Examining the Influence of Hydration Status on Physiological Responses and Running Speed During Trail Running in the Heat With Controlled Exercise Intensity : The Journal of Strength & Conditioning Research

Secondary Logo

Journal Logo

Original Research

Examining the Influence of Hydration Status on Physiological Responses and Running Speed During Trail Running in the Heat With Controlled Exercise Intensity

Lopez, Rebecca M1; Casa, Douglas J2; Jensen, Katherine A2; DeMartini, Julie K2; Pagnotta, Kelly D2; Ruiz, Roberto C2; Roti, Melissa W3; Stearns, Rebecca L2; Armstrong, Lawrence E2; Maresh, Carl M2

Author Information
Journal of Strength and Conditioning Research 25(11):p 2944-2954, November 2011. | DOI: 10.1519/JSC.0b013e318231a6c8
  • Free



For several decades, randomized controlled studies in laboratory settings have shown the importance of hydration in physiological function (namely, core temperature and heart rate [HR]) and exercise performance when exercising in the heat (10,14,22,35,36). Recent field studies have contradicted previous findings and suggested that maintaining fluid balance is not essential to maintaining physiological function in athletic field settings (15,25,26). These inconsistencies between laboratory and recent field studies have resulted in uncertainty among allied health professionals regarding fluid replacement for athletes.

Researchers have shown that prolonged exercise with fluid restriction results in dehydration and thermal strain (8,10,14,18,22,23,28,35,36). Some of the earliest work depicting the relationship between hydration status and thermoregulatory responses to exercise date back to the 1940s (29). Pitts et al. (29) examined the performance of physical work in the heat and found that the best performance during prolonged work was achieved by replacing fluids equal to those lost in sweat. In the 1960s, Buskirk and Beetham and Wyndham and Strydom reported on the negative effects of dehydration during exercise in the heat (5,44). Buskirk and Beetham illustrated the relationship between the percent of body mass (BM) loss and rising core body temperature during various marathons (5). Similarly, >30 years later, several researchers depicted a comparable relationship in a laboratory study showing the effects of graded dehydration on hyperthermia (22,35). Several other researchers have also been able to demonstrate the relationship between hydration status and core body temperature (2,10,17,18,22,28,36,38). Research studies have found increases in body temperature averaging 0.15–0.25°C (14,35) and HR increases of 4–7 b·min−1 (17,35) for every 1% of BM lost.

Despite years of studies supporting the paradigm between dehydration and elevated body temperature during exercise, this continues to be a debated or controversial topic in the literature. Recent field studies have shown that when intensity is not controlled, the effects of dehydration are not as clear (12,15,39). It is well documented that hyperthermia, and exertional heat illnesses, are not caused by dehydration alone (12,18,27,32). However, stating that dehydration does not have an effect on rectal temperature may not be justified, because there is limited research in the field setting. The gap in the literature could potentially be caused by differences in methodology between controlled laboratory studies and more recent in vivo field studies. The main difference in methodology is that the exercise intensity in laboratory studies has been controlled, whereas intensity has not been controlled in field settings.

Similarly, many field studies examining hydration influences have been case studies or observational studies and have therefore lacked the control of intensity and other variables seen in laboratory settings. Because of the nature of these observational studies during races, there was only 1 condition; therefore, the results are never compared with a control condition (13,27,39).

In a recent investigation conducted at our university we sought to clarify some of these inconsistencies. The participants performed both a 7.5-mile (12-km) maximal race and a submaximal exercise bout while either euhydrated or hypohydrated (via 22 hours of fluid restriction and no fluid during exercise). We found that as little as a 2.5% BM difference invoked significant impairments in physiological function (HR and body temperature) and performance (running time). At maximal effort, hydration enhanced performance time while decreasing physiological strain (6). Furthermore, during a controlled submaximal run in which absolute intensity (controlled via matching finishing time) was similar for both trials, improved hydration status enhanced physiological function (namely, body temperature and HR) (6). During the all-out race trials when intensity was not controlled, but rather was a self-paced race, core body temperature was only significantly different immediately after the 7.5-mile run. Others have found that when intensity was not controlled during a race, the percent change in BM was not related to postrace rectal temperature measures (39). On the other hand, when intensity was controlled via matching finishing time in the submaximal trials, hydration status resulted in significantly elevated core temperatures starting at the 5-mile mark and continued to be elevated 20-minutes postexercise.

Although there are many studies on the effect of dehydration on exercise, there is a lack of data regarding the combined effects of dehydration and intensity on core body temperature and perceived exertion, particularly in a field study setting. In this study, the exercise protocol consisted of two 12-km submaximal exercise bouts where intensity was controlled via HR to allow for comparisons in physiological responses between the 2 submaximal trials: euhydrated and hypohydrated.

The purpose of this study was to determine the effects of dehydration on physiological function and running speed when relative intensity was controlled during a 12-km submaximal run. The submaximal running speed was based on each individual's perception of a training-pace at which they were accustomed to running. We controlled relative intensity (via HR) during 2 submaximal trials. We hypothesized that although relative intensity via HR was similar between trials, dehydration would result in slower run times and elevated body temperature.


Experimental Approach to the Problem

The study design consisted of a randomized, crossover, counterbalanced design. The procedures for this study included having the subjects report to a local state park for 1 familiarization session during the 2 weeks before the 2 trials to acquaint participants with the trail course (one 4-km lap). During this familiarization session, the participants completed 2 practice laps on the 4-km course. The 4-km course consisted of typical, single-track trail with rocks, low branches, and tree roots; 25% of the course consisted of hard-packed dirt road (6).


Fourteen (7 men, 7 women) well-trained, experienced runners volunteered to participate in this study. Table 1 contains demographic information for the 14 subjects who participated in this study. We excluded participants for any of the following reasons: a history of exertional heat stroke or heat exhaustion within the last 3 years; any woman who was pregnant; chronic health problems; a history of cardiovascular, metabolic, or respiratory disease; fever or current illness; age not within the range of 18–59 years; any condition that may cause complications because of the ingestion of the ingestible temperature sensor pill (Cor-Temp: HQ, Inc., Palmetto, FL, USA). We also excluded individuals whose physical activity was for <30 min·d−1, 4 times per week of moderate intensity exercise in the last 3 months. The participants completed a medical history questionnaire and a running history questionnaire before enrolling in the study. The participants then read and signed an informed consent form. This study was approved by the University of Connecticut's Institutional Review Board.

Table 1:
Subject demographics.


The trials consisted of participants completing two 12-km runs at a submaximal (training) pace in either a hydrated (HY) or dehydrated (DHY) condition. The participants were randomly assigned to either the HY or DHY condition for the first trial and then assigned to the other condition for the second trial. The submaximal rate varied with each individual as the subjects were asked to run at a comfortable yet challenging (medium) pace. These 2 trials took place a minimum of 4 days apart from each other during the months of July and August. All the subjects were required to perform a typical training run (60-minute run or 90-minutes run or hike) after 2:00 pm the day before each trial to ensure that the subjects were hypohydrated for the DHY trial and for consistency in HYD.

For the DHY trial, the subjects were asked to abstain from ingesting fluids or foods with high water content for 22 hours before their start time. This protocol has been used in our previous studies and found to be effective for eliciting an approximately 2% BM loss (6). This percentage of BM loss has been found in the literature to begin to demonstrate decrements in performance and thermoregulatory strain. The subjects were instructed to consume the same dinner the night before each trial and the same breakfast and snack the day of each trial before testing. A diet log was given to the subjects before the first trial, and a copy of this log was given for their second trial for duplication. The subjects wore typical running attire (shorts and t-shirt, tank top) and were asked to wear the same exact clothing for both trials. The subjects were also instructed to ingest an ingestible temperature sensor (CorTemp; HQ Inc. ) approximately 4–5 hours before their run times on the morning of each trial with 150 ml of water. The only time the DHY group was allowed to consume water was to take the ingestible temperature sensor. Before the HY trial, the participants were instructed to consume sufficient fluids the evening before and the morning of the trial to ensure euhydration upon arrival. The participants were each given a calibrated scale (model HD334; Tanita Corp., Tokyo, Japan) and were instructed to weigh themselves and record their nude BM for the 3 days before each trial and the morning of each trial to determine a euhydrated baseline BM (7). For the HYD trial, the subjects were instructed to drink an extra 16–20 oz. of water the night before to ensure euhydration upon arrival for the run.

Upon arrival at the park between noon and 2:00 pm, the subjects reported for baseline measurements. The subjects first sat in a shaded area and completed a 56-question environmental symptoms questionnaire (34) (ESQ) and a profile of mood states (POMS) questionnaire (21). The subjects then gave a urine sample that was used to measure urine color via a urine color chart (1) (Ucol) and urine specific gravity (Usg) via a refractometer (model A300CL; Spartan Refractometers, Tokyo, Japan). Each of the subjects wore a heart rate monitor (model E40; Polar Electro Inc, Lake Success, NY, USA), and their pretrial BM was recorded (model BWB-800 A; Tanita Corp., Tokyo, Japan). The following baseline measurements were then recorded: HR, body temperature (TGI), thirst sensations (33), thermal sensations (20), perceived hydration (43), perceived muscle pain (9), and ratings of perceived exertion (RPE) (4).

After these baseline measurements, the subjects began the trail run individually with 15-minute intervals separating each subject's start time. During the first trial, research assistants were located at the 800-; 1,600-; 2,400-; and 3,200-m marks for each lap (1 lap = 4 km) to record each subject's HR and time as they ran by. For the second trial, the subjects were instructed to run at a similar intensity as in the first trial. Research assistants located at the same points throughout the course gave the subjects pacing feedback according to their recorded HR from the first trial. The subjects were instructed to slow down or speed up depending on their HR at that respective point from the previous trial.

At the 4-km and 8-km marks (at the end of laps 1 and 2, respectively), there was a 4-minute break where perceptual measures (thirst sensations, thermal sensations, perceived hydration, perceived muscle pain, and RPE), HR, and TGI were measured. During these breaks, the subjects in HY received 400 ml of water, whereas those in DHY received no fluids. At the conclusion of the run, immediate posttrial measurements included the following: HR, TGI, thirst sensations, thermal sensations, perceived hydration, perceived muscle pain, RPE, and blood lactate. Blood lactate was measured using a lancet device (Accu-Chek Softclix; F. Hoffmann-LaRoche Ltd., Basel, Switzerland) and a portable lactate analyzer (Accutrend Lactate; Sports Resource Group Inc., Hawthorne, NY, USA). Ten minutes after the completion of the run, the HR and TGI were measured, and posttrial POMS and ESQ were completed. Thirty minutes after the completion of the run HR, TGI, thirst sensations, thermal sensations, perceived hydration, perceived muscle pain, and RPE were recorded. The subjects in the DHY trial remained on-site after the trial until they were rehydrated to within 2% of their baseline BM measurements. If at any time a subject's TGI exceeded 104°F (40°C), exercise was terminated. In the event that the ingestible thermistor was not functioning properly, a rectal temperature was obtained to ensure the subject's safety.

Throughout the trials, wet bulb globe temperature (WBGT) was measured and recorded every 20 minutes. Each subject's percentage of BM loss at each time point was calculated by the following equation: ([3-day baseline BM − BM at time point]/3-day baseline BM) × 100. Sweat rate was calculated by the following formula: ([pretrial BM − posttrial BM] + fluid consumed − urine output)/time. Percentage of body fat was calculated using 3-site skinfold measurements and calculations before each subject's HY trial (30).

The POMS responses were entered into an electronic scoring system (version 6.6; Educational and Industrial Testing Service, Massachusetts Institute of Technology, Cambridge, MA, USA) to calculate pretrial and posttrial POMS scores.

Statistical Analyses

Data were analyzed with a 2-way (condition × time) repeated measures analysis of variance to examine the differences between conditions and across time. Post hoc Bonferonni corrections were used when appropriate. Greenhouse-Geisser corrections were used when the assumption of sphericity was violated. We used paired-samples t-tests to analyze pretrial and posttrial values during trials and to further evaluate differences. Significance was set at p ≤ 0.05. All statistical analyses were performed using SPSS (version 16.0 for Mac; SPSS Inc, Chicago, IL, USA).


Environmental Conditions

All the trials took place over the course of 4 days. Mean WBGT for the 4 testing days was 27.6 ± 1.3°C. The WBGT during the hydrated condition was 27.8 ± 1.6°C, whereas the WBGT for the dehydrated condition was 26.3 ± 1.1°C (t13 = 2.702; p = 0.018).

Body Mass and Percent Body Mass Loss

Percent BM losses were significantly greater for DHY pretrial (−1.65 ± 1.34%) than for HY (−0.03 ± 1.28%) compared with the 3-day euhydrated baseline BM (p < 0.001; Figure 1). Posttrial, DHY BM losses (−3.64 ± 1.33%) were significantly higher than those for HY (−1.38 ± 1.43%; p < 0.001).

Figure 1:
Body mass changes: 3-day euhydrated baseline, morning (am) of trial, pretrial, and posttrial. *Significant differences between conditions. †Significant time interaction; p < 0.05.

Urinary Hydration Measures and Blood Lactate

Urine and blood lactate values are reported in Table 2. Ucol was significantly different from prerun to postrun in HY (t13 = −4.174; p = 0.001) and in DHY (t13 = −5.259; p = 0.000). USG was not different from prerun to postrun for HY (t13 = −1.117; p = 0.284) or for DHY (t13 = −1.796; p = 0.096). Ucol was significantly different at the prerun time point between conditions (t13 = −4.505; p = 0.001) and postrun (t13 = −5.610; p = 0.000). USG was also significantly different prerun between conditions (t13 = −4.659; p = 0.000) and postrun (t13 = −6.135; p = 0.000) between conditions. Postrun blood lactate levels were not different between HY (4.6 ± 2.6 mmol·L−1) and DHY (5.9 ± 3.0 mmol·L−1; t13 = −1.277; p = 0.224).

Table 2:
Urinary hydration indices and blood lactate.

Heart Rate

There was a significant main effect for time with HR responses (F4.48,111.96 = 29.154; p = 0.00). As expected, no significant time × condition interaction existed in the HR during exercise (F4.48,111.996 = 0.399; p > 0.05; Figure 2). Postrun, the HR was significantly elevated in DHY compared with that in HY 10 minutes postrun (DHY: 111 ± 24, HY: 101 ± 20 b·min−1; t13 = −2.454; p = 0.029) and 30 minutes postrun (DHY: 101 ± 17, HY: 88 ± 19 b·min−1; t13 = −3.290; p = 0.006).

Figure 2:
Heart rate response at pre, 4K, 8K, finish (12K), 10 minutes post, and 30 minutes post.

Gastrointestinal Temperature

Significant time effects existed for TGI in both conditions (F2.6,33.5 = 91.75; p < 0.001). A significant main effect in TGI (F1,13 = 9.261; p = 0.009) for hydration and a significant time × condition interaction (F.3,42.4 = 7.325; p = 0.00) were present. Gastrointestinal temperature was significantly higher in DHY compared with that in HY at the posttrial (DHY: 39.09 ± 0.45°C, HY: 38.71 ± 0.45°C; t13 = −2.432; p = 0.030), 10-minute posttime point (DHY: 38.85 ± 0.48°C, HY: 38.46 ± 0.46°C; t13 = −3.063; p = 0.009), and 30-minute posttime point (DHY: 38.18 ± 0.41°C, HY: 37.60 ± 0.25°C; t13 = −4.607; p = 0.000; Figure 3).

Figure 3:
Gastrointestinal temperature responses during 12K run.

Trail Running Speed

There was a significant main effect for running time (F1.4,18.21 = 18.146; p = 0.00) and a significant main effect for condition (F,13 = 6.236; p = 0.027) but no significant time × condition interaction (F1.28,16.6 = 3.210; p = 0.84). Further tests on individual lap times revealed that running times were not significantly different between conditions after lap 1 (DHY: 1,240 ± 133 seconds, HY: 1,234 ± 128 seconds; t13 = −0.297; p = 0.771), but DHY resulted in significantly slower run times after lap 2 (DHY: 1,311 ± 171 seconds, HY: 1269 ± 162 seconds; t13 = −2.688; p = 0.019) and lap 3 (DHY: 1354 ± 185 seconds, HY: 1302 ± 185 seconds; t13 = −2.536; p = 0.025). The DHY total time was 1 hour 5 minutes 5 seconds, whereas the HY total time was 1 hour 3 minutes 26 seconds. Overall, DHY resulted in the completion of the 12-km run 99 seconds slower than the HY did (t13 = −2.497; p = 0.027, Figure 4).

Figure 4:
Total time and time by lap.

Perceptual Measures

There was an overall time main effect for thirst sensations (F2.681,34.848 = 36.836; p = 0.000). Thirst sensations were significantly greater than prerun measures at all time points (p < 0.05). There was also a significant main effect for condition in thirst sensations (F1,13 = 154.195; p = 0.000) and a significant time × condition interaction (F2.527,32.845 = 5.053; p = 0.008). Pairwise comparisons demonstrated that thirst sensations were significantly greater in DHY compared with that in HY at all time points (p = 0.000; Figure 5). There was an overall time main effect for thermal sensations (F4,52 = 32.210; p = 0.000). There was no significant effect for condition (F,13 = 2.144; p = 0.167) and no significant time × condition interaction (F2.767,35.973 = 1.747; p = 0.178; Figure 5). There was an overall main effect for time (F2.51,32.775 = 15.015; p = 0.000) and condition (F,13 = 89.038; p = 0.000) for perceived hydration status. However, there was no significant time × condition interaction (F2.746,35.696 = 1.354; p = 0.263).

Figure 5:
Thirst sensation, thermal sensation, and perceived hydration.

There was an overall main effect for time for RPE (F1.978,25.712 = 91.177; p = 0.00). The RPE at all time points was significantly greater than prerun RPE scores with the exception of RPE at 30-minutes postrun (p > 0.05; Figure 6). There was also a significant main effect for condition with RPE (F1,13 = 5.073; p = 0.042) but no significant time × hydration interaction (F4,52 = 2.337; p = 0.067). The RPE values were higher in DHY for every time point during running and approached significance at the postrun time point (t13 = −2.105; p = 0.055).

Figure 6:
Rating of perceived exertion and muscle pain.

Perceived muscle pain was significantly greater over time (F.358,30.651 = 14.291; p = 0.000). Pairwise comparisons demonstrated that after lap 2 (p = 0.004) and immediately postrun (p = 0.002), perceived muscle pain was significantly greater than that prerun (Figure 6). There was no significant effect for condition (F,13 = 3.095; p = 0.102) nor a time × condition interaction (F2.053,26.690 = 1.027; p = 0.373).

Environmental Symptoms Questionnaire and Profile of Mood States

There were significant differences from prerun to postrun in ESQ scores for HY (t13 = −4.966; p = 0.000) and DHY (t13 = −6.874; p = 0.00). Prerun ESQ scores were significantly different between conditions (t13 = −4.547; p = 0.001) as were postrun ESQ scores (t13 = −7.094; p = 0.000) (Figure 7).

Figure 7:
Environmental symptoms questionnaire.

Differences in prerun to postrun POMS scores for HY and DHY are shown in Table 3. There were significant differences between conditions for Tension Anxiety (t13 = −2.271; p = 0.041), Depression Dejection (t13 = 2.188; p = 0.048), Vigor Activity (t13 = −2.304; p = 0.038), Fatigue Inertia (t13 = 3.682; p = 0.003), and Total Mood Disturbances (t13 = 2.287; p = 0.040).

Table 3:
POMS pre-post differences.*†


The purpose of this study was to examine the effects of hydration status on physiological function and running speed when relative exercise intensity was controlled during a 12-km submaximal trail run. The main findings of this study were that despite having subjects run both trials at similar relative intensities (a) TGI while running dehydrated was higher than hydrated values after 8-km of running and became statistically significant immediately postrun and continued to be elevated 30-minutes postrun, and (b) subjects ran significantly slower (99 seconds) during a 12-km run when in the dehydrated trial compared with in a hydrated trial, despite running at similar intensities.

Our experimental design aimed to control for exercise intensity in a similar fashion as a coach would ask his or her athletes to exercise at a given heart rate or percentage of their heart rate reserve. Our goal of having subjects complete the 2 trials at similar relative intensities was achieved, because there were no differences in the HR between conditions until after the 12-km run (Figure 2). The control of relative intensity allowed us to compare the effects that body fluid losses would have on running speed and other physiological responses. It is not uncommon for athletes (particularly younger athletes) to carry over fluid deficit from a previous exercise bout or to be in a state of chronic dehydration over the course of a few days (41,42,45,46). Therefore, although an athlete would not purposefully be fluid restricted before an event, this study's design was able to demonstrate the potentially negative effects that dehydration (whether acute or chronic) can have on physiological responses and exercise performance in a field setting.

Our findings support a previous field study that found a relationship between hydration status and thermoregulation in a field setting when intensity was controlled (6). The percentages of BM losses at the end of the 12-km submaximal run in this study (HY: −1.4%; DHY: −3.6%) were similar to the losses incurred in the previous study (−2.0 vs. −4.5%) (6). Casa et al. (6) found that a 2.5% difference in BM losses between 2 submaximal runs resulted in elevated gastrointestinal temperatures and HRs in the dehydrated trial, particularly toward the end of a 12-km run. These researchers controlled for absolute intensity by matching run times throughout a submaximal run in a hydrated and dehydrated trial (6). Similarly, in this study when relative intensity was controlled via heart rate feedback throughout the 12-km run, the difference in BM losses between conditions (2.2%) was sufficient to result in elevated body temperatures toward the later part of the run and elevated HRs up to 30-minutes postrun in the dehydrated condition. The fluid deficit in this study supports the notion that hydration can negatively affect these physiological measures, particularly in situations where exercise intensity is controlled.

Previous researchers have found that in a controlled laboratory setting, BM losses could result in increases of core body temperature of between 0.15 and 0.25°C per 1% of BM lost (Table 4) (14,35). In this field study, postrun temperatures resulted in increases of 0.17 and 0.26°C immediately postrun and at the 30-minutes posttime point for every 1% difference in BM lost between DHY and HY conditions (Table 4). When comparing field and laboratory studies that have investigated the effects of body fluid losses on cardiovascular and thermoregulatory responses, it is evident that a relationship exists, regardless of setting (Table 4). Table 4 demonstrates the similarities in the findings between the laboratory and field studies on the effect that fluid losses can have on physiological measures and running performance.

Table 4:
Changes in physiological values for each additional 1% of BML: A comparison of field and laboratory studies.
Table 5:
Hypothetical meaning of results from this study and Casa et al. (6).*†

A major finding of this study is that despite having similar HRs in both trials, the subjects ran significantly slower when dehydrated after the first lap during a 3-lap 12-km submaximal run (Figure 4). The subjects ran laps 2 and 3 significantly slower (42 and 52 seconds slower), in DHY compared with the HY trial. Overall, there was a 99-second difference in finishing time for the 12-km run with a 2.2% BM loss difference between conditions. These findings are extremely relevant for runners beginning an exercise bout in an already hypohydrated state. These performance differences can be potentially detrimental in longer and higher intensity bouts of exercise. Although some field studies have found runners to be extremely successful despite considerable body fluid losses (13) these runners were not compared with a control condition where these same runners remained more optimally hydrated. Therefore, one cannot conclude that performance in these elite runners may have been enhanced if they had maintained or at least attenuated some of their fluid losses while racing.

Previous laboratory and field studies have found that dehydration may have a negative effect on exercise performance (3,11,19,37,40). On the other hand, some field studies have found no decrements in performance with fluid losses ranging from 2 to 10% (16,24,27,31,39). In fact, some field reports have found that runners who lost the most weight during races were those who finished fastest (24,27,31,39). Body mass losses of up to 10% were found to be unrelated to postrace rectal temperatures and performance in the marathon leg of an Ironman triathalon (39). In another field report, the winner of a marathon race experienced fluid losses of 6.7% of his BM yet ran considerably faster than other runners did (31). However, because these in vivo studies were actual races they lacked a crossover trial or control condition to compare them against. As previously mentioned, it is unknown whether these runners would have run faster had they replaced more of their fluid losses during their race. Furthermore, in this study, the subjects began running in a hypohydrated state, which may have had a greater impact on running speed compared with a gradual increase of fluid losses throughout a race.

This study also revealed that subjects began to have significantly elevated body temperatures in the DHY trial immediately postrun and temperatures continued to be elevated up to 30 minutes postrun compared with HY (Figure 3). These findings are similar to those in Casa et al.'s study, although significant elevations in body temperature were seen sooner (after the second lap or 8 km) in his study (6). These differences in thermoregulatory responses between hydrated and dehydrated runners are clinically significant, particularly when athletes are exercising in the heat and for a prolonged exercise bout. If these runners in this study had been running for a longer duration, higher intensity, and began their run at a higher fluid deficit, their body temperatures would have continued to escalate, as depicted in Table 5.

The hypothetical representation in Table 5 illustrates the differences in the HR, temperature and time between a hydrated and dehydrated condition per every 1% BM loss. Using the empirical data from this study and those from similar studies in Table 4, Table 5 illustrates the impact that body fluid deficits can have on physiological responses and exercise performance. When exercise intensity is controlled (which often happens in athletic settings via pressure from a coach, teammates or personal goal setting) and an athlete is beginning an exercise bout with a fluid deficit or develops a fluid deficit during exercise, the implications on temperature and heart responses and exercise performance are clear, even in a field setting.

It is important to note that field studies reporting no negative impact on body temperature, heart rate or performance (16,24,31,39) were data from actual races or practices. The lack of a control trial and lack of control of exercise intensity are the premise for the differences between this study and previous field studies that have not seen the detrimental effects of fluid loss. However, those who have been actively involved in a competitive atmosphere comprehend that the athlete cannot always control his or her exercise intensity. A hypohydrated athlete may not always be able to use warning signs or voluntarily decrease his or her intensity if it is being controlled by an extrinsic factor (i.e., coach, teammate, competition).

Practical Applications

This study's findings demonstrate that running while dehydrated when relative intensity is controlled may negatively impact run times and elevate body temperature. The 2.2% BM loss difference between conditions is meaningful because it has clinical application for athletes using HR as a means of gauging their exercise intensity, effort, or performance. Therefore, the 99-second difference in run time between conditions despite running at similar heart rates has important clinical implications. The means by which intensity was controlled in this study is similar to how a crosscountry or track coach may advise their athletes to maintain a certain intensity level during a training run.

Table 5 illustrates how influential these differences can be in a clinical setting. Using Casa et al.'s findings of increases of 6 b·min−1 per 1% of BM lost, if an athlete with a 2% fluid deficit is instructed to run while maintaining his or her heart rate between 175 and 180 b·min−1, this athlete would be running approximately 1 minute and 30 seconds slower than if he were to run within this same HR range in a euhydrated state. This athlete would have less of a reserve when there may be a need for a push during an exercise bout, and their ability to adequately pace themselves during a training or competitive run may also be impaired as evidenced by a similar study (40).

This same athlete's body temperature would also be between 0.61 and 0.79°F (0.34 and 0.44°C) higher with just a 2% BM loss. Furthermore, if this same athlete were training with another runner who had <2% BM loss, he or she would have to either slow down to prevent the increase in heart rate or run at a higher intensity to keep up with the euhydrated runner while having significantly higher increases in body temperature the longer they run together at that intensity.

Despite previous field studies demonstrating an unclear relationship between hydration and temperature (15,26) and performance (5,39), it is important to note 2 important variables in methodology: (a) intensity is often not controlled in the in vivo studies, and (b) because these are actual races in previous studies, there is often no crossover design or control condition to compare the differences to. There is a need to incorporate the findings from more realistic but controlled field studies into clinical application. Furthermore, when applying research findings into clinical practice, it is imperative that clinicians take into account the individual athlete to make individualized, realistic, and evidence-based fluid replacement recommendations.


The authors would like to thank the runners who volunteered to take part in a study. This study also would not have been possible without the help of these individuals: Linda Yamamoto, Elaine Lee, Brooke Bailey, Brittanie Volk, Kevin Ballard, Kristyn Hanewicz, Stefania Marzano, Dan Siopa, Emily Hall, Jake Earp, Paula Poh, Sean Wallace, Brian Kupchak, Candice Williams, Lindsey Hom, Meg Van Sumeren, and Michelle Wardwell.


1. Armstrong, LE, Soto, JA, Hacker, FT Jr, Casa, DJ, Kavouras, SA, and Maresh, CM. Urinary indices during dehydration, exercise, and rehydration. Int J Sport Nutr 8: 345–355, 1998.
2. Armstrong, LE, Whittlesey, MJ, Casa, DJ, Elliott, TA, Kavouras, SA, Keith, NR, and Maresh, CM. No effect of 5% hypohydration on running economy of competitive runners at 23 degrees C. Med Sci Sports Exerc 38: 1762–1769, 2006.
3. Baker, LB, Dougherty, KA, Chow, M, and Kenney, WL. Progressive dehydration causes a progressive decline in basketball skill performance. Med Sci Sports Exerc 39: 1114–1123, 2007.
4. Borg, G. Perceived exertion as an indicator of somatic stress. Scand J Rehabil Med 2: 92–98, 1970.
5. Buskirk, ER and Beetham, WP Jr. Dehydration and body temperature as a result of marathon running. Med Sportiva 14: 493–506, 1960.
6. Casa, DJ, Stearns, RL, Lopez, RM, Ganio, MS, McDermott, BP, Yeargin, SW, Yamamoto, L, Mazerolle, SM, Roti, M, Armstrong, LE, and Maresh, CM. Influence of hydration on physiological function and performance during trail running in the heat. J Athl Train 45: 147–156, 2010.
7. Cheuvront, SN, Carter, R III, Montain, SJ, and Sawka, MN. Daily body mass variability and stability in active men undergoing exercise-heat stress. Int J Sport Nutr Exerc Metab 14: 532–540, 2004.
8. Cheuvront, SN and Haymes, EM. Thermoregulation and marathon running: Biological and environmental influences. Sports Med 31: 743–762, 2001.
9. Cook, DB, O'Connor, PJ, Eubanks, SA, Smith, JC, and Lee, M. Naturally occurring muscle pain during exercise: Assessment and experimental evidence. Med Sci Sports Exerc 29: 999–1012, 1997.
10. Costill, DL, Kammer, WF, and Fisher, A. Fluid ingestion during distance running. Arch Environ Health 21: 520–525, 1970.
11. Dougherty, KA, Baker, LB, Chow, M, and Kenney, WL. Two percent dehydration impairs and six percent carbohydrate drink improves boys basketball skills. Med Sci Sports Exerc 38: 1650–1658, 2006.
12. Fowkes Godek, S, Godek, JJ, and Bartolozzi, AR. Thermal responses in football and cross-country athletes during their respective practices in a hot environment. J Athl Train 39: 235–240, 2004.
13. Fudge, BW, Pitsiladis, YP, Kingsmore, D, Noakes, TD, and Kayser, B. Outstanding performance despite low fluid intake: The Kenyan running experience. In: East African Running. Y. Pitsiladis, J. Bale, C. Sharp, and TD. Noakes, eds. New York, NY: Routledge, 2007. pp. 63–84.
14. Gisolfi, CV and Copping, JR. Thermal effects of prolonged treadmill exercise in the heat. Med Sci Sports 6: 108–113, 1974.
15. Godek, SF, Bartolozzi, AR, Burkholder, R, Sugarman, E, and Dorshimer, G. Core temperature and percentage of dehydration in professional football linemen and backs during preseason practices. J Athl Train 41: 8–17, 2006.
16. Godek, SF, Bartolozzi, AR, Peduzzi, C, Heinerichs, S, Garvin, E, Sugarman, E, and Burkholder, R. Fluid consumption and sweating in national football league and collegiate football players with different access to fluids during practice. J Athl Train 45: 128–135, 2010.
17. Gonzalez-Alonso, J, Mora-Rodriguez, R, Below, PR, and Coyle, EF. Dehydration markedly impairs cardiovascular function in hyperthermic endurance athletes during exercise. J Appl Physiol 82: 1229–1236, 1997.
18. Hamilton, MT, Gonzalez-Alonso, J, Montain, SJ, and Coyle, EF. Fluid replacement and glucose infusion during exercise prevent cardiovascular drift. J Appl Physiol 71: 871–877, 1991.
19. Jones, LC, Cleary, MA, Lopez, RM, Zuri, RE, and Lopez, R. Active dehydration impairs upper and lower body anaerobic muscular power. J Strength Cond Res 22: 455–463, 2008.
20. Kobrick, JL and Sampson, JB. New inventory for the assessment of symptom occurrence and severity at high altitude. Aviat Space Environ Med 50: 925–929, 1979.
21. McNair, D, Lorr, M, and Droppleman, L. Profile of Mood States Manual. San Diego, CA: Educational and Industrial Testing Service, 1971. pp. 1–23.
22. Montain, SJ and Coyle, EF. Influence of graded dehydration on hyperthermia and cardiovascular drift during exercise. J Appl Physiol 73: 1340–1350, 1992.
23. Montain, SJ and Coyle, EF. Influence of the timing of fluid ingestion on temperature regulation during exercise. J Appl Physiol 75: 688–695, 1993.
24. Noakes, T. Fluid replacement during marathon running. Clin J Sport Med 13: 309–318, 2003.
25. Noakes, TD. Drinking guidelines for exercise: What evidence is there that athletes should drink “as much as tolerable”, “to replace the weight lost during exercise” or “ad libitum”? J Sports Sci 25: 781–796, 2007.
26. Noakes, TD. Hydration in the marathon: Using thirst to gauge safe fluid replacement. Sports Med 37: 463–466, 2007.
27. Noakes, TD, Myburgh, KH, du Plessis, J, Lang, L, Lambert, M, van der Riet, C, and Schall, R. Metabolic rate, not percent dehydration, predicts rectal temperature in marathon runners. Med Sci Sports Exerc 23: 443–449, 1991.
28. Oliver, SJ, Laing, SJ, Wilson, S, Bilzon, JL, and Walsh, N. Endurance running performance after 48 h of restricted fluid and/or energy intake. Med Sci Sports Exerc 39: 316–322, 2007.
29. Pitts, GC, Johnson, RE, and Consolazio, FC. Work in the heat as affected by intake of water, salt, and glucose. Am J Phys 142: 253–259, 1944.
30. Pollock, ML, Schmidt, DH, and Jackson, AS. Measurement of cardiorespiratory fitness and body composition in the clinical setting. Compr Ther 6: 12–27, 1980.
31. Pugh, LG, Corbett, JL, and Johnson, RH. Rectal temperatures, weight losses, and sweat rates in marathon running. J Appl Physiol 23: 347–352, 1967.
32. Rav-Acha, M, Hadad, E, Epstein, Y, Heled, Y, and Moran, DS. Fatal exertional heat stroke: A case series. Am J Med Sci 328: 84–87, 2004.
33. Riebe, D, Maresh, CM, Armstrong, LE, Kenefick, RW, Castellani, JW, Echegaray, ME, Clark, BA, and Camaione, DN. Effects of oral and intravenous rehydration on ratings of perceived exertion and thirst. Med Sci Sports Exerc 29: 117–124, 1997.
34. Sampson, JB and Kobrick, JL. The environmental symptoms questionnaire: Revisions and new filed data. Aviat Space Environ Med 51: 872–877, 1980.
35. Sawka, MN, Young, AJ, Francesconi, P, Muza, SR, and Pandolf, KB. Thermoregulatory and blood responses during exercise at graded hypohydration levels. J Appl Physiol 59: 1394–1401, 1985.
36. Sawka, MN, Francesconi, RP, Young, AJ, and Pandolf, KB. Influence of hydration level and body fluids on exercise performance in the heat. J Am Med Assoc 252: 1165–1169, 1984.
37. Sawka, MN and Pandolf, KB. Effects of body water loss on physiological function and exercise performance. In: Perspectives in Exercise Science and Sports Medicine. C.V. Gisolfi and D.R. Lamb, eds. Carmel, IN: Benchmark Press, 1990. pp. 1–38.
38. Sawka, MN, Toner, MM, Francesconi, RP, and Pandolf, KB. Hypohydration and exercise: Effects of heat acclimation, gender, and environment. J Appl Physiol 55: 1147–1153, 1983.
39. Sharwood, K, Collins, M, Goedecke, J, Wilson, G, and Noakes, T. Weight changes, sodium levels, and performance in the South African Ironman Triathlon. Clin J Sport Med 12: 391–399, 2002.
40. Stearns, RL, Casa, DJ, Lopez, RM, McDermott, BP, Ganio, MS, Decher, NR, Scruggs, IC, West, AE, Armstrong, LE, and Maresh, CM. Influence of hydration status on pacing during trail running in the heat. J Strength Cond Res 23: 2533–2541, 2009.
41. Stover, EA, Petrie, HJ, Passe, D, Horswill, CA, Murray, B, and Wildman, R. Urine specific gravity in exercisers prior to physical training. Appl Physiol Nutr Metab 31: 320–327, 2006.
42. Stover, EA, Zachwieja, J, Stofan, J, Murray, R, and Horswill, CA. Consistently high urine specific gravity in adolescent American football players and the impact of an acute drinking strategy. Int J Sports Med 27: 330–335, 2006.
43. West, AW, Ganio, MS, Casa, DJ, Lopez, RM, McDermott, BP, Scruggs, IC, and Stearns, RL. Reliability and validity of a novel perceived hydration scale during and after exercise in the heat. J Athl Train 43: S73, 2008.
44. Wyndham, CH and Strydom, NB. The danger of an inadequate water intake during marathon running. S Afr Med J 43: 893–896, 1969.
45. Yeargin, SW, Casa, DJ, Armstrong, LE, Watson, G, Judelson, DA, Psathas, E, and Sparrow, SL. Heat acclimatization and hydration status of American football players during initial summer workouts. J Strength Cond Res 20: 463–470, 2006.
46. Yeargin, SW, Casa, DJ, Judelson, DA, McDermott, BP, Ganio, MS, Lee, EC, Lopez, RM, Stearns, RL, Anderson, JM, Armstrong, LE, Kraemer, WJ, and Maresh, CM. Thermoregulatory responses and hydration practices in heat-acclimatized adolescents during preseason high school football. J Athl Train 45: 136–146, 2010.

hypohydration; hyperthermia; submaximal exercise; field study; performance

© 2011 National Strength and Conditioning Association