Maintaining adequate hydration by replacing body water losses from sweating is essential, especially for individuals who engage in two or more bouts of exercise-heat stress in one day. Hypohydration by as little as 1-2% results in a decreased physical work capacity and maximal aerobic power, and the potential for decrements are greater in a hot environment(2,7,9). It has been suggested that an increased perception of effort may be partly responsible for these decrements(1). The perception of effort involves the integration of multiple sensory and cognitive inputs arising from the cardiopulmonary system and exercising muscles and joints(12,24,25). Dengel(10) found that varying levels of hypohydration did not affect the rating of perceived exertion (RPE) in a neutral environment. Conversely, in a hot environment RPE was higher when subjects were hypohydrated compared with trials in which water or saline ingestion balanced sweat and urinary losses (4).
Fluid replacement after dehydration via intravenous infusion is based on the belief that intravenous fluids restore body water more effectively than oral ingestion. Montain and Coyle (20) rehydrated subjects during cycling exercise in the heat and found a lower RPE with the ingestion of a carbohydrate-electrolyte fluid than with the infusion of 6% dextran in saline. To our knowledge, no data are available comparing the effects of intravenous infusion and oral ingestion using the same type and amount of fluid following dehydration on RPE during subsequent exercise.
The subjective sensation of thirst accompanies fluid deficits greater than 1% of body weight (16), and there is a linear relationship between hypohydration levels and thirst sensations(13). Oropharyngeal and gastric sensations and symptoms that accompany hypohydration function as cues for thirst and drinking behavior(13,22,32) although other physiological pathways, including the sodium-osmotic-vasopressin and renin-angiotensin II systems, also may be involved(15,16,31).
Intravenous infusion of saline during exercise defends plasma volume(22). This may decrease the osmotically-induced vasopressin release and limit hypovolemic activation of the renin-angiotensin II system which, in turn, may result in a decreased urge to drink. However, unlike drinking, saline infusion bypasses oropharyngeal and gastric sensations. Hence, the overall effect of saline infusion on the sensation of thirst is unknown, and we know of no research comparing the sensation of thirst following the infusion and ingestion of fluids subsequent to exercise-induced dehydration.
Thus, the purpose of this study was to compare the effects of IV infusion and oral ingestion on RPE and the sensation of thirst following exercise-induced dehydration. We hypothesized that following rehydration there would be no differences in RPE between IV and oral treatments during a subsequent exercise bout. However, we anticipated higher thirst ratings following IV infusion compared with oral rehydration.
Subjects and procedures. Eight unacclimatized men volunteered to participate in this study. Their physical characteristics (mean ± SEM) were: age, 22.1 ± 0.8 yr; body mass, 73.6 ± 2.4 kg; height, 179.6 ± 1.5 cm; ˙VO2max 57.9 ± 1.6 ml·kg-1·min-1, body fat 7.7 ± 0.9%. Each subject gave informed consent and passed a medical screening procedure. Subjects were paid for their participation.
Prior to the experiment, subjects reported to the laboratory at 0700 h on three occasions to establish baseline levels of urine specific gravity(Usg), plasma osmolality (Plosm), hematocrit, hemoglobin, and body weight. During one visit, each subject's maximal oxygen consumption(˙VO2max) and body fat percentage were determined.˙VO2max was measured on a treadmill using a modified Costill and Fox (8) protocol. Briefly, subjects ran at 160-220 m·min-1 for 4 min at a 0% grade. After 4 min the grade was increased to 4% for 2 min. The grade was then increased 2% every 2 min until the subjects reached exhaustion. Body density was determined from hydrostatic weighing with percent body fat calculated according to Siri(30).
Each subject underwent three experimental sessions consisting of three phases: exercise-induced dehydration (DHY), rehydration (RHY), and exercise performance (EX). DHY and EX protocols were identical during all three trials. RHY treatments consisted of: 0.45% saline infusion (IV), 0.45% saline oral ingestion (ORAL), or no fluid (NF). ORAL consisted of 4 g of a commercial sugar-free flavored beverage (Kool-Aid, Kraft, Inc., White Plains, NY) dissolved in 889 ml of 0.45% saline and 111 ml of distilled, deionized water, maintained at 4°C to optimize palatability. Each trial was separated by at least 14 d, and the order of sessions for each subject was randomly assigned to prevent order effects.
Protocol. Subjects, 12 h post-prandial, reported to the laboratory at 0700 h in an euhydrated state and provided a urine sample. After the placement of a rectal thermistor and indwelling cannula (20 gauge) in a superficial right forearm vein, subjects entered an environmental chamber(33.1 ± 0.6°C) and stood quietly for 20 min to allow their bodies to equilibrate to the hot environment. Following the equilibration period and a baseline blood draw, the subject ate a standard breakfast. After breakfast, a pre-dehydration (PRE-D) body weight (±50 g; model 700 M, SR Instruments, Tonawanda, NY) was taken.
Next, the subject underwent a 2- to 4-h exercise-induced dehydration (33.1± 0.6°C; 47.6 ± 0.5% RH) to reduce body weight by 4%. DHY consisted of alternating 25-min periods of cycle ergometry and treadmill walking (50% ˙VO2max) separated by 5-min rest periods during which body weight was measured. The final exercise bout prior to the 4% weight loss always involved treadmill walking to ensure that the subject was standing for at least 20 min prior to the blood sampling at the end of dehydration.
Upon completion of DHY, the subject returned to a mild environment (25.5± 0.2°C) for rehydration, administered while he was in a semirecumbent posture. IV infusion was administered in the arm opposite to the indwelling cannula via a 21-gauge butterfly cannula. The rate of IV infusion was 0.56 ml·kg body weight-1·min-1. Fluid intake (IV and ORAL) approximated 25 ml·kg-1 PRE-D body weight, and fluids were given over a 45-min period starting 15 min after the subject left the environmental chamber. This volume has been shown to be the upper range for orally ingested fluids following exercise-induced dehydration(23). Following RHY, the subject assumed a standing posture for 55 min, then reentered the chamber (35.9 ± 0.1°C), equilibrated for 20 min, and consumed 1 g carbohydrate/kg PRE-D body weight of a commercial product (Skittles, M & M Mars, Hackettstown, NJ) and 100 ml of distilled, deionized water.
EX (35.9 ± 0.1°C) consisted of treadmill walking at 50%˙VO2max. No fluids were consumed during this exercise period. The duration of EX was intended to be 90 min; however, the trial was terminated early if: 1) heart rate (HR) exceeded 180 beats· min-1 for 5 consecutive min, 2) rectal temperature (Tre) reached 39.5°C, 3) the subject requested termination of the test, or 4) the subject showed signs or symptoms of heat illness. These safety criteria were also used during DHY; however, no subject terminated exercise before the 4% weight loss was achieved.
Perceptual measurements. The subjects were carefully instructed(18) in the use of the Borg 15-point RPE scale(5) and thirst sensation scale (13) during an orientation session and prior to the start of each experimental trial. The thirst scale is a 9-point scale with verbal anchors ranging from 1(not thirsty) to 9 (very, very thirsty). The subject reported local (L-RPE), central (C-RPE) and overall (O-RPE) RPE at minutes 20, 40, 60, and 80 of EX(20EX, 40EX, 60EX, 80EX). The local rating focused on feelings or sensations of strain specifically arising from the muscles and joints in the exercising legs. For the central rating, the subject was asked to report his feelings or sensations of strain pertaining primarily to ventilation. The overall rating integrated local and central feelings. Thirst ratings were reported postdehydration (PD), immediate, 15, 35, and 55 min post-rehydration (PR, PR1, PR2, PR3), pre-exercise (0EX), and at 20-min intervals during EX. The order of requesting differentiated RPE responses and thirst ratings was randomized to prevent an order effect.
Physiological measurements. Tre was monitored continuously during DHY and EX using a thermistor (model 401, Yellow Springs Instrument, Inc, Yellow Springs, OH) inserted 10 cm beyond the anal sphincter and connected to a digital thermometer (± 0.1°C; Odel 8402-00, Cole-Parmer Instrument Co., Chicago, IL). HR was monitored continuously using a transmitting cardiotachometer fitted to the chest (Polar Electro, Port Washington, NY). Oxygen uptake (˙VO2), minute ventilation(˙VE), and respiratory rate (RR) were measured via online, open circuit spirometry (Medical Graphics CPX-D system, Medical Graphics Corp., St. Paul, MN). ˙VO2, ˙VE, and RR values were expressed as the mean of at least four 30-s collections taken at the midpoint of each 25-min exercise period during DHY and at minutes 20, 40, 60, and 80 of EX. Thermocouples (Series 400, Yellow Springs Instruments) secured on the chest, arm, thigh, and calf were used to measure skin temperature(Tsk), computed from a weighted mean of the four local skin measurements (26).
Analyses. Blood samples were taken at PRE-D, PD, PR1, PR2, PR3, 0EX, and every 15 min during EX. Blood samples were analyzed in triplicate for hemoglobin (Hb) using the cyanmethemoglobin method (Kit 525, Sigma Chemical, Inc., St. Louis, MO). Hematocrit (Hct) was determined in triplicate from whole blood by the microhematocrit technique, and measurements were corrected for trapped plasma. Percent change in plasma volume (%ΔPV) was calculated using the method of Dill and Costill (11) using PRE-D as the initial value. Plosm was determined in triplicate by freezing point depression (model 5004 U-osmette, Precision Systems, Inc., Natick, MA). Plasma lactate was measured in duplicate (Model 2300 Stat Analyzer, Yellow Springs Instruments), and plasma sodium was measured in duplicate with ion-specific electrodes (Model 984-2, AVL Scientific Corp., Roswell, GA). A hand-held refractometer was used to visually appraise Usg.
Statistical analysis. Repeated measures ANOVA (treatment × time) was used to evaluate differences among trials. In the event of a significant F ratio, a Newman Keuls post-hoc analysis was used to identify which differences were significant. Selected bivariate correlations were determined using Pearson's product moment correlation. An alpha level of 0.05 was used for all tests of statistical significance. Data are presented as mean ± SEM.
The ambient temperature and relative humidity during DHY (33.1 ± 0.1°C; 42.7 ± 0.5%) and the ambient temperature during EX (35.9±.03°C) were not different among trials. Relative humidity during EX was significantly lower in NF than ORAL (42.6 ± 3.3% vs 48.6± 4.6%), but it is unlikely that this difference was perceptible to subjects.
Values for DHY, RHY, and EX variables are summarized inTable 1. PRE-D Usg was not significantly different among trials (IV = 1.018 ±.003; NF = 1.014 ± 0.004; ORAL = 1.017± 0.003), and all subjects were considered adequately hydrated upon arrival at the laboratory (3). Subjects achieved the 4% weight loss during the DHY session in all trials. The duration of EX was significantly shorter during NF compared with IV and ORAL. There were no significant differences among trials for DHY time,% weight loss during DHY,˙VO2 during DHY or EX, or the amount of commercial glucose product ingested prior to EX.
Figure 1 shows L-RPE, C-RPE, and O-RPE. C-RPE was significantly lower during ORAL than during IV and NF at all time points. L-RPE was higher during NF (vs IV and ORAL) throughout EX, reaching statistical significance at two time points. Only two subjects completed 80 min of exercise during NF; therefore, these values are not shown. L-RPE was higher than C-RPE at every time point in all trials. O-RPE was significantly lower during ORAL compared to NF at 20EX and 40EX.
Figures 2 and 3 illustrate cardiorespiratory and plasma lactate measurements recorded during EX. There were no significant differences among trials for ˙VO2, ˙Ve, or RR. HR was significantly higher during NF compared with IV and ORAL at 0EX, 20EX, and 40EX. Plasma lactate was significantly lower during NF at 15EX and higher at 45EX and 60EX compared with the other trials.
There were no significant differences in Tsk among trials at any time point. The overall means during EX were: IV, 35.3 ± 0.3°C; ORAL, 35.2 ± 0.3°C; and NF, 35.6 ± 0.3°C. Tre during NF was significantly higher compared to IV and ORAL values at 0EX (37.8± 0.1°C vs 37.5 ± 0.1°C and 37.4 ± 0.1°C, respectively) and 20EX (38.4 ± 0.1°C vs 38.1 ± 0.2°C and 37.8 ± 0.1°C, respectively). Tre remained higher during NF throughout the remainder of EX, but differences were not significant.
Correlations between C-RPE or O-RPE and selected physiological data are presented in Table 2. Tsk was significantly correlated with O-RPE in the IV and NF trials and with C-RPE in the NF trial. Thirst, HR, and Tre also had moderate to strong correlations, albeit nonsignificant in some cases, with O-RPE and C-RPE in all trials with the exception of Tre during NF. The correlation between L-RPE and lactate was low (IV, r = 0.16; ORAL, r = 0.01; and NF, r = 0.15).
Thirst ratings are presented in Figure 4. Thirst ratings were not different among trials at PD. Following RHY, thirst was rated significantly higher during NF (vs IV and ORAL) at all time points and significantly lower during ORAL (vs IV) at all time points except 80EX.
Changes in Plosm and plasma sodium (Na+)(Fig. 5) and the percentage change in plasma volume(%ΔPV) (Fig. 6) from pre- to post-DHY were not different among trials. Plosm was greater during NF (vs IV and ORAL) throughout RHY and EX, reaching statistical significance at three time points. Similarly, plasma Na+ was significantly higher in NF (vs IV and ORAL) at every time point throughout RHY and EX. The%ΔPV was significantly lower during NF (vs IV and ORAL) at PR1, PR2, PR3, and 15EX.
RPE. One purpose of this investigation was to compare the effects of rehydration via saline infusion and ingestion on RPE following exercise-induced dehydration. Since the amount of fluid ingested or infused was the same, we hypothesized that ORAL and IV would result in similar RPEs during subsequent exercise in the heat and that RPE would be greater in the NF trial compared with ORAL and IV. Despite exercising at the same relative intensity, L-RPE, C-RPE, and O-RPE were higher at all time points during NF although differences did not always reach significance. However, IV and ORAL exhibited differences in RPE, with the most notable difference being the significantly lower C-RPE during ORAL compared with IV.
The higher RPEs found during the NF trial are in agreement with the study conducted by Barr et al. (4), which reported higher RPEs during 6-h of cycle ergometry (30°C) at 55% ˙VO2max when in a hypohydrated (vs euhydrated) state. Montain and Coyle(20) found lower RPEs in subjects who were rehydratedvia ingestion compared with infusion during cycling exercise, which is in agreement with our investigation. However, their study was confounded by the use of different fluids for infusion (6% dextran in saline) and ingestion(Gatorade, Quaker Oats, Chicago, IL), as well as a large difference in the volume of fluid given (ingestion, 2405 ± 103 ml; infusion, 398 ± 24 ml). The present investigation controlled these confounding factors.
The central physiological parameters that have been linked to RPE include HR, ˙VE, RR, and ˙VO2 (19,25). The lack of significant differences in˙VE, RR, and%˙VO2 during the three treatments used in this study suggests that these parameters were not responsible for the differences found in C-RPE and O-RPE among trials. In general, it appeared that subjects were not receiving strong cues from ˙VE and RR for C-RPE or O-RPE, as demonstrated by low correlations among these variables. It is possible that feelings associated with heat and thirst superseded these factors, modifying the variables that contributed to RPE. The significantly higher HR during NF may have contributed to the higher C-RPE and O-RPE found during that trial. In addition, HR was the only central variable demonstrating a moderate, although statistically insignificant, correlation with O-RPE in every trial (r = 0.57 - 0.68).
Local sensory cues for RPE include lactate and general muscle and joint sensations (12,19). Plasma lactate may have contributed to the higher L-RPE and O-RPE values found during NF as significantly higher values were found at 45EX and 60EX compared with all other trials. L-RPE was higher than C-RPE at every time point during all trials. This suggested that local factors dominated the sensations of effort during exercise, which is in agreement with several investigations(17,24,27). Although the correlation between plasma lactate and O-RPE was low, other local effects such as mechanoreceptor and Golgi tendon organ activity may have contributed. Since the subjects exercised for an average of 4.3 h (DHY and EX) during each trial, it is likely that sensations arising from the muscles and joints of the lower body influenced O-RPE.
It has been suggested that Tsk is a variable that may readily be perceived (19,21). Noble et al.(21) demonstrated that Tsk was an important predictor of RPE during exercise in a hot environment. In our study Tsk had the highest correlation with O-RPE in all trials when compared with other physiological variables and likely contributed to O-RPE. However, Tsk was not different among trials and therefore could not account for the higher O-RPE found in the NF trial. Although Tre was not strongly correlated with O-RPE, it was higher during NF compared with IV and ORAL at all time points, reaching significance at 0EX and 20EX. Therefore, Tre may have contributed to differences among trials in O-RPE.
Results from this study suggest that thirst may be an underlying cue for O-RPE during exercise in a hot environment. Following rehydration, thirst was rated lowest during ORAL at all time points, followed in order by IV and NF. The same pattern of response emerged for O-RPE. In addition, thirst was moderately correlated with O-RPE and C-RPE, reaching significance in the IV trial. Interestingly, the correlation between thirst and O-RPE was lowest (r = 0.49) during NF. This may be explained by the fact that the subjects were extremely thirsty throughout EX during this trial, and they only used the most extreme end of the thirst scale. In fact, the highest rating (9-very, very thirsty) was used most frequently, with several subjects rating their thirst as nine-plus. Therefore, the lack of variability in thirst scores probably resulted in this low correlation.
Thirst. A second purpose of this study was to compare thirst ratings following the three treatment protocols. There were no differences in thirst ratings among trials immediately following dehydration, and subjects received the same amount of fluid during rehydration in ORAL and IV. However, immediately following rehydration, the dramatic decline in thirst ratings during ORAL was not matched in IV. The lower ratings of thirst persisted throughout the remainder of ORAL. As expected, thirst was rated highest during NF.
Increases in Plosm and plasma Na+ concentrations that accompany hypohydration have been identified as mechanisms that control thirst(16). In the present investigation, Plosm was generally higher and plasma Na+ was significantly higher during NF throughout rehydration and exercise, likely contributing to the higher thirst ratings in that trial.
The significantly lower thirst ratings during ORAL may have been the result of oropharyngeal and gastric mechanisms, as these mechanisms were bypassed during IV and NF. Changes in oropharyngeal sensations that accompany hypohydration and rehydration serve as cues that produce differential feelings of thirst and drinking behavior (13). Engell et al.(13) found that while changes in Plosm and plasma volume contributed to fluid intake in hypohydrated individuals sensations and symptoms associated with thirst also contributed to differential fluid intake. Gargling tap water leads to a temporary reduction in thirst with no accompanying changes in Plosm or Na+ (29), which further implicates oropharyngeal mechanisms in the sensation of thirst. Gastric distension and the subjective feeling of “stomach fullness” that accompanies drinking may have contributed to the lower ratings of thirst reported during ORAL (28). The cold temperature (4°C) of the fluid consumed during ORAL also may have influenced thirst ratings. It has been demonstrated that humans prefer to drink cool fluids and subjectively rate warm fluids as“unpleasant” (6). However, following dehydration, maximum water intake has been observed at a fluid temperature of 15°C with colder and warmer water ingested to a lesser extent(6).
Although oropharyngeal mechanisms were not engaged in IV or NF, ratings of thirst were higher during NF compared with IV. This may have been a result of the lower plasma volume found during NF. Following the rehydration treatments, the percent change in plasma volume was smaller during NF compared with IV. Hypovolemia activates the renin-angiotensin II system, which has been associated with increases in thirst (14). It is possible that plasma volume changes were partly responsible for the greater perception of thirst during NF compared to infusion trials.
In summary, we have shown that IV and oral saline rehydration resulted in different perceptual responses, with O-RPE and C-RPE responses generally lower in the oral saline rehydration trial compared with IV and no fluid trials. Lower RPE ratings may influence performance as the subjects were able to exercise for the longest period of time during ORAL, followed by IV and NF, respectively. It also appears from this study that thirst may be an underlying cue for RPE. These results support the importance of drinking before, during, and after exercise. Future studies comparing IV and oral rehydration should examine RPE and thirst responses to higher exercise intensities which might better reflect those used by athletes.
Figure 4-Ratings of thirst at post-dehydration (PD), immediate post-rehydration (PR), 15, 35, and 55 min post-rehydration (PR1, PR2, PR3), pre-exercise (0EX) and during exercise (minutes 20, 40, 60, 80) with intravenous 0.45% saline (IV), no fluid (NF), and oral 0.45% saline (ORAL). Symbols indicate significant differences (
Figure 5-Change in plasma osmolality (mOsm·kg-1) and sodium (mEq·l-1) at post-dehydration (PD), 15, 35, and 55 min post-rehydration (PR1, PR2, PR3), pre-exercise (0EX) and during exercise(minutes 15, 45, 60, 75) with intravenous 0.45% saline (IV), no fluid (NF), and oral 0.45% saline (ORAL). Symbols indicate significant differences(
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Keywords:©1997The American College of Sports Medicine
DEHYDRATION; HEAT STRESS; HYPOHYDRATION; FLUID REPLACEMENT