Should Carbohydrate Concentration of a Sports Drink Be Less Than 8% During Exercise in the Heat?

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1 International Journal of Sport Nutrition and Exercise Metabolism, 2005, 15, Human Kinetics Publishers, Inc. Should Carbohydrate Concentration of a Sports Drink Be Less Than 8% During Exercise in the Heat? Mindy L. Millard-Stafford, Phillip B. Sparling, Linda B. Rosskopf, and Teresa K. Snow Our purpose was to determine if sports drinks with 6 and 8% CHO differentially affect physiological responses or run performance in the heat. Ten men ran 32 km while ingesting: placebo (P), 6% carbohydrate-electrolyte (CE6), and 8% carbohydrate-electrolyte (CE8). At 15 km, a 250 ml drink labeled with deuterium oxide (D 2 O) was ingested. Blood glucose and respiratory exchange ratio were significantly higher (P < 0.05) for CE6 and CE8 compared to P. Rectal temperature (T re ) at 32 km was higher for CE8 (40.1 ± 0.2 C) compared to P (39.5 ± 0.2 C) but similar to CE6 (39.8 ± 0.2 C). D 2 O accumulation was not different among drink trials. Run performance was 8% faster for CE8 (1062 ± 31 s) compared to P (1154 ± 56 s) and similar to CE6 (1078 ± 33 s). Confirming the ACSM Position Stand, 8% CE are acceptable during exercise in the heat and attenuate the decline in performance. Key Words: hyperthermia, sports drinks, electrolytes, fluid replacement Exercise in the heat is an important concern for athletes because of an increased risk for life-threatening heat illnesses (1). Increased cardiovascular and thermal strain during exercise in heat stress compromise physical performance (19, 37). This is practically relevant because elite competitions are hosted in warm and humid conditions (i.e., Summer Olympics in 1992, Barcelona, Spain; 1996, Atlanta, GA, and Athens, Greece 2004). Thus, factors that enhance athletic performance, while maintaining safety during competition in the heat, remain of interest. While it is well documented that carbohydrate (CHO) improves performance in cool conditions (1, 10, 16, 38), evidence regarding the efficacy of CHO in the heat is less compelling. CHO has improved prolonged cycling performance in the heat, but not always with a clear metabolic explanation (2, 5, 30). The benefits of fluid and CHO are additive (each contributing 6%) towards improving performance during 1 h of high-intensity cycling (2). Greater muscle glycogenolysis but reduced oxidation of ingested CHO has been observed during exercise in the heat (19, 23). In contrast, muscle glycogen levels are often higher at the point of fatigue in hot conditions compared to cold (37). Consequently, CHO ingestion does not always The authors are with the Exercise Physiology Laboratory, School of Applied Physiology, Georgia Institute of Technology, Atlanta, GA

2 118 Millard-Stafford et al. improve exercise performance in a hot laboratory (13, 16, 27). In fact, several reviews indicate CHO availability is not the limiting factor for exercise performance in the heat (15, 22). Historical dogma suggests that a lower CHO content is optimal during exercise in the heat since the need for fluid is greater and gastric emptying slows as CHO content of the beverage increases (4, 9). In 1996, the revised American College of Sports Medicine position stand (1) recommended that solutions containing 4 to 8% CHO could be ingested during exercise in the heat to aid performance without compromising fluid delivery. The upper end of this optimal range, however, is not clearly defined in the literature and has recently been questioned. In 2000, the National Athletic Trainers Association position stand (6) stated during events when a high rate of fluid intake is necessary to sustain hydration, the [CHO] composition should be kept low (e.g. < 7%) to optimize gastric emptying and fluid absorption. Murray et al. (29) observed 8% CHO significantly reduced mean gastric emptying rate for 90 min of cycling compared to plain water, 4%, or 6% CHO; concluding, 8% CHO is not optimal for rapid fluid replacement. In another study, water absorption in the proximal small intestine was slower with 8% CHO compared to 6% CHO during cycling when subjects initiated exercise hypohydrated (40). A 1998 review (42) considers solutions containing 5 to 7% CHO optimal when selecting an effective beverage during high-intensity, intermittent exercise (though heat conditions were not specified). Moreover, Rehrer (39) recently stated to enhance water absorption, glucose or glucose-containing CHO (e.g. sucrose, maltose) at concentrations of 3 to 5% weight/volume are recommended. Collectively, these studies suggest that the cut-point for CHO content of sports drinks designed for hot weather usage or high intensity exercise should be revisited. Few studies have been conducted on commercially available sports drinks, but a meta-analysis (8) concluded there is little evidence supporting an ideal CHO formulation. A limitation of research on this topic is that few studies are conducted in the field to simulate application of laboratory findings. Previously, our group (26) found during a 42 km field run in the heat, a commercially available 7% sports drink (5% maltodextrin, 2% fructose) improved run performance compared to placebo (P). A mixture of CHO sources is recommended to maximize absorption (41). The capacity for fructose absorption is limited in most humans, however, because of reduced ion transport that facilitates water absorption (when compared to glucose) (41). The limit for fructose content in a sports drink is, however, unknown. Fructose content of 6% might elicit greater gastrointestinal distress and reduce exercise capacity (30), and was recommended to be limited to about 2-3% (6). Whether fructose content above 2% CHO differentially impacts fluid uptake and performance during competitive running in the heat is not clear. Therefore, the purpose of this study was to determine if commercially available 6% sucrose/glucose and 8% fructose/glucose/maltodextrin carbohydrate-electrolyte (CE) drinks would differentially affect physiological responses, relative fluid uptake, and run performance compared to water placebo. Our hypothesis was that, in accordance with the ACSM Position Stand, 6 and 8% CE beverages would both improve exercise performance and maintain higher blood glucose compared to P but otherwise result in similar physiological effects.

3 Sports Drinks During Running in the Heat 119 Subjects Methods Ten highly-trained male distance runners volunteered to participate in the study and signed an informed consent statement approved by the institutional review board. Physical characteristics are presented in Table 1. Criteria for participation were: age < 40 y, training of 110 km/wk for the last 3 months with no anticipated change in training throughout the duration of the study, use of programmed drinking during training, and completion of a 32 km training run in the month before the study. All subjects had been training in the ambient summer weather for at least 6 wk prior to the study. Testing Procedures Approximately 1 wk after an initial session where VO 2max and other descriptive data were obtained, subjects reported to the laboratory following an overnight fast. A double-blind, counterbalanced test design was used. Subjects served as their own control by completing three 32 km run trials while consuming 2 different CE beverages and a control: 1) 8% high fructose corn syrup (3.5% fructose/2.5% glucose)/2% maltodextrin sports drink, CE8 (PowerAde, The Coca-Cola Co., Atlanta, GA); 2) 6% sucrose/glucose drink, CE6 (Gatorade, The Quaker Oats Co., Chicago, IL); and 3) an artificially-sweetened placebo (P) which utilized similar lemon-lime flavoring and coloring. A minimum of 14 d separated each of the 32 km run tests. Runners were instructed to follow a standard normal-mixed diet (55% carbohydrate, 15% protein, 30% fat) and to duplicate their training for 3 d before each field test. Run Protocol Subjects reported to the laboratory at 0700 h and consumed 400 ml of the test beverage 15 min before initiating the run. During the 32 km run on an outdoor course, 250 ml of the test beverage was provided at 5 km intervals. This fluid replacement regimen is consistent with that recommended by the American College of Sports Medicine (1). Subjects were instructed to run within 20 s per km of their marathon pace except for race efforts at the 1.6 km immediately preceding the Table 1 Physical Characteristics of Subjects (n = 10) Age (y) 23.7 ± 3.6 Height (cm) ± 9.2 Weight (kg) 63.9 ± 5.0 % body fat (skinfolds) 7.0 ± 2.2 VO 2max (ml/kg/min) 76.9 ± 6.1 Run distance (km/wk) ± 30.9 Note. Values are means ± standard deviation.

4 120 Millard-Stafford et al. 16 km break and the final 5 km (27 to 32 km). At 16 km, nude dry body weight and blood samples were obtained during a 10 min break. The final 5 km of the run was designed to examine run performance, with subjects putting forth maximal effort. Prize money was awarded based on the total time for all 3 trials. Additional monetary incentives were given for the final 5 km of running and 14.4 to 16.0 km split to motivate subjects for all-out efforts. Environmental measurements (dry bulb, wet bulb, and black globe temperatures) were obtained hourly during each trial to calculate WBGT index (= 0.7 wet bulb black globe dry bulb). Physiological Measurements Heart rate (HR), ratings of perceived exertion (RPE), and core temperature (T re ) were measured throughout the run. HR was obtained every 1.6 km by telemetry using a Polar monitor (Polar CIC, Inc., Port Washington, NY) worn on the chest. T re was measured using a disposable thermistor (YSI model 4491E, YSI, Inc., Yellow Springs, OH) connected to a telethermometer. The thermistor was inserted 15 cm past the anal sphincter by subjects after the 16 km break to monitor T re during the remaining 16 km of the run. Subjects gave their RPE according to the traditional Borg scale (3). Before the run and following the recovery, subjects rated their tolerance and acceptability of the beverages using a 5-point Likert scale. Respiratory gases were collected at 5, 15, 21, and 27 km on a treadmill placed adjacent to the run course. Subjects ran at their 5 km run pace for approximately 1.5 min. Expired air was collected for 1 min using a Daniels valve and the Douglas bag method. Carbohydrate oxidation rate was calculated from VO 2 and respiratory exchange ratio, assuming a nonprotein R. Nude dry body weight was obtained immediately before the start of the run, after 16 and 32 km, and following the recovery. Blood samples were obtained before exercise, at selected intervals during, and immediately after the run via venipuncture at an antecubital vein. Serum glucose, electrolytes, and hemoglobin were analyzed via reflectance spectrophotometry (Kodak Ektachem DT60, DTE, Eastman Kodak Co., Rochester, NY). Whole blood lactate was measured using an YSI Model 27 analyzer (YSI Inc.). Hematocrit was obtained using the microhematocrit method. Percentage change in plasma volume was calculated from hematocrit and hemoglobin values (14). A qualitative indicator of relative fluid absorption was obtained during the run using a method described previously (12). The 250 ml bolus of fluid consumed 1 km prior to the 16 km break was labeled with 10 ml of 99.8 atom percent deuterium oxide (D 2 O). The blood samples obtained at 10 and 40 min post-ingestion were frozen and subsequently analyzed for deuterium in the serum using an isotope-ratio mass spectrophotometer (Metabolic Solutions, Inc., Merrimack, NH). Excess deuterium (actual minus baseline enrichment) was used for making comparisons. Statistical Analysis A two-way analysis of variance (ANOVA) with repeated measures was used to determine differences between the 3 drink trials. A priori planned comparisons of individual means were done using orthogonal contrasts. The possibility of a Type II error might result from uncontrolled environmental conditions in field studies.

5 Sports Drinks During Running in the Heat 121 In an effort to minimize error, an analysis of covariance (ANCOVA) was utilized on run performance data to account for different WBGT heat indices among testing days. An alpha level of 0.05 was utilized to indicate statistical significance. All values reported are means ± standard deviation. Results Because of the field nature of this study, mean WBGT index could not be controlled and was not similar (P < 0.05) during all testing days (n = 11). The WBGT index averaged over time during the test trial was 25.6 C but ranged from 22.8 to 27.6 C among test days. Mean WBGT index increased from 23.2 C at the start to 27.7 C by the finish; environmental conditions considered high risk for conducting sporting events in the heat (1). Figure 1 illustrates the WBGT index (± standard deviation) over time by beverage. On all test days except two, one of each beverage treatments was tested. The conditions on one day were significantly milder (mean WBGT= 22.8 C) for P and 6% CE compared to the test day for 8% CE (WBGT = 26.1 C). Because ambient temperature directly affects performance (19) and there was variability among trials, mean WBGT index for each trial day was used as a covariate to account for the influence of temperature fluctuations on run performance. Exercise Performance The final 5 km of the 32 km run (from 27 to 32 km) was designed to be the measure of performance, because ergogenic benefits of CE would not be expected until late in an endurance event. Using the adjusted means via ANCOVA, 5 km run time for Figure 1 Mean (± standard deviation) increase in environmental heat stress during 32 km run test sessions by beverage (placebo, 6% carbohydrate-electrolyte (CE6) and 8% carbohydrate-electrolyte drinks (CE8). Note: WBGT index is weighted for wet bulb, black globe, and ambient temperature. CE6 symbol lies underneath Placebo values.

6 122 Millard-Stafford et al. CE8 (1062 ± 98 s) was significantly faster (P < 0.05) by 8.5% compared to P (1154 ± 177 s) and tended to be faster (by 7%) with CE6 (1078 ± 102 s) compared to P. There was no performance difference between CE8 and CE6. Without adjustment for WBGT, mean 5 km run times tended to be faster (P = 0.08) with CE8 (1068 s) and CE6 (1075 s) compared to P (1150 s). A line of best fit for unadjusted run times plotted against daily mean WBGT indices is presented in Figure 2. As illustrated, there are clearly individuals who were responders that could not maintain run speed in the heat without CHO ingestion. As mean heat index increased, CE provided benefit by attenuating the decline in performance compared to P. There were no significant effects as the result of training, learning, or acclimatization because 5 km times were not significantly different based on trial order (1128 ± 104, 1084 ± 174, and 1083 ± 111 s for trial 1, 2, and 3, respectively). The overall 32 km run time was not significantly different among CE8 (127.0 ± 7.7 min), CE6 (126.1 ± 6.8 min) and P (128.4 ± 7.8 min). Physiological Effects Blood glucose was significantly higher (P < 0.05) when consuming CE6 and CE8 compared to P (Figure 3). Blood glucose was higher at rest (because of preexercise drink ingestion) and at the completion of 32 km. Two subjects were hypoglycemic (< 3.3 mmol/l) with P compared to none with CE drinks at the end of the run. RER was significantly higher at 27 km for CE6 and CE8 (P < 0.05) compared to P (Figure 3), indicating greater reliance on CHO oxidation prior to the performance run. There was no significant beverage effect on percent of maximum heart rate (% HR max ) during the run, with values ranging from 83 to 88% HR max during the steady Figure 2 Lines of best fit plotted for average wet bulb-black globe heat index (WGBT) versus run performance while ingesting placebo (P: bold solid line), 6% carbohydrateelectrolyte (CE6: fine dashed line) and 8% carbohydrate-electrolyte drinks (CE8: bold dash/dotted line).

7 Sports Drinks During Running in the Heat 123 pace portion to 95% HR max for the time trial. There was no significant difference in relative oxygen uptake among the 3 drink treatments during the initial 27 km of the run (~ 65% VO 2max ). Subjects RPE increased significantly over the course of the run (from 11 at 5 km to 18 at 32 km) but was unaffected by beverage treatment. There were no significant effects of the beverage treatments on the relative change in plasma volume, plasma osmolality, or serum electrolytes (Table 2). Although mean plasma volume losses appear to be reduced for CE6, the variability attributable to large interindividual differences resulted in nonsignificance (P = 0.5). There was a significant (P < 0.05) treatment effect for blood lactate. Lactate was significantly lower with P at the end of the run compared to CE6 and CE8. The amount of weight lost during the run was similar between CE8 Figure 3 Mean (± standard deviation) blood glucose and respiratory exchange ratio (RER) during the 32 km run in the heat while ingesting placebo (P), 6% carbohydrate-electrolyte (CE6) and 8% carbohydrate-electrolyte drinks (CE8). *Significantly higher for both CE 8 and CE6 (P < 0.05) compared to P.

8 124 Millard-Stafford et al. ( 4.0 ± 1.0%), CE6 ( 4.1 ± 1.2%), and P ( 4.4 ± 1.6%) indicative of a similar level of dehydration. There was no significant difference in mean accumulation of serum deuterium oxide above baseline among beverages (Figure 4), suggesting a similar relative fluid uptake of the drinks during running. Core Temperature Effects There was a significant treatment effect (P < 0.05) for T re (Figure 5). T re was higher at 32 km with CE8 (40.1 ± 0.6 C) compared to P (39.5 ± 0.6 C) but not different from CE6 (39.8 ± 0.6 C). At 16 km (following the 1.6 km higher effort run prior to Table 2 Serum Electrolytes, Lactic Acid, Osmolality, and Plasma Volume Changes Compared to Baseline Rest (% PV Change) During the 32 Km Run in the Heat While Ingesting Placebo (P), 6% Sports Drink (CE6), and 8% Sports Drink (CE8) Rest 16 km 21 km 32 km Na+ (mmol/l) P ± ± ± ± 2.4 CE ± ± ± ± 2.7 CE ± ± ± ± 3.2 K+ (mmol/l) P 4.8 ± ± ± ± 0.3 CE6 4.7 ± ± ± ± 0.3 CE8 4.6 ± ± ± ± 0.3 Cl (mmol/l) P ± ± ± ± 3.1 CE ± ± ± ± 2.7 CE ± ± ± ± 2.4 Lactic acid (mmol/l) P 1.7 ± ± ± ± 2.3* CE6 2.0 ± ± ± ± 2.5 CE8 1.9 ± ± ± ± 2.3 Osmolality(mosmol/kg) P ± ± ± ± 6.0 CE ± ± ± ± 10.0 CE ± ± ± ± 9.4 % PV change P 5.3 ± ± ± 4.7 CE6 3.9 ± ± ± 5.1 CE8 5.1 ± ± ± 7.0 Note. Values are means ± standard deviation. *Indicates a significant beverage effect (P < 0.05) for P compared to CE6 and CE8.

9 Sports Drinks During Running in the Heat 125 the break), T re was significantly higher by 0.4 C in both CE treatments compared to P. None of the runners required any type of medical assistance or treatment for heat illness other than recovery in an air-conditioned room. Drink Acceptability Using a 5-point rating scale, there was no difference in stomach bloating between the beverages. The mean score for postrun stomach upset tended to be lower (P < 0.05) for CE8 (2.1 ± 0.6) compared to P (2.7 ± 1.0) but was not different from CE6 (2.2 ± 0.6). The mean rating for nausea was lower (P < 0.05) for CE8 (1.9 ± 1.2) compared to P (2.5 ± 1.0) but not different from CE6 (2.2 ± 1.0). Discussion Consistent with our hypothesis, there were no significant differences in physiological responses, relative fluid uptake, or exercise performance during competitive running in the heat when comparing a 6% and 8% CE sports drink. This is congruent with the previous 1996 ACSM guideline (1) and meta-analysis (8) that commercially available sports drinks are similar in their overall effectiveness during exercise. Our data contradict recent suggestions that 8% CHO is not optimal for athletes during high-intensity exercise in the heat (6, 29, 39, 42). Furthermore, a fructose content slightly above 3% did not elicit gastrointestinal discomfort or adversely impact performance. Ingestion of CE drinks in the present study might have improved performance by maintaining blood glucose and CHO availability during the latter stages of running because these variables were higher during both CE trials. Higher CHO oxidation/plasma glucose paralleled a 21% performance improvement during cycling in the heat following a high versus low CHO diet (38). Cycling performance for 1 h in the heat was improved with CE drinks but the mechanism responsible was not entirely clear (2). Greater muscle glycogenolysis occurs during exercise in the heat (17, 23). Yet, laboratory studies involving cycling (15, 37) indicate glycogen is not limiting at the point of fatigue. Moreover, improved performance could be related to the attenuation of a centrally-mediated fatigue. The provision of exogenous CHO might reduce sensations of fatigue by reducing serotonin in the brain, increasing tolerance of rising deep body temperature, or lower perceived effort (5, 35). Mechanisms that explain improved performance resulting from CHO supplementation in the heat may, thus, be mode-specific and related to factors other than the sparing of muscle glycogen. The optimal concentration of CHO (and limit for fructose) in a sports drink to maximize fluid uptake during exercise in the heat is not well documented, in part because no single measurement of total fluid uptake into the circulation exists that is without limitations in an exercise setting. Fluid uptake is influenced by gastric emptying rate (GE) and intestinal absorption and both of these processes are influenced by numerous factors as well. Although CHO content of a solution effects gastric emptying (4, 9), the maintenance of an optimal volume in the stomach also enhances GE (34). No difference was observed in percent fluid emptied between 6 and 8% sports drink (7, 8) with serial feedings during cycling, although differences have been reported elsewhere. Using a modified double-sampling technique, gross

10 126 Millard-Stafford et al. Figure 4 Accumulation of deuterium oxide (D 2 O) in blood over the middle 40 min of the 32 km run in the heat while ingesting placebo, 6% carbohydrate-electrolyte (CE6) and 8% carbohydrate-electrolyte drinks (CE8). Figure 5 Mean (± standard deviation) core temperature (T re ) during a 32 km run in the heat while ingesting placebo, 6% carbohydrate-electrolyte (CE6) and 8% carbohydrateelectrolyte drinks (CE8). *Indicates significantly higher (P < 0.05) for CE8 compared to P; the solid box indicates CE6 is higher compared to P. gastric volume and overall GE were impaired with 8% compared to 6% CE (29). Conversely, although GE was not different among 6, 8, and 9% CE drinks, intestinal water absorption was greater with water and 6% compared to 8 and 9% CE (40). Segmental perfusion via a triple-lumen tube is an excellent technique to determine intestinal absorption (20). Segmental perfusion, however, is also limited because it cannot account for intestinal absorption in segments distal to the measurement site or ascertain the overall efficacy of a beverage during high-intensity

11 Sports Drinks During Running in the Heat 127 exercise (40). For example, although more water was absorbed in the first 25 cm of the small intestine with P compared to 6% CE, there was no improvement in fluid homeostasis or thermoregulatory function (24). The authors stated segmental perfusion data does not indicate overall efficacy of a beverage and recommend including additional clinical measures (i.e., fluid retention and plasma volume changes). To provide a composite measure of both GE and intestinal absorption (because neither in isolation gives comprehensive results), a heavy-water tracer (D 2 O) has been used as a relative marker to compare the appearance of different beverages in the blood over time (11, 12). Its primary limitation is that it does not provide a quantitative measure of fluid uptake and exchange. The technique also has been criticized because results do not always agree with those studies that include net water movement via triple-lumen technique (20). The technique does, however, discriminate CHO beverages in the range of 12 to 15% as appearing more slowly into the bloodstream (12, 13). Moreover, deuterium accumulation in the plasma decreases proportionally with increased exercise intensity (25). Thus, the D 2 O technique, although not quantitative, appears valid for a relative comparison of beverages ingested during conditions encountered in competitive sports. A true upper limit for CHO content tolerable under heat stress conditions is not well documented in the literature. It is stated that solutions > 10% CHO cause net movement of fluid into intestinal lumen because of high osmolality (1). A review of the literature, however, reveals there are few studies examining the efficacy of beverages with CHO content in the range just above the ACSM guidelines (i.e., > 8% but < 12% CHO). Both a 10% glucose and 10% glucose polymer beverage elicited similar GE and core temperature compared to P during a 2 h treadmill run in heat (36). Using the accumulation of D 2 O in plasma, Davis et al. (11) observed a similar relative fluid uptake profile for water, 6% maltodextrin and 6, 8, and 10% glucose/fructose beverages during cycling. Cycling performance in the heat, however, was only improved with a 6% sucrose beverage versus P and 10% sucrose but not different than 8% sucrose (31). Thus, recommendations for an 8% CHO still allow for a margin of safety (~ 2 to 4%) below published reports where adverse physiological effects have been observed in the heat. A higher T re at the end point of exercise was unexpectedly observed when our subjects ingested CE8 in the heat, unlike other investigations (2, 5, 27). If CE8 adversely influenced fluid availability to the blood, proportional increases in both T re and HR would be expected (28, 29, 40). Similar HR, plasma volume changes, and accumulation of D 2 O in the blood combined with lower ratings for stomach discomfort suggest, however, CE8 was comparable to both P and CE6 in fluid delivery. This is consistent with the aforementioned study (11) using the D 2 O technique. Moreover, a concentration well above 8% (12% CHO) resulted in slower D 2 O accumulation and increased incidence of gastrointestinal upset during cycling in the heat (13). Thus, we believe higher T re with CE8 was not related to adverse absorption of the drink. Ours was not the first study to report differential core temperature related to CHO ingestion. A high CHO precompetition diet resulted in higher T re at the point of exhaustion during cycling compared to a low CHO diet (38). Fritzsche et al. (18) reported CHO increased heat production, final core temperature, and whole body sweating during 110 min of cycling in the heat (35 C, 50% RH). In another cycling study performed in 35 C, CHO improved performance by 14% (5) but

12 128 Millard-Stafford et al. subjects tended to fatigue at a higher T re. Thus, the attainment of the same critical core temperature was not the limit to performance in our study in contrast to what is observed in shorter duration (<1 h), heated laboratory experiments (15, 21, 34). To better understand what factors constrain human performance in the heat, it seems logical to study individuals who routinely perform exercise in hot environments and are accustomed to experiencing exercise-induced hyperthermia. In this study, a CE drink facilitated improvement in run performance in the heat compared to P and that, in turn, the faster run speed elicited a significantly higher peak temperature and blood lactate at the end point of exercise. Exercise intensity is the major determinant of peak body core temperature (33). Thus, it follows that if exercise intensity (run pace) was higher during the final 5 km, greater T re would be elicited. If runners can sustain exercise of sufficient duration and intensity in the heat, CHO availability could become limiting, in contrast to other laboratory findings (15). Our data, in contrast to Nielsen, (32) indicate that, in highly-trained, heat-acclimatized runners, a consistent critical core temperature does not always limit performance in the heat. Whether body temperature per se is the critical variable or if temperature merely coincides with another limiting variable(s) causing fatigue remains to be determined in an athletics setting. Congruent with ACSM guidelines, we observed an 8% CHO sports drink (containing 3.3% fructose) is clearly within the range for optimal CHO content, and can enhance performance during competitive distance running in the heat. No meaningful differences were observed in the relative fluid uptake or other measures reflecting state of hydration between 6% and 8% commercially available sports drinks during running. Moreover, these data suggest that among acclimatized distance runners, CHO availability (versus achieving a similar peak core temperature) limits prolonged running performance in the heat. Acknowledgments This study was supported by a grant from The Coca-Cola Co., Atlanta, GA and the first author has served as a consultant to company. The results of the present study do not constitute endorsement of the products by the authors. We recognize Dr. Shirley Darracott for providing the statistical analyses. Special thanks are extended to the subjects and to Alan Drosky, coach of the Georgia Tech cross-country team, for permitting his athletes to participate in this study. References 1. American College of Sports Medicine. Position stand: exercise and fluid replacement. Med. Sci. Sports Exerc. 28:i-vii, Below, P.R., R. Mora-Rodriquez, J. Gonzalez-Alonzo, and E. F. Coyle. Fluid and carbohydrate ingestion independently improve performance during 1 h of intense exercise. Med. Sci. Sports Exerc. 27: , Borg, G. Perceived exertion: a note on history and methods. Med. Sci. Sports Exerc. 19: , Brouns, F.J. Heat-sweat-dehydration: a praxis oriented approach. J. Sports Sciences. 9: , Carter, J., A.E. Jeukendrup, T. Mundel, and D.A. Jones. Carbohydrate supplementation improves moderate and high-intensity exercise in the heat. Pflugers Arch. Eur. J. Appl. Physiol. 446: , 2003.

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