ORIGINAL RESEARCH International Journal of Sport Nutrition, 1991, 1, 225-239
Consumption of Carbonated and Noncarbonated Sports Drinks During Prolonged Treadmill Exercise in the Heat Alan J. Ryan, Amy E. Navarre, and Carl V. Gisolfi University of Iowa These studies were done to determine the effect of carbonation and carbohydrate content on either gastric emptying or ad libitum drinking during treadmill exercise in the heat. Four test drinks were used: a 6% carbohydrate, noncarbonated; a 6%carbohydrate, carbonated; a 10% carbohydrate, noncarbonated; and a 10% carbohydrate, carbonated drink. For gastric emptying studies, subjects completed four I-hr treadmill runs in the heat. They were given 400 mL of test drink at 0 rnin and 200 mL at 15, 30, and 45 min of exercise. For ad libitum drinking studies, subjects completed four 2-hr treadmill runs in the heat. Gastric residual volumes were similar during the four 1-hr runs. During the 2-hr runs, ad libitum drinking of the four beverages was also similar. Mean values for sweat rate, percentage of body weight lost, and percentage of fluid replaced by ad libitum drinking were similar for the four trials. Similar changes in heart rate, rectal temperature, and ratings of perceived exertion were also observed during the four 2-hr treadmill runs. We conclude that the presence of carbonation in a carbohydrate drink did not have a significant effect on either gastric emptying or ad libitum drinking.
Adequate fluid intake is considered the primary preventive measure against the onset of heat illness during prolonged exercise in the heat. During physical training and competitive events, athletes commonly experience body fluid losses ranging from 2.5 to 7.0% body weight, and core body temperatures exceeding 40.0 "C (1 1, 23, 34). Inadequate fluid intake during physical training and/or competition may predispose an athlete to heat illness by reducing sweat rate (3I), skin blood flow (8), and gastric emptying of fluids (21, 22), and by increasing the extent of dehydration and excessive rise in body core temperature (1 1). Surprisingly, the factors that influence the amount of fluids ingested during exercise are not well understood (13). This work was supported by the Quaker Oats Company. The authors are with the Department of Exercise Science at the University of Iowa, Iowa City, IA 52242.
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Typically, men working in the heat will not drink sufficient fluids to replace the water lost in sweat (1,5, 12, 32). This phenomenon, termed voluntary dehydration, may be due to an inadequacy of the thirst mechanism to detect and correct body fluid deficits (1, 12, 13). Factors that may influence the degree of voluntary dehydration include oropharyngeal cues (3, 7), degree of gastric distention (28), and perception of thirst (13,30). Few studies have examined the factors relating to voluntary dehydration during exercise in the heat (13). Hubbard et al. (13) demonstrated that the extent of voluntary dehydration sustained during a simulated 14.5 km desert walk (40 "C db, 26 "C wb) can be markedly reduced by cooling and flavoring water. In contrast, Carter and Gisolfi (5) reported that their subjects tended to drink more water than a 7.5% carbohydrateelectrolyte drink during cycling exercise (60% V0,max) in the heat. Gastric emptying rate is a factor that may affect the amount of fluid ingested during exercise. The rate of emptying of fluids from the stomach is directly related to drink volume (intragastric pressure) and inversely related to drink osmolality and caloric density. During exercise, repeated ingestion of sports drinks will help maintain fairly high gastric emptying rates (25, 29), presumably as a result of maintaining elevated intragastric pressures (6). During exercise in the heat, repeated ingestion of fluid is essential in preventing dehydration and hyperthermia and an associated reduction in gastric emptying rate (21, 22). The factors that contribute to cessation of drinking during exercise are unknown; however, sensations of stomach fullness (elevated intragastric pressure) can contribute to termination of drinking in humans at rest (28). Thus, three factors that have the potential to influence ad libitum drinking during exercise in the heat include drink palatability, drink composition, and the presence of carbonation. Carbonation could influence drinking behavior by altering palatability, sensations of stomach fullness (intragastric pressure), and/or gastric emptying. The present studies were designed to examine the influence of carbonation and carbohydrate content on either gastric emptying or ad libitum drinking during prolonged moderate intensity (60-65 % V02max) treadmill exercise in the heat. To our knowledge, there is currently no information on the effect of carbonation on ad libitum drinking during exercise.
Methods Nine healthy men (ages 19 to 40 years) volunteered to participate in these studies. Each was informed of the experimental procedures and gave his signed informed consent. Before the experimental trials, each subject completed (a) a test for maximal aerobic capacity (VO,max), (b) a test to identify the treadmill speed required to elicit 60 or 65 % V02max, and (c) a heat acclimation protocol. To determine VO,max, each subject ran at an initial treadmill speed of 8 km-h-' up a 2 % grade; at every Zmin interval thereafter, the treadmill speed and grade were increased by 0.8 km-h-' and 2%, respectively. Maximal aerobic capacity was identified by a plateau of VO, with an increase in exercise intensity and a respiratory exchange ratio greater than 1.O. To determine treadmill speed required to elicit 60 or 65% VO,max, each subject ran at preselected speeds for 6 min and his VO, was measured during the
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Consumption of Sports Drinks During Exercise 1 227
last 3-min interval. To induce acclimation to heat stress, each subject completed five to seven Zhour treadmill runs (approximately 60% V02max) in the heat (32-35 "C db, 15-25% rh). Heat exposures were conducted on different days within a period of 14 days. The four test drinks used in this study were 6% carbohydrate, noncarbonated (6% NC) drink; 6 % carbohydrate, carbonated (6% C) drink; 10% carbohydrate, noncarbonated (10 % NC) drink; and 10% carbohydrate, carbonated (10 % C) drink. The 6 % NC and 6 % C drinks contained sucrose and glucose whereas the 10% NC and 10% C drinks contained high fructose corn syrup (Table 1). Drinks were analyzed for Naf and K+ (Instrumentation Laboratory, Model 943) and osmolality (Wescor, Model 5300). Carbonation of drinks was quantified using a Model D-T Piercing Device (Terriss-Consolidated Industries, Inc., Asbury Park, NJ). Briefly, a capped bottle containing 250 mL of drink was placed into the device and pierced with a pressure gauge. The device and drink were then shaken as a unit until no further increases in pressure were observed. The quantity of carbonation was determined as the volumes of gas pressure per volume of liquid (Table 1). Experiments were performed using a balanced design under single-blind conditions. Subjects were asked to limit their exercise during the 24-hour period prior to experiments. Subjects were also encouraged to drink at least 1 L of fluid the evening before the experiments to ensure hydration. Each subject performed experiments at the same time of day, after an 8- to 10-hour fast.
Table 1 Composition of the Four Test Drinks
Drink
6% NC 6% C 10% NC 10% C
CHO type
Na+ (mEqlL)
Sucrose, glucose Sucrose, glucose Fructose corn syrup, sucrose Fructose corn syrup, sucrose
21.1 20.0 2.6 11.5
K+ Osmolality Carbonation (mEqlL) (mOsm1Kg) (volumes)
2.8 3.0 0.01 4.5
373 355 622 608
0 2.3 0 2.0
Gastric Emptying Studies
Six subjects (V0,max = 56.9A4.9 mL-Kg-'-min-l) participated in the gastric emptying experiments. Each subject completed four 1-hour treadmill runs (65 % V02max) in the heat (35 "C db, 30% rh) drinking either 6% NC, 6 % C, 10% NC, or 10% C. During each trial the subjects were given 1 L of test drink; 400 mL at 0 min and 200 mL at 15, 30, and 45 min of the run. All test drinks contained polyethylene glycol (PEG), a nonabsorbablemarker, at a concentration of 7.6 mg-rn~-'.On the day of an experiment, subjects voided and then weighed
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228 1 Ryan, Navarre, and Gisolfi
nude (Acme scale, f log). After a resting heart rate (HR) and rectal temperature (T,d were obtained, the subject entered an environmental chamber and then consumed 400 mL of test drink (4 "C). Immediately after drinking the test drink, subjects mounted the treadmill and ran at a preselected speed. Heart rates (ECG) and T, (clinical thermometer inserted 7 cm into rectum for 2 min) were obtained at 20, 40, and 60 min of exercise. Subjects were allowed to straddle the treadmill during consumption of drinks (to avoid spilling) and to accommodate the insertion of a clinical thermometer. Typically the subjects required approximately 20 sec to straddle the treadmill, insert the clinical thermometer, and consume the drink. Immediately following exercise the subject passed a nasogastric tube (French-Levine, no. 12) into his stomach. Gastric contents were quickly removed by aspiration with a 60-mL syringe; removal was facilitated by systematic movement of the tube as well as by having the subject assume different body positions. This procedure was followed by infusion of 60 mL of distilled water into the stomach, which was then mixed thoroughly with any remaining residue and again aspirated. Subjects then removed the tube, towel dried, and a nude body weight was obtained. Ad Libitum Drinking Studies
Seven subjects (V0,max = 61.4k2.3 m ~ - ~ ~ - ' - r n i )n participated -' in the ad libitum drinking experiments. Each subject completed four 2-hour treadmill runs (60% V0,rnax) in the heat (32 "C db, 30% rh) drinking either 6 % NC, 6 % C, 10% NC, or 10% C. Upon reporting to the laboratory, the subject voided and then weighed nude. After resting HR and T, were obtained, the subject entered an environmental chamber, mounted a treadmill, and ran at a predetermined speed. Heart rates (ECG) and T, (clinical thermometer inserted 7 cm into the rectum for 2 min) were obtained every 15 min. Ratings of perceived exertion were obtained every 20 min (2). Cool (4 "C) drinks were placed in a styrofoam container within easy reach. To maintain drink temperature and carbonation, open containers of drink were provided every 10-20 min. Subjects were allowed to straddle the treadmill during consumption of drinks (to avoid spilling) and insertion of clinical thermometers. Immediately after the Zhour treadmill run, subjects towel dried and then weighed nude. Subjects subsequently left the environmental chamber and responded to a two-page questionnaire designed to provide a subjective assessment of drink characteristics and gastrointestinal symptoms. Total residual volume (V,) was determined by using the following calcuwhere P = PEG lations (17): V, = V, V, and Pinitial-V,= Pdilutee(V2 VffZO); concentration (mg-mL-'), V, = gastric residue (mL) recovered during first aspiration, V, = gastric residue (mL) remaining after first aspiration, and V, = water volume (mL) infused. Total body sweat rate was calculated from the net change in nude body weight corrected for fluid consumed and urine voided. Percent body weight replaced was calculated as the fluid consumed divided by the sum of net change in body weight, fluid consumed, and urine excreted. Fluid consumed was determined by the net change in weight of the drink container. Subjective assessment of drink characteristics was quantified by using a visual scale (145 mm line).
+
+
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Consumption of Sports Drinks During Exercise 1 229
Similar visual analog scales have been used in prior investigations to quantify thirst perception (28,33). These studies have documented a close relationship between plasma osmolality, vasopressin release, and the onset of thirst (33). Sample questions addressed with the visual scale included "not carbonatedlvery carbonated," "not sweetlvery sweet," and "not mouthcoatinglvery mouth coating." Subjects were asked to mark a point on the line to indicate their response. Responses were quantified by measuring the distance (in mm) of the subject's mark on the 145-mm line, which extended from 0 mm (not -) to 145 mm (very -1. Gastrointestinal symptoms surveyed included abdominal cramps, sideache, and diarrhea. Data were analyzed using either a one-way or a two-way ANOVA with repeated measures. Significant differences identified by these analyses were determined using the Scheffk test. Data are expressed as mean f standard error. Nonparametric data (frequency of gastrointestinal distress) were analyzed by using a chi-squared test. The level of significance for all comparisons was P0.05) gastric residual volumes (GRV). Values for GRV were 220f 54 mL, 243 f79 mL, 376f 87 mL, and 306 +65 mL for the 6% NC, 6 % C, 10%NC, and 10%C trials, respectively. Corresponding mean values for percentage of drink emptied were 78.0, 75.7, 62.4, and 69.4%. The four 1-hr treadmill runs also resulted in similar changes in HR and Tre. At the end of exercise, HR ranged from 152f6 to 156f 6 bpm, and Tre ranged from 39.1 +0.2 to 39.4 f0.2 "C. Sweat rates were also similar for the four trials and ranged from 1.87 0.1 to 2.03 f0.2 Kglh.
+
Ad Libitum Drinking Studies
Ad libitum drinking of the 6 % NC, 6 % C, 10% NC, and 10% C drinks resulted in similar changes in HR, T,, and RPE during the 2-hr treadmill exercise bouts. Mean HR increased (P
Consumption of Carbonated and Noncarbonated Sports Drinks During Prolonged Treadmill Exercise in the Heat Alan J. Ryan, Amy E. Navarre, and Carl V. Gisolfi University of Iowa These studies were done to determine the effect of carbonation and carbohydrate content on either gastric emptying or ad libitum drinking during treadmill exercise in the heat. Four test drinks were used: a 6% carbohydrate, noncarbonated; a 6%carbohydrate, carbonated; a 10% carbohydrate, noncarbonated; and a 10% carbohydrate, carbonated drink. For gastric emptying studies, subjects completed four I-hr treadmill runs in the heat. They were given 400 mL of test drink at 0 rnin and 200 mL at 15, 30, and 45 min of exercise. For ad libitum drinking studies, subjects completed four 2-hr treadmill runs in the heat. Gastric residual volumes were similar during the four 1-hr runs. During the 2-hr runs, ad libitum drinking of the four beverages was also similar. Mean values for sweat rate, percentage of body weight lost, and percentage of fluid replaced by ad libitum drinking were similar for the four trials. Similar changes in heart rate, rectal temperature, and ratings of perceived exertion were also observed during the four 2-hr treadmill runs. We conclude that the presence of carbonation in a carbohydrate drink did not have a significant effect on either gastric emptying or ad libitum drinking.
Adequate fluid intake is considered the primary preventive measure against the onset of heat illness during prolonged exercise in the heat. During physical training and competitive events, athletes commonly experience body fluid losses ranging from 2.5 to 7.0% body weight, and core body temperatures exceeding 40.0 "C (1 1, 23, 34). Inadequate fluid intake during physical training and/or competition may predispose an athlete to heat illness by reducing sweat rate (3I), skin blood flow (8), and gastric emptying of fluids (21, 22), and by increasing the extent of dehydration and excessive rise in body core temperature (1 1). Surprisingly, the factors that influence the amount of fluids ingested during exercise are not well understood (13). This work was supported by the Quaker Oats Company. The authors are with the Department of Exercise Science at the University of Iowa, Iowa City, IA 52242.
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226 1 Ryan, Navarre, and Gisolfi
Typically, men working in the heat will not drink sufficient fluids to replace the water lost in sweat (1,5, 12, 32). This phenomenon, termed voluntary dehydration, may be due to an inadequacy of the thirst mechanism to detect and correct body fluid deficits (1, 12, 13). Factors that may influence the degree of voluntary dehydration include oropharyngeal cues (3, 7), degree of gastric distention (28), and perception of thirst (13,30). Few studies have examined the factors relating to voluntary dehydration during exercise in the heat (13). Hubbard et al. (13) demonstrated that the extent of voluntary dehydration sustained during a simulated 14.5 km desert walk (40 "C db, 26 "C wb) can be markedly reduced by cooling and flavoring water. In contrast, Carter and Gisolfi (5) reported that their subjects tended to drink more water than a 7.5% carbohydrateelectrolyte drink during cycling exercise (60% V0,max) in the heat. Gastric emptying rate is a factor that may affect the amount of fluid ingested during exercise. The rate of emptying of fluids from the stomach is directly related to drink volume (intragastric pressure) and inversely related to drink osmolality and caloric density. During exercise, repeated ingestion of sports drinks will help maintain fairly high gastric emptying rates (25, 29), presumably as a result of maintaining elevated intragastric pressures (6). During exercise in the heat, repeated ingestion of fluid is essential in preventing dehydration and hyperthermia and an associated reduction in gastric emptying rate (21, 22). The factors that contribute to cessation of drinking during exercise are unknown; however, sensations of stomach fullness (elevated intragastric pressure) can contribute to termination of drinking in humans at rest (28). Thus, three factors that have the potential to influence ad libitum drinking during exercise in the heat include drink palatability, drink composition, and the presence of carbonation. Carbonation could influence drinking behavior by altering palatability, sensations of stomach fullness (intragastric pressure), and/or gastric emptying. The present studies were designed to examine the influence of carbonation and carbohydrate content on either gastric emptying or ad libitum drinking during prolonged moderate intensity (60-65 % V02max) treadmill exercise in the heat. To our knowledge, there is currently no information on the effect of carbonation on ad libitum drinking during exercise.
Methods Nine healthy men (ages 19 to 40 years) volunteered to participate in these studies. Each was informed of the experimental procedures and gave his signed informed consent. Before the experimental trials, each subject completed (a) a test for maximal aerobic capacity (VO,max), (b) a test to identify the treadmill speed required to elicit 60 or 65 % V02max, and (c) a heat acclimation protocol. To determine VO,max, each subject ran at an initial treadmill speed of 8 km-h-' up a 2 % grade; at every Zmin interval thereafter, the treadmill speed and grade were increased by 0.8 km-h-' and 2%, respectively. Maximal aerobic capacity was identified by a plateau of VO, with an increase in exercise intensity and a respiratory exchange ratio greater than 1.O. To determine treadmill speed required to elicit 60 or 65% VO,max, each subject ran at preselected speeds for 6 min and his VO, was measured during the
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Consumption of Sports Drinks During Exercise 1 227
last 3-min interval. To induce acclimation to heat stress, each subject completed five to seven Zhour treadmill runs (approximately 60% V02max) in the heat (32-35 "C db, 15-25% rh). Heat exposures were conducted on different days within a period of 14 days. The four test drinks used in this study were 6% carbohydrate, noncarbonated (6% NC) drink; 6 % carbohydrate, carbonated (6% C) drink; 10% carbohydrate, noncarbonated (10 % NC) drink; and 10% carbohydrate, carbonated (10 % C) drink. The 6 % NC and 6 % C drinks contained sucrose and glucose whereas the 10% NC and 10% C drinks contained high fructose corn syrup (Table 1). Drinks were analyzed for Naf and K+ (Instrumentation Laboratory, Model 943) and osmolality (Wescor, Model 5300). Carbonation of drinks was quantified using a Model D-T Piercing Device (Terriss-Consolidated Industries, Inc., Asbury Park, NJ). Briefly, a capped bottle containing 250 mL of drink was placed into the device and pierced with a pressure gauge. The device and drink were then shaken as a unit until no further increases in pressure were observed. The quantity of carbonation was determined as the volumes of gas pressure per volume of liquid (Table 1). Experiments were performed using a balanced design under single-blind conditions. Subjects were asked to limit their exercise during the 24-hour period prior to experiments. Subjects were also encouraged to drink at least 1 L of fluid the evening before the experiments to ensure hydration. Each subject performed experiments at the same time of day, after an 8- to 10-hour fast.
Table 1 Composition of the Four Test Drinks
Drink
6% NC 6% C 10% NC 10% C
CHO type
Na+ (mEqlL)
Sucrose, glucose Sucrose, glucose Fructose corn syrup, sucrose Fructose corn syrup, sucrose
21.1 20.0 2.6 11.5
K+ Osmolality Carbonation (mEqlL) (mOsm1Kg) (volumes)
2.8 3.0 0.01 4.5
373 355 622 608
0 2.3 0 2.0
Gastric Emptying Studies
Six subjects (V0,max = 56.9A4.9 mL-Kg-'-min-l) participated in the gastric emptying experiments. Each subject completed four 1-hour treadmill runs (65 % V02max) in the heat (35 "C db, 30% rh) drinking either 6% NC, 6 % C, 10% NC, or 10% C. During each trial the subjects were given 1 L of test drink; 400 mL at 0 min and 200 mL at 15, 30, and 45 min of the run. All test drinks contained polyethylene glycol (PEG), a nonabsorbablemarker, at a concentration of 7.6 mg-rn~-'.On the day of an experiment, subjects voided and then weighed
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228 1 Ryan, Navarre, and Gisolfi
nude (Acme scale, f log). After a resting heart rate (HR) and rectal temperature (T,d were obtained, the subject entered an environmental chamber and then consumed 400 mL of test drink (4 "C). Immediately after drinking the test drink, subjects mounted the treadmill and ran at a preselected speed. Heart rates (ECG) and T, (clinical thermometer inserted 7 cm into rectum for 2 min) were obtained at 20, 40, and 60 min of exercise. Subjects were allowed to straddle the treadmill during consumption of drinks (to avoid spilling) and to accommodate the insertion of a clinical thermometer. Typically the subjects required approximately 20 sec to straddle the treadmill, insert the clinical thermometer, and consume the drink. Immediately following exercise the subject passed a nasogastric tube (French-Levine, no. 12) into his stomach. Gastric contents were quickly removed by aspiration with a 60-mL syringe; removal was facilitated by systematic movement of the tube as well as by having the subject assume different body positions. This procedure was followed by infusion of 60 mL of distilled water into the stomach, which was then mixed thoroughly with any remaining residue and again aspirated. Subjects then removed the tube, towel dried, and a nude body weight was obtained. Ad Libitum Drinking Studies
Seven subjects (V0,max = 61.4k2.3 m ~ - ~ ~ - ' - r n i )n participated -' in the ad libitum drinking experiments. Each subject completed four 2-hour treadmill runs (60% V0,rnax) in the heat (32 "C db, 30% rh) drinking either 6 % NC, 6 % C, 10% NC, or 10% C. Upon reporting to the laboratory, the subject voided and then weighed nude. After resting HR and T, were obtained, the subject entered an environmental chamber, mounted a treadmill, and ran at a predetermined speed. Heart rates (ECG) and T, (clinical thermometer inserted 7 cm into the rectum for 2 min) were obtained every 15 min. Ratings of perceived exertion were obtained every 20 min (2). Cool (4 "C) drinks were placed in a styrofoam container within easy reach. To maintain drink temperature and carbonation, open containers of drink were provided every 10-20 min. Subjects were allowed to straddle the treadmill during consumption of drinks (to avoid spilling) and insertion of clinical thermometers. Immediately after the Zhour treadmill run, subjects towel dried and then weighed nude. Subjects subsequently left the environmental chamber and responded to a two-page questionnaire designed to provide a subjective assessment of drink characteristics and gastrointestinal symptoms. Total residual volume (V,) was determined by using the following calcuwhere P = PEG lations (17): V, = V, V, and Pinitial-V,= Pdilutee(V2 VffZO); concentration (mg-mL-'), V, = gastric residue (mL) recovered during first aspiration, V, = gastric residue (mL) remaining after first aspiration, and V, = water volume (mL) infused. Total body sweat rate was calculated from the net change in nude body weight corrected for fluid consumed and urine voided. Percent body weight replaced was calculated as the fluid consumed divided by the sum of net change in body weight, fluid consumed, and urine excreted. Fluid consumed was determined by the net change in weight of the drink container. Subjective assessment of drink characteristics was quantified by using a visual scale (145 mm line).
+
+
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Consumption of Sports Drinks During Exercise 1 229
Similar visual analog scales have been used in prior investigations to quantify thirst perception (28,33). These studies have documented a close relationship between plasma osmolality, vasopressin release, and the onset of thirst (33). Sample questions addressed with the visual scale included "not carbonatedlvery carbonated," "not sweetlvery sweet," and "not mouthcoatinglvery mouth coating." Subjects were asked to mark a point on the line to indicate their response. Responses were quantified by measuring the distance (in mm) of the subject's mark on the 145-mm line, which extended from 0 mm (not -) to 145 mm (very -1. Gastrointestinal symptoms surveyed included abdominal cramps, sideache, and diarrhea. Data were analyzed using either a one-way or a two-way ANOVA with repeated measures. Significant differences identified by these analyses were determined using the Scheffk test. Data are expressed as mean f standard error. Nonparametric data (frequency of gastrointestinal distress) were analyzed by using a chi-squared test. The level of significance for all comparisons was P0.05) gastric residual volumes (GRV). Values for GRV were 220f 54 mL, 243 f79 mL, 376f 87 mL, and 306 +65 mL for the 6% NC, 6 % C, 10%NC, and 10%C trials, respectively. Corresponding mean values for percentage of drink emptied were 78.0, 75.7, 62.4, and 69.4%. The four 1-hr treadmill runs also resulted in similar changes in HR and Tre. At the end of exercise, HR ranged from 152f6 to 156f 6 bpm, and Tre ranged from 39.1 +0.2 to 39.4 f0.2 "C. Sweat rates were also similar for the four trials and ranged from 1.87 0.1 to 2.03 f0.2 Kglh.
+
Ad Libitum Drinking Studies
Ad libitum drinking of the 6 % NC, 6 % C, 10% NC, and 10% C drinks resulted in similar changes in HR, T,, and RPE during the 2-hr treadmill exercise bouts. Mean HR increased (P
