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Glycerol hyperhydration and endurance running performance in the heat.

INTRODUCTION

Voluntary Dehydration

Dehydration of approximately 2% of body mass has been shown to negatively affect endurance performance in both temperate and hot environments regardless of the mode of dehydration (2,12,21). The American College of Sports Medicine (ACSM) recommends ingesting fluids in a fashion to avoid a greater than 2% body weight loss during exercise (18). With sweat rates reaching and exceeding 1.8 l/hour when conditions are hot and the intensity is high, a 70 kg man would need to consume a minimum of 400 ml/hour (18). Armstrong has shown that a 1% loss of body weight would have a meaningful decrease in performance, while Montain suggests that the optimal rate of fluid replacement to attenuate cardiovascular drift is to match sweat loss (2, 12). These studies suggest that the necessary fluid intake be increased to 1.1-1.8 l/hour. Unfortunately, the fluid intake of an athlete is seldom more than 500 ml/hour (15). Recently, experienced marathoners given ample fluids to ingest during a 10 mile race, voluntarily dehydrated by 1.9% on average (16). It appears that even seasoned runners may not be aware of sweat loss during running and cannot gauge necessary fluid intake. Even if runners are aware of sweat loss and attempt to match it, their efforts may not be enough as gastric emptying is limited to 1-1.2 l/hr (5).

Glycerol and Hyperhydration

In 1987, a novel approach to induce hyperhydration over prolonged periods of time during endurance performance was demonstrated by utilizing the osmotic properties of glycerol (17). Since then there have been numerous studies providing mixed results on cardiovascular, thermal and performance outcomes while using glycerol to hyperhydrate subjects prior to exercise. The majority of investigations suggest that glycerol and large volumes of fluid will hyperhydrate to a greater extent than equal volumes of water alone if taken 2-2.5 hours prior to exercise (1, 22). These hyperhydration protocols have seen increases in body water of 0.3-1.3 liters and frequently measuring 500 ml more than water alone. The consensus that dehydration can have negative effects on performance (2, 12, 18, 21) would lead one believe that glycerol's ability to increase body water would be advantageous during exercise, particularly when fluid ingestion during exercise cannot meet sweat rates. Currently the performance advantage of glycerol hyperhydration during competitive endurance events is equivocal (1, 3, 6-8, 14, 22). One difference that may have lead to conflicting results in the literature may be the difference in protocol selection used to measure variable outcomes. To determine whether glycerol hyperhydration is beneficial for competitive athletes, it should be studied using a protocol that is the most consistent with their competitive event. To our knowledge no study has measured physiological and psychological variables during a long distance run of competitive nature (set distance, self-selected pace, finishing as quickly as possible).

The aim of the present study was to compare the influence of glycerol ingestion and hyperhydration on the physiological (cardiovascular and thermoregulatory), performance (overall run time), and psychological (perceived exertion) variables during a self-selected pace running protocol in a hot environment with endurance trained runners. To achieve this aim, we tested the hypotheses that following glycerol hyperhydration runners would have a.) greater fluid retention; and b.) higher running velocities; while maintaining c.) similar heart rates, core temperatures, and RPE's.

METHODS

Subjects

Nine healthy, endurance-trained males volunteered for the study. Subjects had been accustomed to running 5 days or more a week and 35 miles (56.3 km) or more a week and had run competitive races throughout the previous 3 years. Subjects had no history of heat injuries, prior use of glycerol, history of abnormalities in swallowing, esophageal or bowel strictures, fistulas, or gastrointestinal obstructions. Testing occurred primarily in the fall, winter and spring months. Subjects gave their informed consent. The study was approved by The Ohio State University Biomedical Institutional Review Board.

Procedures

A double-blind, randomized, crossover (glycerol or placebo) design was employed. Subjects underwent two treadmill performance runs estimated to last 1 hour. Prior to each run subjects ingested either a solution containing glycerol or a placebo solution of equal volume.

Body composition was determined using 3-site skinfolds as described by Jackson and Pollock (9). Chest, abdomen and thigh were measured twice in a circuit fashion. Measurements not within 2 mm were measured a third time, an average of the measurements were taken. The Siri equation was used to calculate percent fat from body density (20).

Oxygen Consumption

Maximal oxygen consumption was determined using a maximal incremental treadmill protocol to volitional exhaustion. The test consisted of 1 minute stages with an initial stage set at 6 mph and 0% grade. Every minute the speed was increased 0.5 mph. When subjects were capable of running beyond 13 min (max 12 mph), the speed was held constant and the grade was increased by 1% every minute until exhaustion. Oxygen consumption was calculated at 15 second intervals using a computerized system (Parvo Medics True One Metabolic System, Sandy Utah). The system was calibrated for volume and gas concentration prior to each test.

Based on the results from Schabort where 8 trained runners completed a one hour time trial on three occasions and averaged 80-83% of V[O.sub.2peak] based on heart rate (19), we believed that the present subjects could maintain an intensity of 83% of V[O.sub.2peak] for one hour. Using the speed that elicited 83% of V[O.sub.2peak] during the incremental treadmill test, we calculated the distance they would run by multiplying the speed (miles/hour) by the one hour we wanted the test to last. For example: 83% of subject two's V[O.sub.2peak] was elicited by running at 10 miles/hour so it was estimated that subject two could run 10 miles in one hour. A fixed workload to exhaustion protocol was abandoned due to the lack of similarity to which competitive races are run. A set distance protocol was also abandoned due to the likelihood of large variations in finishing time and the subsequent effect that such variation would have on levels of dehydration and fatigue thus undermining any possible effect of glycerol on measured variables.

Diet and Training

Subjects were given a 3-day food diary to record all food and fluids consumed during the 72 hours prior to each performance run. To ensure a similar nutritional state, a copy of the first 3-day diary was given to the subjects and they were encouraged to eat the same or similar items before returning for their second performance run. Each subject was told to ingest 5 ml of water per kilogram of body weight upon awakening the morning of the performance runs to ensure proper hydration. Subjects were also told to maintain their current training practices and to do similar training the 3 days prior to each performance run.

Performance Run

Upon arrival subjects voided their bladder and were weighed on a platform scale (Model BWB-627-A, Tanita Corporation, Japan) wearing only their shorts. The shorts were then weighed so that nude body weight could be calculated. Subjects were then seated while a blood sample was drawn from a superficial forearm vein using a 21-guage needle. Blood was then inverted in a heparin vacutainer and immediately spun (15 minutes x 3400 RPM). Following centrifugation, plasma osmolality was measured (uOsmette Model 5004, Precision Systems, Natick, MA) to ensure subjects were euhydrated according to ACSM standards (18). Ten minutes after arriving at the lab, subjects began the hyperhydration phase with ingestion of either the glycerol containing drink or the placebo. The glycerol drink was a 20% by weight solution of glycerol (NOW Foods, Bloomingdale, IL) in water equal to 1.2 grams glycerol per kilogram of body weight. To mask the sweetness of glycerol, both the glycerol drink and the placebo drink contained 1 gram of artificial sweetener (Equal, Merisant US, Inc., Chicago, IL) per 120 ml of solution and 1 gram of a colored artificial sweetener (Great Value raspberry ice, Wal-Mart Store, Bentonville, AR) per 60 ml of solution. The placebo solution was an equivalent volume of fluid, flavored and colored identically to the glycerol solution. At the time of fluid ingestion subjects also swallowed the core temperature pill (VitalSense Integrated Physiological Monitoring System, Mini Mitter) to provide sufficient time for the pill to pass into the gastrointestinal tract, ensuring minimal interference of temperature readings from fluid ingested during the performance run.

The hyperhydration phase began 2 hours 20 minutes prior to the beginning of the performance run. Ingestion of the glycerol or placebo solution was completed in 30 minutes after which subjects were given 1 hour 20 minutes to ingest enough water so the entire fluid consumption equaled 26 ml per kilogram of body weight. During the hyperhydration phase subjects remained seated in a thermoneutral environment at 23.5 [+ or -] 0.5 [degrees]C and 35 [+ or -] 5 % relative humidity. Subjects were allowed to get up to urinate as necessary. All urine was collected and measured. Two hours after entering the lab, subjects finished fluid consumption and urinated if necessary. Post-hydration phase weights were then taken and subject was fitted with a heart rate monitor. At 2 hours and 20 min subjects began a 5 minute warm-up at 60% of V[O.sub.2peak] in an environmental chamber (Tescor, Inc) set at a temperature and humidity of 30 [+ or -] 1 [degrees]C and 50 [+ or -] 5 % relative humidity. Following the warm-up, subjects exited the chamber and were towel dried and weighed. At 2 hours and 30 min subjects entered the chamber to begin the performance run.

Each subject was instructed on how far they had to run, each distance calculated from individual V[O.sub.2peak] data. Subjects began the performance run from a running start equal in speed to the warm-up. Subjects were able and encouraged to increase or decrease the speed as they saw fit and reminded that they were to complete the run as quickly as possible. Subjects were capable of seeing the distance run; however, they were blinded to speed, time and heart rate data. Core temperature and heart rate measures were taken at every mile. RPE was measured at every other mile.

To mimic a competitive race, at each mile subjects were offered water to ingest from a squeezable water bottle. Subjects were allowed to ingest up to 500 ml of water over the course of the run. Water was kept at room temperature to further reduce any effects it may have on core temperature monitoring. Subjects were handed water for ingestion after core temperature data was recorded. The volume of water ingested at each mile interval was recorded. The drinking pattern of the second run was determined by the first run such that during the second run a volume of water, in an amount equal to that ingested at that particular mile interval during the first run, was handed to the subject and the subject was instructed to ingest it in full.

Immediately following completion of the run subjects were towel-dried and weighed. Their shorts were also weighed due to absorbance of sweat so calculation of nude body weight could be determined. Following the weight measurements, subjects sat for 5 minutes and a blood draw was taken for plasma osmolality measurements.

Fluid retention during the hyperhydration phase was calculated as the difference between fluid ingested and urine volume prior to the warm-up. Body mass increase during the hyperhydration phase was measured as the difference between the post-hydration weight and the initial weight. Sweat loss was determined by differences in body weight prior to and after the performance run, correcting for the amount of fluid ingested during the run. Dehydration as percent weight lost was calculated as the difference between the initial weight and the post-run weight divided by the initial weight.

Statistical Analyses

Descriptive statistics were performed on demographic data (Table 1). Three subjects did not complete the study; one dropped out due to illness and two were stopped during the run due to excessive core temperatures. The two subjects who were stopped prematurely were done so during the first performance run and each run was of a different trial. Paired t-tests were performed to determine significant differences between the two conditions for all dependent variables (hyperhydration volume, core temperature, heart rate, RPE, performance run time). Level of significance was set to p < 0.05. All statistical calculations were done via computer based statistical software (SPSS ver. 16.0, Chicago, IL). Data are reported as mean [+ or -] standard deviation.

RESULTS

Pre-hyperhydration Phase Data

Baseline testing was conducted for plasma osmolality, initial body weight, average 3-day fluid and caloric intakes (Table 2). No differences were found for any variable between trials, suggesting an equal level of euhydration and energy status prior to each hyperhydration phase.

Hyperhydration Phase Data

Results for urine outputs, fluid retention, and weight gain are presented in Table 2. Although urine outputs were higher during the placebo trial, they did not reach statistical significance. Likewise, the glycerol trial was associated with greater fluid retention and weight gain, yet failed to reach significance. Two of the six subjects experienced nausea or bloating following the glycerol ingestion. These symptoms subsided by mile 2 of the run.

Performance Run Data

Prior to starting the run, core temperatures (glycerol = 37.0 [+ or -] 0.4 [degrees]C, placebo = 37.1 [+ or -] 0.5 [degrees]C) and body weight (glycerol = 69.8 [+ or -] 9.9 kg, placebo = 69.3 [+ or -] 9.7 kg) were not different between trials (p = 0.251, p = 0.645, respectively).

Based on the estimation of performance run distance from the V[O.sub.2peak] testing; one subject completed 8 miles, one subject completed 9 miles, three subjects completed 10 miles, and one subject completed 11.25 miles.

The time to complete the performance run (Figure 1.A) was not different between the two trials (p = 0.908). The average time was 4074 [+ or -] 229 s and 4079 [+ or -] 295 s for the glycerol and placebo trials, respectively. There was no order effect for performance between the two trials (p = 0.883). Subjects ingested an average of 274 [+ or -] 121 ml of water during the run. Core temperature, heart rate and RPE (Figures 1.B, 1.C and 1.D, respectively) were not different between trials at any given mile interval or at the end of the run. The estimated 1 hour sweat rate was similar between trials, allowing for a lower, but not significantly different, percent weight loss from sweating in the glycerol trial (Table 2). The post-run plasma osmolality was higher in the glycerol trial than the placebo trial (294 [+ or -] 7 mOsm x [kg.sup.-1], 284 [+ or -] 12 mOsm x [kg.sup.-1], respectively), but not significantly (p = 0.069).

DISCUSSION

The main finding of the present study was a lack of a difference in endurance running performance in a hot (30[degrees]C) and dry (50% humidity) climate with glycerol hyperhydration when compared to hyperhydration with water alone. Previous literature has suggested that subjects capable of regulating RPE and thermal strain via a self-selected pace protocol showed no differences in performance with glycerol hyperhydration versus placebo (11). This is perplexing as the results of dehydration during exercise, including earlier onset of fatigue, increased cardiovascular and thermoregulatory parameters as well as perception of effort, are well established (12). The current belief that heart rate, core temperature and RPE would be similar between trials was based on the thought that glycerol would augment total body water, promote a faster pace but result in similar physiological responses due to a decreased overall running time promoted by the glycerol hyperhydration. This idea has been verified by Coutts et al. as they saw a 2.1% increase in performance with no difference in cardiovascular or thermoregulatory parameters during an Olympic distance triathlon (lasting > 2 hours) (3). Coutts et al. attributed the increase in performance to the greater fluid retention and postponement of dehydration with glycerol hyperhydration. Other studies on dehydration and performance have also not seen changes in cardiovascular or thermal strain during the first hour of exercise with similar temperatures and intensities as the current study but have led to improved performance (12, 21). The protocol by Marino and the one used in the current study had athletes only exercising for ~1 hour, possibly limiting the time that 2% dehydration was present and reducing the potential for performance differences.

Glycerol hyperhydration did not result in a larger fluid retention than water hyperhydration. The hyperhydration protocol was similar to those used in previous studies that had significantly higher fluid retentions and increased performances with glycerol (7,13). The volume of fluid retained with glycerol (977 ml), as well as the difference between glycerol and placebo (586 ml), are similar to values reported in previous literature (3,13,14). Glycerol is said to be reabsorbed from the distal and proximal tubules in the kidney, producing an osmotic gradient that allows for the reabsorption and retention of water (4). The trend for higher post-run plasma osmolality in the glycerol trial suggests a higher concentration of glycerol in the blood, as has been noted by Lyons (10). It cannot be said for certain whether this higher osmolality was present in the hyperhydration phase to induce water retention because plasma osmolality was not measured during that phase. However, a reasonable assumption can be made that glycerol was in high concentration in the blood because of the similarities with previous studies. Therefore, we believe that the hyperhydration protocol was not a factor in the lack of significance for fluid retention. Baseline hydration status also cannot explain the lack of difference in fluid retention as all subjects reported similar 3-day fluid intakes, weighed the same and had plasma osmolalities that suggested they were euhydrated similarly prior to each trial (Table 2). In light of these similarities, it is difficult to explain the lack of significance for fluid retention beyond a failure of statistical achievement. This result is likely attributable to the small number of subjects from which the data was collected and the high variability between individuals, creating a need for larger fluid retention than most studies in order to see significance. Functionally, the 400 ml difference may be physiologically significant but the difference may have been negated by the performance event duration which may have required more than an hour to reveal the advantage of the additional water.

Despite the availability of 500 ml of water, fluid intake during the run only averaged 274 ml, well below the already small volume of 500 ml that is suggested that runners ingest per hour (15). The self-imposed limit on fluid intake, along with the high rate of sweat loss (1.8 L/hour), allowed dehydration to exceed 2% of body weight in the placebo trial, an amount that has been associated with decreases in performance (2, 21). When this amount of dehydration occurred during the present study was surely different for each subject and not known. It is possible that there was insufficient time for it to become a limiting factor for performance. If the decrease in performance due to dehydration occurs along a continuum, it is also possible that the 1.6% dehydration during the glycerol trial was sufficient to reduce performance in a similar manner to the placebo trial. It may also be argued that the ingestion of fluids was capable of washing out any minor differences in physiological response between the trials. Fluid is available during competition and although glycerol hyperhydration may lessen the need to drink, it is possible that some instances may still warrant fluid ingestion (i.e., dry mouth). If runners prefer to drink, albeit small amounts, and it negates any ergogenic benefits, then the ergogenic benefits were clearly not great enough to make glycerol hyperhydration worth-while.

Subjects that become dehydrated experience increased cardiovascular and thermoregulatory parameters and perception of effort at a given intensity compared to euhydrated subjects (12). This results in an earlier fatigue, or during self-selected pace protocols, a tendency to maintain perception of effort via slowing down, resulting in similar cardiovascular and thermoregulatory parameters but performance differences (7). Our hypotheses that glycerol hyperhydration will not alter heart rate, core temperature and RPE were based on these ideas. Because the overall time was not different at any point during the self-selected pace run, the run can be seen as an externally fixed load. Marino has stated that an externally fixed load should have proportional increases in physiological responses (11). In our original hypotheses, the increased running speed that accompanied glycerol hyperhydration was thought to happen despite similar heart rates, core temperatures and RPE's. These hypotheses are also in line with Marino's idea that if the work load was fixed then the physiological parameters would differ, and because the running times were the same the data was instead analyzed in the following way: core temperature, heart rate and RPE are expected to be lower with glycerol hyperhydration than with water ingestion alone during an endurance run of similar intensity and time.

Core temperatures were not different between trials (Figure 1.B). With similar run times suggesting similar workloads, it was postulated that glycerol hyperhydration would result in larger body water stores capable of decreasing the thermal strain seen during placebo trials. The lack of core temperature differences suggests that glycerol did not decrease thermal strain and is in contrast to our hypothesis and previous finding (1,10). This may be explained by the lack of difference in sweat rate (Table 2), eliminating differences in evaporative potential. Only Lyons and Marino have shown increases in sweat rate with glycerol hyperhydration (10,11). Both authors suggest glycerol alters the thermoregulatory set point allowing for earlier and more sweating, even separately from hyperhydration. The current results do not support that notion.

Glycerol hyperhydration did not alter the cardiovascular strain that occurred during one hour of exercise in the heat as shown by the similar heart rates between trials (Figure 1.C). An increase in total body water should attenuate the cardiovascular strain associated with dehydration by prolonging the time before dehydration occurs (12). As stated above, it is possible that dehydration did not occur early enough during the run or to a great enough degree for loss of body water to subsequently affect heart rates. Studies that have seen performance differences have lasted long enough to see differences in dehydration, although significance is not reported (7,12,13,21). It is recommended that in order to see differences in dehydration, cardiovascular strain, and eventually in performance, protocols need to be longer (> 1.5 hours) and fluid intake during exercise limited.

RPE was not different between trials at any given point during the run (Figure 1.D). This suggests equal levels of overall physiological strain, most likely a result of the similar cardiovascular and thermal strain that was present during both trials. The lack of difference in RPE also suggests that subjects exerted a similar effort despite the type of fluid ingested, which allows us to eliminate effort as a means for lack of physiological difference between trials.

Two of the six subjects complained of nausea or bloating following glycerol hyperhydration. These symptoms were not felt during the water hyperhydration protocol. Gastrointestinal distress has been reported in previous literature on glycerol hyperhydration (3,8). It appears that the response to the high gastric volumes is dependent on the individual as the fluid retention was not higher than other subjects during the glycerol trial nor was their initial plasma osmolality different. Both subjects reported that the discomfort had passed with the first 2 miles of the run. Importantly, the subject discomfort did not result in significantly decreased performance run times. Thus, despite the low subject number, these subjects were not responsible for the overall outcomes. It is suggested that glycerol hyperhydration be practiced prior to use during competitive events so any modifications can be made to limit stomach discomfort.

CONCLUSIONS

In the present study, subjects did not retain more fluid when ingesting glycerol and water than when they simply ingested an equal volume of water alone. We were also unable to find a performance difference with glycerol hyperhydration when running for an hour in hot conditions. The average time that the subjects took to run the set distance only differed by 5 seconds. With the pace being similar between both trials we expected glycerol hyperhydration to lower the strain of running in the heat. However, glycerol hyperhydration was unable to reduce heart rate, core temperatures or perceived exertion, meaning subjects experienced similar strain during each run. Two of six subjects did experience some stomach discomfort for the first couple of miles of the run with glycerol ingestion, so use of glycerol should be practiced prior to use during competitive events.

Although glycerol hyperhydration showed no beneficial effect for running for 1 hour in the heat, it is possible that the 2% dehydration that occurred during the placebo trial did not emerge soon enough to show the effectiveness of the increased water load due to glycerol ingestion. It is believed that had the run been of longer duration, dehydration differences would have caused increased cardiovascular, thermoregulatory strain and perceived effort. This could have led to reductions in pace and decreased performance. We believe additional studies need to be done with subjects running for a longer amount of time in both hot and race-like conditions, because it is in these conditions that fluid intake is minimal and dehydration is most likely to reach performance-lowering levels.

Address for correspondence: Devor ST, PhD., Health and Exercise Science, The Ohio State University, Columbus, Ohio, United States, 43210. Phone (614)688-8436; FAX: (614)688-3432; Email. sdevor@ehe.osu.edu.

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CORY SCHEADLER, MATTHEW GARVER, TIMOTHY KIRBY, STEVEN DEVOR

Exercise Science Laboratory, Health and Exercise Science Department, The Ohio State University, Columbus, OH, USA
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Title Annotation:Environmental Exercise Physiology
Author:Scheadler, Cory; Garver, Matthew; Kirby, Timothy; Devor, Steven
Publication:Journal of Exercise Physiology Online
Article Type:Report
Date:Jun 1, 2010
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