehydration potentiates hyperthermia, increases cardiovascular strain, and elevates perceived exertion. Specifically, a body mass loss of 3–4% induced by dehydration may decrease muscular strength by 2%, muscular power by 3%, and high-intensity endurance exercise by 10%.^1,10 As the body water deficit increases (ie, due to unreplaced fluid losses during outdoor competition, training sessions, “making weight” in combat sports, or routine daily activities), both detrimental physiological effects (ie, which often influence exercise) and clinical signs and symptoms proliferate.^11–13

The following documented scenarios illustrate the wide range of fluid deficits that are experienced by athletes in various team and individual sports. During hot summer months, collegiate American football linemen can lose 4–6 kg of body water during a single summer practice session,⁵ and the sweat rates of tennis players and marathon runners during competition reach extremes of 3.4 and 3.7 L/h, respectively.^14,15 In moderate or cool environments, ultraendurance athletes can experience extreme body water losses (7.8–8.5 kg, 11–12% of body mass) when competing in a 12.3 h Ironman triathlon event.¹⁶ During 1.5–2.5 h training sessions across a variety of sports (eg, European soccer, American football, tennis, cross country), body mass deficits typically range from 0.5–3.2% when fluids are readily available.^17–20 Other athletes purposefully dehydrate to qualify for lower weight classes (eg, wrestling, martial arts); this regimen can result in body water losses of 3–5%.^21,22 Even on days that are void of training or competition, it is possible for healthy young adults to incur a 2.1% body mass loss during a single day of routine activities if no fluid is consumed.²³

At the opposite end of the hydration continuum, some endurance athletes consume a large volume of fluid that greatly exceeds the volume of fluid which he/she lost as sweat. Because sodium is lost in sweat, the outcomes are dilution of body fluids and reduced blood concentration of sodium.²⁴ This phenomenon is appropriately named exertional hyponatremia or water intoxication. Not only does exertional hyponatremia degrade exercise performance (eg, due to undue fatigue), but severe cases may result in coma, pulmonary edema, cerebral edema, or death.⁴ The prominent signs and symptoms include dilute urine, swollen hands, disorientation, nausea, vomiting, skeletal muscle twitching, and grand mal seizure.²⁵ Documented cases of symptomatic exertional hyponatremia^26,27 have shown that fluid volumes of 10.3–14.7 L were consumed during exercise lasting 4–10 h. However, exertional hyponatremia has also been verified among non-endurance athletes,^4,24 soldiers,²⁸ and individuals who erroneously believed that excessive water intake was beneficial to performance or health.²⁶ To reduce the risk of exertional hyponatremia, athletes are advised to gain no weight during exercise (ie, signifying fluid retention), and use a digital floor scale before and after exercise to confirm body weight change.³

The aforementioned large range of body water changes, coupled with the dynamic/complex physiological effector responses that occur when the brain senses altered body water volume or concentration,^29–33 represents great complexity that makes it difficult (ie, for clinicians, physiologists, athletes, or coaches) to accurately predict in advance the fluid deficit or excess which any athlete might experience during training or competition. This complexity explains, in part, why no single hydration biomarker allows a conclusive clinical diagnosis of dehydration for all athletes in all sport scenarios.^34–39 Thus, we recommend creating an individualized hydration plan (IHP) for each athlete which incorporates multiple biomarkers, in agreement with numerous investigators and professional organizations.^{3,4,27,40–43} Specifically, we recommend three hydration indices which are inexpensive, valid, and require minimal technical expertise: body mass change, urine specific gravity, and rating of thirst. This approach provides athletes with data that is unique in the existing literature because most publications^4,44–50 provide only a single hydration biomarker measurement (eg, urine specific gravity of 1.020) or a narrow range of values (eg, body mass loss of 3–4%) for all athletes, to indicate the threshold of dehydration’s detrimental effects. Using only one biomarker to assess hydration state is inferior, however, because (a) interindividual variability of fluid–electrolyte imbalances is large and is influenced by many factors;^31,51–54 and (b) research studies^31,55 have determined that combining two or three hydration indices (ie, body mass change, urine concentration, and thirst rating) improves the likelihood of distinguishing hypohydration from euhydration (ie, an optimal body water balance without underhydration or overhydration). In addition, our three criterion biomarker approach contributes importantly to the literature because no inventory of hydration biomarker data exists to guide interpretation of the unique body water changes (ie, deficits or excesses) which athletes experience in various sports.

Therefore, the purpose of this review is to provide reference values for three field expedient biomarkers: body mass change, urine specific gravity, and rating of thirst. We have assembled a novel inventory of reference values for these three hydration biomarkers from field studies that involved outdoor competitions, training sessions, “making weight” in combat sports, and performing routine daily activities with no exercise. This collection of reference values is intended for use by clinicians, athletes, coaches, and physiologists, (a) as one of multiple resources that can be utilized to create and evaluate IHP for athletes and (b) to interpret hydration status before physical training or competition, post-exercise dehydration or excess fluid intake/retention, rehydration that optimizes recovery and health, and hydration status during routine daily activities.

Search Methods

Manual searches for relevant articles were performed using advanced search operators (eg, “and”, “or”, “not”). These searches focused on field studies that incorporated measurements of three human hydration biomarkers: body mass, urine specific gravity, and subjective ratings of thirst. Search inclusion criteria included English language, full-text, peer-reviewed articles with adult test participants, and availability through May 1, 2024. These searches were performed in the PubMed and Google Scholar databases; they employed numerous combinations of the following keywords, during separate explorations of the title/abstract and all text fields: “athlete”, “athletic”, “sports”, “hydration”, “hydrate”, “hydrated”, “hydrates”, “hydrating”, “biomarker”, “biomarkers”, “index”, “indexes”, “indices”, “indicators”, “urine specific gravity”, “thirst”, “body mass”, and “body weight”. These online searches acknowledged that the authors of hydration-relevant articles rarely distinguish the term “body mass” from “body weight”.

Using these search terms within PubMed and Google Scholar databases, initially 3,224 articles were identified as potentially relevant (Figure 1). After duplicate records were removed, we reviewed titles and abstracts (ie, seeking studies that reported body mass change, urine specific gravity, and a subjective thirst rating as hydration indices) to identify potentially appropriate full-length articles which should be assessed further. Also, we performed manual searches of the reference lists in these relevant articles, as well as the bibliographies found in germane systematic reviews and meta-analyses. Finally, we investigated full text (.pdf) versions of 385 articles to ascertain if they met our inclusion criteria. Figure 1 illustrates the steps in this selection process.

Figure 1 Selection process for the 16 field research studies which appear in the present review.

Results

Using the keywords, inclusion criteria, and methods described above, we accepted 16 articles for analysis within the present review. All of these 16 articles measured body mass with a floor scale and urine specific gravity with a handheld refractometer; subjective ratings of thirst were measured using one of the two methods (eg, category scale or visual analog scale) described in footnotes b and e of Table 1. Thus, the following tables present measurements of only body weight change, urine specific gravity, and subjective rating of thirst during a variety of athletic and routine daily activities. For example, Table 1 presents reference values which can be utilized by clinicians, coaches, physiologists, and athletes to develop an IHP for use during outdoor athletic competitions and training sessions.

Table 1 Hydration Biomarker Values Associated with Outdoor Athletic Competitions and Training Sessions

Table 2 focuses on combat sport (ie, wrestling, boxing, judo) athletes who attempt rapid body mass loss to qualify for a weight class. Although our extensive literature search identified only a few studies of this type, values are provided for both pre- and post-weigh-in. Table 3 describes the changes of body mass, urine specific gravity, and subjective thirst ratings of two men who developed exertional hyponatremia during a 164-km summer ultraendurance cycling event.²⁷ In column 4, mean (± SD) values are presented for a comparison group of 31 normonatremic cyclists who participated in the same event.

Table 2 Hydration Biomarker Values Associated with Rapid Body Mass Loss to “Make Weight” in Combat Sportsa

Table 3 Change of Body Mass, Urine Specific Gravity, Thirst, and Serum Sodium Concentrations of Two Ultraendurance Road Cyclists (Columns 2 and 3) Who Experienced Exertional Hyponatremia

In Table 1 and 2, the relationship between average body mass change and urine specific gravity (r = 0.50, p = 0.013), as well as the relationship between average body mass change and rating of thirst (r = 0.68, p = 0.001), were significantly correlated.

Table 4 and 5 describe our three criterion hydration biomarkers, as they were measured during the routine daily activities of representative, healthy young adults. The test participants in these field studies^23,68–70 were nonathletes who either performed recreational exercise (Table 4) or refrained from exercise (Table 5). The experimental design of the former study^69,70 allowed a comparison of the average differences between morning and afternoon measurements. The latter field study (Table 5) illustrates hydration biomarker perturbations that resulted from a 24-h period of water restriction, a 30-min unrestricted water intake, and a 24-h ad libitum water rehydration. It is noteworthy that the average changes of body mass, urine specific gravity, and rating of thirst in Table 5 (ie, involving no exercise, column 4) are greater than many of the changes that are reported in Tables 1–3 which involved exercise.

Table 4 A Comparison of Morning and Afternoon Body Mass, Urine Specific Gravity, and Thirst Valuesa for 24 healthy, Young Adultsb Who Conducted Routine Daily Activities (Free Living) Including Eating, Drinking, and Recreational exercise

Table 5 The Effects of Water Restriction and Rehydration on Body Mass, Urine Specific Gravity, and Thirst During Routine Daily Activities. These Measurements Were Obtained During Five Laboratory Visits on Three Consecutive Days (Columns 2–6). At All Other Times, These 14 healthy Young Mena Were Free Living and Refrained From Exercise. On Days 1 and 2 (Columns 2–4), They Consumed No Water and Ate Only Dry Food Items for 24 h. During the 30-min Unrestricted Water Intake Period (Day 2, Column 4 to 5), Participants Drank as Much Water as Desiredb, Then Consumed Water and Food Ad Libitum for 24 h (Columns 5 and 6)

Figure 2 provides a visual summary of the hydration biomarker data (group averages) contained in Tables 1 and 5. As such, it illustrates net body water loss during athletic competition or training, as well as water restriction during daily activities that include no exercise. All thirst ratings and urine specific gravity values in Figure 2 increased when a net body water loss occurred (ie, body mass decreased). Figure 3 further illustrates the association among the criterion biomarker data (group averages) from the research studies presented in Tables 1–5.

Figure 2 A visual summary of body mass, thirst rating, and urine specific gravity changes due to net body water loss during various activities. The 10 color-coded symbols represent the athletic competitions and training sessions described in Table 1. White symbols denote the 24-h period of water restriction during routine daily activities (n = 14) with no exercise, as described in Table 5 (columns 2 to 4). All symbols represent group averages (range, n = 6–57). Abbreviation: VAS, visual analog scale.

Figure 3 Relationships among the three hydration-relevant variables of this review: thirst, body mass loss, and urine specific gravity. Data points are mean values from the studies cited in Tables 1–5. Test participants rated thirst using either a 100-mm visual analog scale (left vertical axis) or a 9-point category scale (right vertical axis) in panels (A) and (B). Percent female sex: panel (A), 21%, panel (B), 8%, panel (C), 8%.

Discussion

The evidence gathered by the present narrative review illustrates a wide range of hydration biomarker changes (ie, shown as standard deviations from the mean) which men and women experience during competition, training, and routine daily activities. Although not within the scope of the present study, from the assessed literature it is evident that this sizeable variability results from interactions of (a) sport-specific exercise mode, duration, and intensity; (b) individual differences of heat acclimatization status, fluid-electrolyte losses in sweat, physical training history, personal knowledge or beliefs about rehydration during exercise; (c) intentional water loss by athletes who attempt to “make weight”; (d) gear or clothing insulation; and (e) environmental conditions.^{31,51–54,71,72} This complexity and variability make it difficult to accurately predict in advance the fluid deficit or excess which any athlete might encounter during training or competition.^19,42,73,74 Thus, investigators and professional organizations recommend creating an individualized plan for each athlete.^{3,4,19,30,34,56,73,75–77} However, previous investigations have not succeeded in providing practical, individualized hydration guidelines for athletes,⁷⁶ few inventories of hydration biomarker reference values have been published, and none of those studies examined the hydration biomarker responses of athletes during training, competition, overhydration, or purposeful weight loss for multiple sports^76,78–80 as are presented in Tables 1–3. Also, in most publications, a single hydration biomarker measurement (eg, urine specific gravity of 1.020) or a narrow range of values (eg, body mass loss of 3–4%) represents the threshold of detrimental effects for all athletes.^{1,4,46,81–83} This approach does not take into account the large variability of hydration biomarker values shown in Tables 1–5.

Optimal IHPs incorporate biomarkers that are easy-to-use, field expedient, noninvasive, safe, portable, and inexpensive.^{3,4,30,34,75,77} In view of these criteria, several hydration biomarkers were initially considered for inclusion in the present review,^34,37,39,84 including clinical assessment methods.³⁵ The following five points influenced our decision to employ three hydration biomarkers: body mass change, urine specific gravity, and subjective rating of thirst. First, the 2022 consensus statement of the International Olympic Committee Medical and Scientific Commission recommended that athletes monitor their body weight, urine specific gravity (or urine color), and thirst during periods of training or competing in hot environments.⁸² Second, an evaluation of 13 hydration assessment methods determined that body mass change, urine specific gravity, and rating of thirst were among the top five most suitable field techniques;³⁴ the criteria for this finding were cost and time of analysis, technical expertise required, portability, and likelihood of an adverse event. Third, acute body mass change is recognized as the simplest and most accurate index of hydration status when serial measurements are collected in close proximity (eg, pre- and post-exercise, ≤ 4h) and when corrected for the mass of urine excreted and fluid consumed during the activity.^4,34,85–88 Fourth, human body water balance primarily depends on two parameters: thirst that influences input, and urine excretion that determines output. When a deficit or excess of total body water exists, the kidneys act to restore euhydration. As a result, a change of urine concentration (eg, specific gravity) can distinguish a net loss or gain of body water (Figure 2), discriminate between euhydration and dehydration,^4,27,89,90 and determine whether daily total water intake is adequate.^68,91–93 Fifth, measurements of body mass, urine specific gravity, and thirst each have limitations when they are employed alone to assess hydration state, due to factors such as time of day, internal circadian rhythms, first morning hydration state, and dietary osmolar load.^3,94–96 For example, consumption of a large bolus of water or a high protein diet may alter urine specific gravity acutely.^30,97,98 Also, the intensity of thirst may be difficult to replicate consistently on a rating scale due to the interactions of numerous internal physiologic (eg, plasma osmolality and volume, oropharyngeal sensations) and external (eg, fluid characteristics, learned preferences, air temperature) influencing factors which are integrated by the brain.^{32,33,59,94–106} Therefore, we conclude that a single hydration assessment method (a) is not valid in all scenarios or for all individuals,^34–39 and (b) does not provide unequivocal evidence of fluid intake inadequacy.³¹ However, research studies have shown that the combination of two hydration biomarkers makes identification of inadequate fluid intake or dehydration likely, and the combination of three indices makes this determination very likely.^31,55,70

Hydration Biomarker Interrelationships

Figure 2 provides a visual summary of body mass changes (%), subjective thirst ratings, and urine specific gravity values from 12 studies which focused on outdoor athletic competitions or training sessions (Table 1), and 24-h water restriction during routine daily activities (Table 5). All symbols represent group means (n = 6–28) and illustrate consistent agreement among these three hydration indices, in that all thirst ratings increased and all urine specific gravity values increased when a net body water loss occurred (ie, body mass decreased).

Figure 3 illustrates the relationships of the three criterion biomarkers presented in Tables 1–5. The calculated Spearman’s rho correlation coefficients of these relationships (R² = 0.41–0.88) are considered to be moderate (Panel C) and strong (panels A and B).¹⁰⁷ Thus, we propose the following theoretical interpretation of Figure 3 that arises from widely recognized physiological responses. Specifically, both urine concentration within the kidneys (ie, represented here by increased specific gravity) and the sensation of thirst are strongly influenced by a body water deficit (ie, elevated extracellular fluid concentration). These effects are regulated by the release of vasopressin (antidiuretic hormone) from the brain into the circulation,^108–111 in response to increased plasma concentration, decreased plasma volume, and/or reduced arterial blood pressure.^101,112 Additionally, thirst, urine concentration, and vasopressin release are influenced by oropharyngeal (ie, rear of the tongue, soft palate, and throat) nerve signals to the brain which rapidly reduce total fluid intake and modulate satiety,^113,114 often before blood volume–concentration–pressure changes occur. We believe that these interrelated responses account for the strong correlation shown in Figure 3.

Although an acute body mass change is recognized as the simplest and most accurate index of hydration status when serial measurements are collected in close proximity (as noted above), multiple factors influence the extent of this change.^115,116 To explore the possibility that body mass change may not be significantly correlated with urine specific gravity or thirst, we analyzed the data in Tables 1 and 2. Both the relationship between body mass change and urine specific gravity (r = 0.50, p = 0.013), as well as the relationship between body mass change and rating of thirst (r = 0.68, p = 0.001), were significantly correlated. The strength of these two relationships are interpreted as fair and moderately strong, respectively.^107,117

The findings described in the three previous paragraphs demonstrate that our three criterion hydration biomarkers responded consistently when a body water deficit occurred; this suggests satisfactory internal validity. Two individual exceptions to this observation appear in Table 3, in the form of cyclists who experienced exertional hyponatremia during a 164-km summer ultraendurance event. Row 1 of Table 3 shows that the comparison group with normal serum sodium levels (column 4) consumed an average of 5.9 L of fluid during 9.0 h of cycling. In contrast, the hyponatremic cyclists A and B consumed 13.7 L/8.9 h and 14.7 L/10.6 h, respectively, which resulted in serum sodium concentrations of 130 mmol/L for both²⁷ – well below the range observed among healthy adults (136–145 mmol/L).⁶⁷ In terms of body mass, the comparison group lost 1.3%, cyclist A gained 4.3%, and cyclist B experienced virtually no change of body mass (+0.1%). This obvious difference among the two hyponatremic cyclists (ie, who had identical pre- and post-ride serum sodium values) likely resulted from a large difference in their sweat rates and the fact that cyclist B rode 1.7 h longer than cyclist A. It is noteworthy that both cyclists A and B exhibited post-ride urine specific gravities and thirst ratings that were well below the mean values of the comparison group (column 4). This illustrates an important concept. Due to the complex interactions of many factors, some cases of exertional hyponatremia involve body mass gain (ie, overhydration and water retention that exceeds sweat, urine, and respiratory water losses),^118,119 whereas other cases are associated with a large unreplaced sweat sodium loss and partially-replaced water loss.^120–122 Thus, the development of exertional hyponatremia in cyclists A and B (Table 3) and the responses of hydration biomarkers were idiosyncratic.²⁷

Considerations for Developing an IHP

Previous publications describe (a) the negative effects which fluid–electrolyte imbalances can have on athletic performance, physiological functions, and health;^3,4,46,51 and (b) the importance of developing an IHP for each athlete,¹²³ as opposed to utilizing a single hydration biomarker value or a narrow range of values to indicate the threshold of detrimental effects for all athletes.^84,98 In its simplest form, an IHP involves measuring an athlete’s sweat rate (described in the section titled “Body mass”) and planning the appropriate volume and timing of fluid intake in view of the anticipated rehydration opportunities of the sport. More sophisticated IHP models take into account an athlete’s tolerance to consuming a relatively large fluid volume,⁴¹ the concentration of electrolytes in sweat (ie, sodium, chloride, potassium),^124,125 or the events that require sodium and carbohydrate in rehydration fluids.^51,122,126 An IHP that incorporates most or all of these factors increases the likelihood that an athlete will attain the desirable middle ground between underhydration and overhydration, thereby optimizing performance.

Interpreting Biomarker Data

The hydration biomarker data in Tables 1–5 are intended to be used concurrently with other information to interpret changes of hydration status. Because no single biomarker accurately indicates hydration status for all athletes, activities, and environments, and because different biomarkers sometimes provide conflicting information (ie, due to the dynamic complexity of human fluid–electrolyte regulation^29–33), it is best to employ information from multiple sources to determine hydration status.^31–55 One common technique involves comparing an athlete’s biomarker information to a single static value that represents the threshold of detrimental effects attributed to dehydration; in fact, most sport nutrition and human hydration publications^4,44–50 provide a single biomarker value (eg, urine specific gravity of 1.020) or a narrow range of values (eg, body mass loss of 3–4%) for all athletes, activities, and environments. As a second source of information, each athlete can maintain a personal diary of body weight, urine specific gravity, and thirst measurements, before and after exercise; with this data, he/she can monitor trends across days and weeks to develop personalized reference values representing usual body weight changes, urine specific gravities, and thirst ratings. This diary of past hydration experiences can then be compared to each new competition or daily training session. The biomarker data of the present review represents a third source of information. Tables 1–5 provide activity-specific values which can be utilized by athletes, coaches, clinicians, and physiologists as one source of information to interpret IHP effectiveness. Changes (pre- versus post-exercise) of an athlete’s body mass, urine specific gravity, and thirst can be compared to the hydration biomarker averages in Tables 1–5, to determine if those changes are typical of that sport or activity; if dehydration, euhydration (ie, optimal body water balance), or overhydration exist; and the extent of body water loss or gain experienced (ie, departure from average values). Further, athletes and coaches who have not collected hydration biomarker data previously can refer to the values in Tables 1–5 for a general idea of what to expect when they begin measurements.

With regard to the practical application of data in Tables 2–5, we offer four additional observations. First, the various methods of making weight in combat sports (Table 2) all result in dehydration and loss of body mass. Thus, during the process of restoring body mass to the pre-exercise level, we recommend that measurements of our three criterion biomarkers be scheduled throughout the refeeding and rehydration process. Second, in cases of overhydration (Table 3), changes of biomarker values may differ from one athlete to the next, depending on the volume and rate of fluid intake, sweat rate, body size, duration of exercise, and other factors.²⁷ We recommend that a sports medicine physician be consulted if body mass increases and if urine specific gravity is unusually low post-exercise, suggesting retention of excess water; also refer to the section titled “Avoiding overhydration”. Third, body mass tends to be lower in the morning, whereas first morning urine specific gravity and rating of thirst tend to be greater than midday and afternoon values, as shown in Table 4. If measurements are recorded at various times of the day, they may be confounded by water and food intake, voiding, exercise, and internal circadian rhythms.⁹⁵ Thus, we recommend day-to-day consistency in the time of measurements.^31,76 Fourth, as illustrated in Table 5, ordinary daily activities with no exercise can influence an athlete’s body water balance. To ensure that she/he begins a competitive event or daily training session well-hydrated, morning measurements of the three criterion variables should guide pre-exercise fluid intake.

The following points present additional insights regarding the interpretation of biomarker data.

• To discover an athlete’s typical patterns of fluid loss and fluid intake, several observations should be made in conjunction with training sessions and relevant field simulations of competitive events.
• Because our three criterion biomarkers reflect the responses of different organ systems, it is possible that one hydration index indicates a water excess whereas the other two indices suggest a water deficit. In this case, the majority indication should be utilized until another assessment is completed a few hours later.¹²⁴
• When extreme or chronic biomarker changes are observed, consider possible causes and repeat measurements at regular intervals. If values do not return to pre-exercise levels within a reasonable time frame, consult a physician.

Body Mass

• During exercise lasting 0.5–4.0 h, body mass change is the most commonly used representation of body water loss or gain^36,39 because muscle mass and adipose tissue changes are negligible.
• We recommend using the same digital floor scale each time that body mass is measured. Many floor scales can be calibrated by following the manufacturer’s instructions and loading the scale with objects of known mass. Identical clothing and footwear should be worn during each measurement. A digital floor scale that reads to ±100 g has a measurement resolution of ±0.1 L (eg, 100 mL out of a 42–47 L total body water volume).
• An athlete’s sweat rate (ie, the volume of sweat produced per hour, expressed as L/h or qt/h) is determined by measuring pre-exercise nude body mass and exercising for 60 min (ie, in an environment, at an exercise intensity, and wearing clothing/equipment that simulates a future event). After exercise, nude post-exercise body mass, with sweat wiped from the skin, is subtracted from the pre-exercise body mass and corrected for fluid consumed and urine eliminated, if any.^41,51,124 Additional details regarding the calculation of sweat rate, plus the methods for measuring sweat electrolyte losses (eg, sodium, potassium), appear elsewhere.^41,124
• Sweat electrolyte losses become problematic during events lasting more than 4 h (ie, due to an increased risk of exertional hyponatremia) or across consecutive days of heavy sweating with inadequate dietary water and sodium replacement.^24,41,51
• To assess the day-to-day change of body mass, measurements should be compared to the baseline body mass for that individual. To determine this baseline, measure morning body mass on 3–5 consecutive days.⁸⁵ The median (middle) or average body mass serves as a meaningful baseline for that individual.¹²⁴ If the body mass value on the morning after an endurance event is notably less or greater than baseline, fluid intake should be adjusted accordingly (ie, avoiding overconsumption and underconsumption) for 24–48 h.^75,127
• Two other considerations are relevant. First, if dietary “carbohydrate loading” is employed during the days prior to competition, the baseline body mass may increase (0.5–1.5 kg) because water is stored with glycogen in skeletal muscles. Second, body mass measurements may not be valid after an event lasting 7–24 h because cellular water is generated during metabolic reactions, and skeletal muscle or adipose tissue is degraded.^{51,115,128,129}

Urine Specific Gravity

When a total body water deficit exists, the kidneys retain water by concentrating urine and decreasing urine volume; in contrast, when excess fluid has been consumed and retained, a larger volume of dilute urine is produced. These changes (ie, dehydration or fluid overload) can be detected by observing urine samples.

• Urine specific gravity is recommended as a practical and valid biomarker for identifying suboptimal fluid intake or underhydration by professional sports medicine organizations,^3,4,46 physiologists, and researchers.^{6,76,89,97,130–132}
• Urine specific gravity (see Tables 1–5, Figures 2 and 3) is measured quickly and accurately with a refractometer.^75,133 This instrument is relatively inexpensive, simple to use, and is available from online retail platforms as a handheld device which can be calibrated before use.

• An euhydrated urine specific gravity baseline value (ie, usually between 1.015 and 1.025^26,27) should be determined for each athlete, using a method similar to the one described above for body weight. Changes from this baseline value will assist the interpretation of hydration status.

• Hydration assessment via urine specific gravity has limitations when used alone,^90,98,134 as do all hydration biomarkers.
• Many athletes consume nutritional supplements to gain a performance edge or health benefit.^135–138 When consumed in the manufacturer’s recommended dosage, supplements such as vitamin C, beetroot, and riboflavin may or may not alter urine color.^139,140 However, even if they visibly alter urine color, it is unlikely that nutritional supplements will measurably influence urine specific gravity because the mass and osmolar content of nutritional supplements is small in comparison to the total osmolar load of the food items in an athlete’s daily diet.³⁰
• During normal daily activities, men may^78,79,141 or may not^69,70 exhibit a greater urine specific gravity than women.
• An analysis of data from the National Health and Nutrition Examination Survey (NHANES; n = 3,634) indicated that urine specific gravity is influenced by body size and body composition.¹⁴² Thus, athletes who have a large lean body mass or body mass index can be expected to have greater urine specific gravity values than their smaller counterparts.
• If an athlete consumes a large volume of pure water or dilute beverage rapidly (eg, 1.4 L in 30 minutes), urine specific gravity decreases (eg, < 1.010) in both well-hydrated and dehydrated athletes as a defense against fluid overload.⁹⁷ In contrast, if the same volume of water or dilute fluid is consumed gradually and in small aliquots, the change of urine specific gravity will be minor.^30,115,119 The sensation of thirst exhibits a parallel phenomenon.

Rating of Thirst

As noted above, thirst is a sensation that regulates total body water and the concentration of body fluids. Coordinated by the brain, thirst responds in concert with renal responses to correct fluid and electrolyte perturbations. Figure 3 illustrates the relationships between thirst, urine specific gravity, and body mass loss. In particular, panel A supports our recommendation to assess subjective ratings of thirst.

• In research studies involving a variety of activities, thirst ratings have consistently identified an existing body water deficit. See Figure 2.
• Thirst is initially sensed when a body mass loss of 1–2% occurs during exercise-heat stress,^11,32 or when a 0.5% body mass loss occurs during routine daily activities involving no exercise,²³ and intensifies as the body water deficit increases.
• Research has shown that the morning sensation of thirst upon waking is a reliable indicator of hydration status.^76,143 However, an intense morning sensation of thirst may not affect body mass, total water intake, or hydration status later in the day.⁶⁹
• We recommend measuring thirst with a 9-point intensity rating scale⁶³ because of its simplicity. This scale offers the following options: 1, not thirsty at all; 3, a little thirsty; 5, moderately thirsty; 7, very thirsty; and 9, very very thirsty. See Tables 1–4 and Figure 3.
• An euhydrated baseline thirst rating should be determined for each athlete, using a method similar to the one described above for body weight. Changes from this baseline value will assist the interpretation of hydration status.

• Field studies have reported that thirst was not correlated with the hydration status of youth campers¹⁴⁴ and high school football players.¹⁴⁵ This may be due to an age-related difference of thirst (ie, influenced by puberty and maturation). Alternatively, these observations may be due to the difficulty of controlling pre-exercise hydration state or fluid intake throughout the day, in a field setting. This matter requires further investigation to determine its relevance to the thirst responses of young athletes.

Strategies for Drinking During Exercise

• An important component of each IHP is the amount of fluid which an athlete can gain access to during training or competition.⁴¹ If an athlete has unrestricted access, consuming the appropriate amount of fluid is relatively easy. However, the rules of many sporting events (eg, tennis) do not allow continuous access to fluids; this challenges the athlete to appropriately time his or her fluid intake. Also, if the athlete is participating in an endurance running event, the number of aid stations are predetermined. Thus, it is essential for the athlete to determine his/her sweat rate (described above) and consider the total exercise time, to calculate how much he or she should consume at each station.
• When developing an approach to fluid intake during prolonged or endurance events, athletes have five options: (1) drink only when thirst is sensed, (2) drink whenever and in whatever volume is desired (ie, ad libitum), (3) drink according to an IHP, (4) drink nothing during exercise, and (5) drink as much as possible, in excess of thirst. A detailed comparison of these options, including pros and cons, appears elsewhere.⁵¹ To our knowledge, no professional sports medicine or sport nutrition organization recommends options 4 and 5. The present authors support option 3.
• A controlled, single-blinded research study¹⁴⁶ reported that prolonged ad libitum fluid replacement (option 2 above), after a 110-min intermittent running session, left a small degree of hypohydration at 20 h post-exercise. Consistent with this finding, endurance athletes consistently replace ≤ 50% of sweat losses when allowed to consume liquids ad libitum during exercise.^147–149 These publications support the development and application of an IHP for athletes (option 3 above).
• In sports where athletes have few, defined opportunities to drink (eg, soccer, field hockey, rugby), it is essential that athletes begin competition well-hydrated but not overhydrated. The values of body mass change, urine specific gravity, and subjective rating of thirst in Tables 1–5 can be employed to interpret an athlete’s hydration status prior to an event.
• Utilizing the reference values in Tables 1–5 to develop an IHP, each athlete can seek a hydration middle ground between dehydration and overhydration that allows him/her to optimize physical performance and reduce the risk of illness.

Avoiding Overhydration

As discussed above, the development of exertional hyponatremia and the changes of hydration biomarkers in Table 3 are idiosyncratic.²⁷ We therefore believe that creating an IHP is the most straightforward pre-event action that athletes can take to avoid excessive fluid intake. This involves measuring personal sweat rate (L/h) and considering the event duration (h), to calculate a rate of fluid intake (L/h) that results in no increase of body mass (ie, indicating retention of excess fluid).

• Body mass, measured before and after exercise with a digital floor scale, informs post-exercise fluid consumption.³
• A low specific gravity value (eg, less than 1.010) or pale yellow urine color (see below) in multiple consecutive urine samples typically indicates overhydration (ie, the kidneys are releasing excess body water) and suggests that fluid intake should be curtailed and monitored until specific gravity values moderate.
• In field settings, when a refractometer or floor scale is not available, urine color can be used to estimate hydration status and the adequacy of daily water intake.^{89,97,150,151} This field expedient technique is accomplished in outdoor daylight or a well-lit room by holding urine samples against a white background and rating them as whole numbers, after comparison to a previously validated chart (scale colors ranging from 1 [lightest] to 8 [darkest]).^89,97 This chart, including interpretative guidelines, can be requested from the first author via Email (see title page). Two important precautionary notes are in order. First, computer screens and printed pages often produce copies of online color charts that have invalid colors. Second, numerous urine color charts can be found online but few have been validated in human research studies.
• Before exercise, a low urine specific gravity (eg, 1.003 or 1.005; see Table 3, row 5) increases the risk of developing exertional hyponatremia.^26,27
• During exercise, being alert for the signs and symptoms of exertional hyponatremia provides additional protection. For example, mild exertional hyponatremia exhibits nonspecific symptoms (eg, nausea, lightheadedness, headache), whereas severe symptomatic hyponatremia may involve vomiting, altered mental status, frothy sputum, collapse, seizure, or coma.^3,66
• Two previous analyses of triathlete and endurance runner data^25,51 showed that those who consumed fluids at a rate < 700 mL/h had a decreased risk of symptomatic exertional hyponatremia (serum sodium < 130 mmol/L). This 700 mL/h threshold is consistent with the fluid intake recommendations of the International Marathon Medical Directors Association,¹⁵² the American College of Sports Medicine,⁴ and a mathematical model that was designed to clarify the etiology of exertional hyponatremia.²⁴

Weight Category Sports

Various methods are employed to “make weight” in rowing, powerlifting, body building, and combat sports (ie, boxing, wrestling, taekwondo, judo, mixed martial arts). These methods include fasting, fluid restriction, exercise-induced sweating, avoiding foods with high carbohydrate content (ie, to avoid water retention in muscle and liver tissues), exercise-induced sweating, sauna, hot salt baths, vomiting, diet pills, laxatives, and diuretics.^{71,72,153–158}

• Each of these weight loss methods results in a unique combination of water, fat, and/or lean tissue loss; also, the effects of each on urine specific gravity and thirst are unique.
• The majority of purposeful weight loss methods involve a body water deficit, increased urine specific gravity, and increased thirst. Water loading (ie, consumption of large fluid volumes for days, followed by sudden fluid restriction on the day before weigh-in) is an exception, however this practice increases the risk of fluid overload hyponatremia (see above).
• The rapid body mass losses of rowing and combat sport competitors average 3.2–5.2% in 3–24 h.¹⁵⁹ Also see Table 2.
• The short-term body mass reductions (3–5 d) among boxing, wrestling, and taekwondo athletes range from 3.0–6.2%.¹⁵⁹
• Gradual, long-term weight management regimens (2–8 wk) of combat sport athletes reportedly result in average body mass losses up to 18.1%.¹⁵⁹

• The time that elapses between weigh-in and competition varies among sports, and affects the ability of athletes to recover from mass loss. Sports that offer a long recovery time (eg, up to 32 h) between the weigh-in and competition allow athletes to be well-hydrated and well-fueled. However, these sports also support extreme body mass manipulation, as evidenced by an average mass recovery of 9.1–10.1% by mixed martial arts competitors,¹⁶⁰ whose recovery values ranged up to 20%.¹⁵⁹
• During the week following competition, an investigation of Australian Olympic combat sports¹⁶¹ revealed that athletes regained considerable body weight (eg, boxing, 4.1%; judo, 3.6%; taekwondo, 4.6%; and wrestling, 4.8% of their baseline body mass).
• We recommend downloading a copy of the 2021 American College of Sports Medicine consensus statement which focuses on weight-category sports.¹⁵⁹ This document contains a wealth of information regarding the benefits and disadvantages of weight-making practices, pre-event weigh-in procedures, opportunities for recovery after weigh-in, sport rule changes to minimize potential harmful effects of making weight, and recommendations for safer weight-making practices in various sports.

Routine Daily Activities

• During ordinary daily activities, the average body mass may be lower in the morning (whereas urine specific gravity and thirst values tend to be greater) than during afternoon hours (Table 4). However, because exceptions have been reported,⁶⁹ the morning body mass of each athlete should be recorded in a personal diary each day, to determine his/her unique responses.
• Most humans do not experience severe dehydration (> 5% body mass loss) during routine daily activities but may reach a state of mild dehydration (1–2% of body mass lost as water) multiple times each week.³⁰ For example, an average body mass loss of 1.1% was observed in healthy young men after only 8.5 h of water restriction (Table 5).

• For reasons that have not been clarified, some individuals habitually consume little water each day.³⁰ These low volume drinkers (plain water + beverages + food moisture < 1.5 L/24 h) represent 25–33% of all adults in the United States and Europe.^162–164 This 1.5 L volume is considerably less than the Adequate Intakes for water recommended by the European Food Safety Authority¹⁶⁵ and the US National Academy of Medicine¹⁶⁶ for men (2.5, 3.7 L/d) and for women (2.0, 2.7 L/d).
• A 2024 review paper¹⁶⁷ reported that chronic low volume drinking behavior, and/or urine biomarkers that signal chronic dehydration, are strongly associated with two disease states: type 2 diabetes and kidney stones.
• The four preceding observations emphasize that hydration practices during routine daily activities are an important aspect of an athlete’s hydration state. To ensure that she/he begins a competitive event or training session well-hydrated, the pre-exercise biomarker values in Tables 1–4 can be utilized to interpret an athlete’s hydration status prior to exercise.

Summary

The hydration biomarker changes in Tables 1–5 provide reference values that can be compared to an athlete’s body weight, urine specific gravity, and thirst sensation. These reference values are intended to be used as one component of a comprehensive IHP. Such comparisons enhance the interpretation of both pre- and post-exercise measurements uniquely, by providing a window into the past experiences of athletes during outdoor events, weight category sports, training sessions, and routine daily activities. This repository of sport-relevant hydration biomarker data is unique in the extant literature because it does not rely on single threshold values (ie, one value fits all). We propose that consistent use of these reference values, as part of a multi-faceted IHP, will clarify the extent of dehydration or overhydration, guide rehydration efforts, and optimize subsequent exercise performance, recovery, and health.