The ingestion of dilute carbohydrate-electrolyte drinks during prolonged physical activity is recommended in order to delay the fatigue process (18). The ingested exogenous carbohydrate has been shown to prevent hypoglycemia and maintain carbohydrate oxidation to meet the energy demands during the later stages of endurance exercise (8,9). The water component of drinks can compensate for the exercise-induced sweat loss, thereby sustaining thermoregulatory and cardiovascular function and exercise capacity (7,17). Little benefit can be derived from ingestion of a drink until its components have been incorporated into the relevant body pool. This, in turn, depends on the rate at which the ingested drink is emptied from the stomach, absorbed by the small intestine and transported to the appropriate body pools (17). The rate of gastric emptying is thought to be the major determinant of the availability of ingested drinks and will therefore have a significant effect on their efficacy (6,17).
The volume and the composition of fluids in the stomach are known to be important factors that regulate the rate of gastric emptying (6,17,30), but until recently it has been assumed that the exercise intensity at which most competitive team sports are played has little effect on gastric emptying (6,30). The pattern of exercise in most team sports and other games is one of intermittent exercise characterized by short bursts of high-intensity sprinting interspersed by variable periods of lower intensity exercise or rest (1,30). It has been shown that gastric emptying is not noticeably slowed during prolonged constant power output exercise until exercise intensity exceeds about 70% V̇O2max, (14,15), although there are a small number of cycling studies that have noted a slight reduction in emptying at lower intensities (21,27). Although exercise intensity varies in most competitive sports, the overall level of intensity averages about 70% V̇O2max and the time spent in sprinting is relatively short (1). Therefore, it has been considered that there are sufficient periods of low-intensity exercise during competitive sports to allow ingested fluids to empty from the stomach and for individuals to benefit from the carbohydrate and water supplied by ingested drinks during a game (1,30). Indeed several studies have shown improved endurance capacity (10,25) and exercise performance (5,22) during intermittent exercise when carbohydrate-containing drinks have been consumed compared with that when no drinks or flavored water has been ingested.
Using an isotopic water tracer method, we have previously demonstrated that cycle exercise at 40% V̇O2max and above can reduce the rate of appearance in the circulation of ingested fluids, and that this effect is proportional to the exercise intensity (21). In that study, we could not identify the role of gastric emptying in this effect. In two more recent studies we have shown that intermittent variable-intensity exercise does slow gastric emptying. In the first of these investigations, gastric emptying of a carbohydrate-electrolyte sports drink was significantly slowed when subjects carried out cycling exercise at 60% V̇O2max interspersed at 10-min intervals with three 30-s sprints at 100% V̇O2max (15). Cycling at a constant power output (66% V̇O2 max) equivalent to the average power output in the intermittent variable-intensity trial had no measurable effect on the rate of gastric emptying. In the second study, gastric emptying of the same carbohydrate-electrolyte sports drink as that used in the cycling trials was inhibited during a competitive five-a-side soccer match compared with the volume emptied during low-intensity walking exercise (16). Heart rate recorded throughout the soccer match demonstrated that the intensity of exercise varied widely between individuals and over time but the calculated average power output was equivalent to only 54 ± 23% V̇O2peak for the first 15-min period of the game and 63 ± 20% V̇O2peak for the second period. The average exercise intensity of these studies would not normally be considered enough to slow gastric emptying; therefore, the effect of the intermittent high-intensity sprints must be sufficient to cause a noticeable inhibition of gastric emptying. During the cycling study, subjects exercised for a total of 9 min at 100% V̇O2max and 51 min at 60% V̇O2max, whereas in the soccer trial, subjects exercised for an average total of 467 s at between 70 and 79% V̇O2peak and 218 s at 80% V̇O2peak or above throughout the 30 min playing period.
The purpose of the present study was to investigate whether gastric emptying is slowed during a standardized, variable-intensity shuttle running test designed to simulate the activity pattern of soccer. The second outcome was to identify whether the faster rates of gastric emptying of low-energy fluids compared with that of carbohydrate solutions might allow dilute electrolyte-containing solutions to be more effectively emptied than carbohydrate-electrolyte sports drinks during intermittent exercise.
Eight trained healthy male games (soccer, rugby, or tennis) players were recruited for this study. Subjects were provided with full details of the study protocol before giving written informed consent to participate. This study was approved by Loughborough University Ethical Committee. The physical characteristics (mean ±SD) of the subjects were: age 20.4 ± 1.2 yr, height 1.79 ± 0.06 m, body mass 82.9 ± 10.2 kg, and V̇O2max 53.2 ± 4.6 mL·kg−1·min−1. The volumes of test drink ingested during each trial were body mass adjusted using the nude body mass of the overnight fasted subjects determined during these preliminary measurements before the subjects undertook the familiarization trial.
Initially, all potential subjects were screened to identify individuals who could be successfully intubated with the oro-gastric aspiration tube used to determine gastric emptying. The aspiration tube was inserted orally to allow subjects to intubate themselves, which in our hands appears to reduce the stress of being intubated, increases the number of individuals who can successfully pass the tube and speeds up the process when more than one subject is required to be repeatedly intubated. Maximum oxygen uptake (V̇O2max) was estimated by means of a progressive shuttle run test (26). From this estimate of V̇O2max, running speeds corresponding to 55 and 95% V̇O2max were calculated for each subject using tables for predicted V̇O2max values (26). Subjects then performed the LIST protocol (23) for 30 min, in order to familiarize themselves with the required running speeds and experimental procedure.
Each subject undertook four separate trials, each of which involved exercising for two 15-min periods with a break of approximately 4 min between the two exercise periods. In two of the trials the exercise consisted of walking slowly on a level floor (WE) and in the other two trials subjects carried out the LIST. Before the first exercise period of each trial, subjects ingested a bolus of 5 mL·kg−1 body mass volume of either a 6.4% carbohydrate-electrolyte sports drink (CHO drink) or an artificially sweetened carbohydrate-free placebo drink containing electrolytes (Plac drink). Both drinks were prepared by the same company (GlaxoSmithKline, Brentford, UK) and were of similar color, mouth feel, and taste but differed in osmolality and in carbohydrate and electrolyte composition. The mean ±SD osmolality of the placebo drink was 61 ± 1 mosmol· kg−1 and that of the carbohydrate sports drink was 279 ± 1 mosmol·kg−1. The mean measured concentrations (mmol·L−1) of electrolytes of the CHO drink were sodium 21 ± 2, potassium 2.5 ± 0.3, and chloride 0 ± 2, whereas those of the Plac drink were sodium 1 ± 3, potassium 2.9 ± 0.4, and chloride 0 ± 3. The drinks were administered in a double-blind procedure to ensure that neither the subject nor the individual carrying out the gastric emptying procedure and assay knew which solution was being tested. The treatment order was randomized using a Latin square order design and all subjects successfully completed the four trials. For 24 h before each of the four trials, subjects consumed the same diet that they had eaten before the first trial and did not drink any alcohol. Subjects were asked to refrain from strenuous physical activity for two days before each LIST. For each subject, a minimum of 24 h separated one trial and a WE trial, and at least 48 h separated one trial and a LIST trial. All trials were carried out indoors at an ambient temperature maintained at between 15 and 17°C, and the ingested drinks were served at ambient temperature.
Subjects reported to the laboratory on the morning of each trial after an overnight fast. Each subject emptied his bladder before his nude body mass was measured to the nearest 50 g using a beam balance (Avery, Birmingham, UK). Subjects then dressed appropriately for the LIST or WE trial. A standardized 10-min warm-up was performed before each LIST trial but not before the WE trials. Subjects were fitted with a telemetric heart rate monitor (Polar PE3000; Bodycare Products, Southam, UK) that recorded heart rate every 15 s and stored the values.
The subjects then sat down, passed the gastric aspiration tube (French Levine, 14 gauge: Vygon Ltd., France) orally, and positioned it in their stomach. The tube was positioned such that the stomach could be effectively emptied and the length of tube that each subject required to swallow was established during the familiarization stage. The fasting gastric contents were emptied from the stomach via the tube using a 50-mL catheter-tipped syringe (Becton Dickinson, Drogheda, Ireland), and the stomach was washed with 100 mL of distilled water and a recovery test was carried out to ascertain that the aspiration tube was correctly positioned (11). Subjects then rapidly ingested (43 (15–115) s; median (range)) a body mass adjusted volume (5 mL·kg−1 BM) of the appropriate test drink containing 25 mg·L−1 phenol red (water-soluble; BDH, Poole, UK). The stomach contents were thoroughly mixed by repeated aspiration and reinjection of the stomach contents using the 50-mL syringe. A 2.5-mL aliquot of gastric contents was collected. The gastric tube was removed and subjects began exercising after a median (range) time of 125 (61–367) s after ingesting the drink. Exactly 15 min after starting exercising, they sat down and repositioned the gastric tube. The stomach contents were mixed as before, and a 2.5-mL aliquot of gastric fluid was collected. One milliliter of phenol red at a concentration of 1000 mg·L−1 was injected into the stomach, and the contents mixed again before a second 2.5-mL sample was collected. The gastric tube was removed, the subjects then rapidly ingested (38 (10–96) s; median (range)) a further body mass adjusted volume (2 mL·kg−1 BM) of the appropriate test drink containing 25 mg·L−1 phenol red, and they began exercising within a median (range) time of 226 (180–364) s of finishing the first exercise period. Subjects continued to exercise as previously for a further 15 min before sitting down and repositioning the gastric tube. The gastric contents were mixed once more and a 2.5-mL aliquot collected. Phenol red solution (1 mL) at a concentration of 1000 mg·L−1 was then injected into the stomach, and the contents mixed again before a second 2.5 mL sample was collected. Distilled water (100 mL) was then injected into the stomach, mixed with the gastric contents and the total fluid volume of the stomach was emptied as completely as possible by aspiration.
Comparisons were made of the values for total gastric fluid volume at the end of the experiment as calculated by three different methods: 1) the method of Beckers et al. (2), 2) as estimated from the dilution of the phenol red concentration of the stomach contents by the 100 mL of distilled water wash, and 3) as measured by the actual volume aspirated from the stomach minus the 100 mL of distilled water wash.
As phenol red dye is poorly absorbed by the stomach (3), the difference in concentration of the dye in the original test drink and the collected samples can be used to calculate the total volume in the stomach and the volume of test drink remaining in the stomach at the specific time points (2). The difference between the total gastric volume and the test drink volume is the volume of gastric secretions and swallowed saliva.
Subjects stood and walked slowly along a level floor for each of the two 15-min periods of low-intensity exercise during the WE trials. The LIST trials followed a set activity pattern comprising three by 20-m walks, one 20-m sprint at maximum running speed, an approximately 4-s standing rest period, three 20-m jogs at 55% V̇O2max, and three 20-m cruises at 95% V̇O2max (23). This block of exercises is repeated throughout each 15-min period of the LIST. The 20-m distance was marked on the floor, and walking and running speeds were dictated by an audio signal from a microcomputer (BBC Master series, Cambridge, UK) with sprint times measured in one direction by two infrared photoelectric cells (RS Components, Michigan). The same pattern was repeated during each of the two 15-min periods of exercise during this trial. Approximately 22.3% of the total exercise time in the LIST is spent at an intensity at or greater than 95% V̇O2max, whereas the activity levels for the remainder are at or below 55% V̇O2max (23).
During the LIST trials, a record was made of the time taken to complete each cycle of intermittent activity, the distance covered and the sprint times. Subjective ratings of perceived exertion were recorded (4) at the midpoint of each cycle of intermittent exercise during the LIST trial. The subject’s nude body mass was recorded after exercise and the amount of evaporative water loss was calculated from the decrease in body mass that occurred during the trial plus the volume of test drink emptied from the stomach.
The phenol red concentration of test solutions and aspirated samples was measured spectrophotometrically at a wavelength of 560 nm after dilution 1:10 with NaOH-NaHCO3 buffer (250:500 mmol·L−1, pH 9.7). The carbohydrate content of drinks and aspirates was determined using a glucose oxidase method (GOD perid; Roche-Boehringer, East Sussex, UK) after acid hydrolysis of the sample (12) and is therefore expressed as millimolar glucosyl units. Sodium and potassium concentration of the test drinks were measured using a flame photometer (Clinical Flame photometer 410C; Corning Ltd., Halstead, Essex, UK); chloride concentration was determined using a coulometric titrator (PCLM 3; Jenway, Dunmow, Essex, UK). Osmolality was determined by freezing point depression (Gonotec Osmometer 034: Clanden Scientific, Hants., UK).
All data were initially tested for distribution and for homogeneity of variance. The assumption that the data were normally distributed was reasonably met for all the study data except for sweat loss data and the elapse times between finishing drinking and beginning both exercise bouts. Statistical analysis of the normally distributed data was carried out using ANOVA, and where appropriate this was followed by application of Fisher’s least significant difference test to assess any differences between treatments or times. Statistical analysis of the nonparametric data was carried out using the Kruskal–Wallis test and pairwise differences were assessed using Wilcoxon’s matched-pairs signed ranks test where appropriate. All tests were two-tailed, and the conventional 5% level was used to determine statistical significance. Data are reported in the text and figures as mean ±SD values, except for the nonparametric data that are reported as median and range values.
Nude body mass was similar at the beginning of each of the four trials (Table 1). Whereas body mass increased due to the volume of beverage emptied into the small intestine during the walking trials, body mass decreased (P < 0.01) by similar amounts in the two intermittent high-intensity trials (Table 1). Heart rate, averaged over the exercise periods on the trials, was similar in the two low-intensity walking trials and lower (P < 0.01) than on the LIST trials, which were similar to each other (Table 2). Average heart rate recorded over the 4-min rest periods before starting exercise (66 ± 12 bpm) was similar on both walking trials before the first (P = 0.73) and second (P = 0.92) 15-min exercise bouts. No differences in average heart rate were detected between rest and exercise periods on the walking trials (P = 0.27). Ratings of perceived effort were also comparable between the two LIST trials, as were average sprint time and total distance covered over the 30 min of exercise (Table 2). The total time spent sprinting during the 30 min of exercise was similar (P = 0.57) on the Plac (56.6 ± 0.4 s) and CHO trials (56.5 ± 0.3 s), and no difference could be detected (P = 0.93) for the overall time spent running at 95% V̇O2max on the Plac (346.3 ± 0.1 s) and CHO trials (346.3 ± 0.1 s).
The test drink volume in the stomach was the same at the start of all four trials (Table 3). During the first 15 min of exercise on the LIST trials, similar volumes of the placebo drink and the CHO drink were emptied from the stomach, and these volumes were less than that emptied on the WE trials when the placebo drink was ingested (P < 0.002; Fig. 1). On the WE trials, there was a tendency for a greater volume of the placebo drink to be emptied than of the CHO drink (P = 0.09). The volume of the placebo drink emptied on the LIST trial was similar to that of the CHO drink on the WE trial.
At the start of the second period of exercise, there was less test solution volume in the stomach after ingestion of the placebo drink on the WE trials than on any of the other trials (P < 0.05; Table 3). The volume of CHO test drink present in the stomach at the start of the second exercise period was less (P < 0.02) on the WE trial than on the LIST trial. No other differences were detected in these volumes at the start of the second exercise period. The volume of test drinks emptied during the second 15 min of exercise was similar on all four trials (Fig. 1).
The test drink volume emptied over the 30 min of exercise (Fig. 1) on the LIST trials was similar for both test drinks and was less than that after ingestion of the placebo drink during the WE trial (P < 0.001). A greater volume of the placebo drink was emptied than of the CHO drink over the whole of the WE trial (P < 0.02). The test drink volume emptied over the entire WE trial after ingestion of the CHO drink was greater than that on the LIST trial when the same drink was ingested (P < 0.05) but similar to that on the LIST trial when the placebo drink was consumed.
The same total volume of test drink (587 ± 67 mL) was ingested on all four trials. The volume of test drink emptied over the whole 30 min of exercise, expressed as a percentage of the total volume ingested, was greater on the WE trial when the placebo was the test drink (67.3 ± 9.4%) than on the WE trial when the CHO drink was ingested (49.7 ± 9.9%; P < 0.005) or on either the LIST trials (P < 0.001). A greater percentage of the total CHO drink volume intake was emptied over the whole WE trial than of CHO drink on the LIST trial (35.6 ± 13.8%; P < 0.035) or of placebo drink on the LIST trial (35.2 ± 15.5%; P < 0.05). There was no difference in the percentage of the total test drink intake that was emptied during the two LIST trials.
There was no difference between trials in the median elapsed time after the end of ingestion of the initial drink bolus and the beginning of exercise (125 (61–367) s; P = 0.27), nor between finishing ingestion of the second drink and the start of the second bout of exercise (226 (180–364) s; P = 0.19). However, one subject did not start the first exercise bout until 367 s after finishing the initial bolus of the CHO drink on the LIST trial, and another subject did not start the second bout of exercise until 364 s after ingesting the placebo drink on the LIST trial. In neither case was the total fluid or drink volume emptied faster over that specific exercise bout of the LIST trial. In addition, only a weak negative correlation was found between the elapsed time before starting the first bout of exercise and the total fluid or drink volume emptied during the first exercise period (Spearman’s rank correlation coefficient ρ = −0.310 and ρ = −0.244, respectively), and no association was shown between the elapsed time before starting the second bout of exercise and the total fluid or drink volume emptied during the second exercise period (ρ = 0.048 and ρ = 0.009, respectively).
Carbohydrate delivery to the small intestine.
The amount of carbohydrate ingested at the start of the two CHO trials was 26.9 ± 3.0 g (149 ± 17 mmol glucosyl units), and the total amount ingested in each of these trials was 37.6 ± 4.3 g (208 ± 24 mmol glucosyl units). The amount of ingested carbohydrate delivered to the small intestine over the first 15 min of exercise was greater (P < 0.01) on the WE trial (10.2 ± 4.0 g; 57 ± 22 mmol glucosyl units) than on the LIST trial (4.6 ± 2.7 g; 26 ± 15 mmol glucosyl units). The total amount of carbohydrate delivered to the small intestine was greater (P < 0.05) during the 30 min of the WE trial (19.0 ± 4.8 g; 105 ± 27 mmol glucosyl units) than on the LIST trial (13.3 ± 5.3 g; 74 ± 29 mmol glucosyl units).
Although most of the fasting contents of the stomach were removed at the beginning of each trial, there was a residue of 28 ± 17 mL that was similar for all trials. The total fluid volume in the stomach was greater than the test drink volume due to the presence of the fasting residual volume at the start of a trial, and to the addition of gastric secretions and swallowed saliva throughout each trial. The total fluid volume in the stomach was similar at the start of all four trials (Table 3). However, total fluid volume in the stomach at the start of the second period of exercise was less (P < 0.05) after ingestion of the placebo drink than the CHO drink on the WE trials and was also less than the total volume at the same time point on either of the LIST trials (Table 3). There were no other differences detected in these volumes at the start of the second exercise period. During the first 15 min of exercise, there was a similar (P = 0.31) secretion volume of 90 ± 40 mL on each of the four trials. Over the second 15 min of exercise, the gastric secretion volume was again similar between trials (58 ± 33 mL; P = 0.45) but slightly less than that produced in the initial exercise period (P < 0.001). Gastric emptying of the total fluid volume in the stomach followed a similar pattern to that of the corresponding test drink on the same trial over the same time period (Fig. 2).
The mean total gastric volume measurements at the end of the 30 min of exercise were essentially the same whether calculated by dye dilution using the formula of Beckers et al. (2) or from dilution of the dye by the 100 mL of distilled water wash (P = 0.92), and both were similar (P = 0.41) to that aspirated from the stomach at the end of the trials (Table 4).
Sweat loss (mL) was assumed to be equivalent to the change (g) in nude body mass over the period of each experimental trial plus the total volume of test solution (mL) that had been emptied into the small intestine (Table 1). Most of the gastric secretions produced during the trials were emptied into the small intestine; therefore, no correction was made, when calculating sweat loss, for the small volume of secretion that was removed when the stomach contents were aspirated at the end of each exercise period. Sweat loss during the WE trials was essentially the same (P = 0.40) and was much less (P = 0.001) than that on the two LIST trials, which were similar (P = 0.20) to each other (Table 1).
Gastric emptying was slowed during the LIST trials although the overall exercise intensity would not normally be considered sufficient to affect gastric emptying had the physical activity been carried out at a constant power output (30). The subjects in the present study were exercising at or above 70% of their V̇O2 max for only about 3 min 21 s during each 15-min period of the LIST (23), and during the remainder of the time exercise intensity was at or below 55% of their V̇O2max. During the LIST trials, average heart rate of the subjects was approximately 87% of their peak heart rate, which approximates to a mean exercise intensity of about 70% V̇O2max. This is slightly greater than that measured in students playing a five-a-side soccer game (16) but similar to that measured in professional soccer players competing in outdoor soccer matches (1). The volume of the carbohydrate-containing test drink emptied into the small intestine during the two exercise periods of the LIST trial approximated to that emptied of a similar sports drink during the first and second halves, respectively, of a competitive five-a-side soccer match (16). This finding suggests that even a small amount of high-intensity sprinting can slow gastric emptying during variable-intensity exercise such as occurs during many sports. The amount and level of high-intensity sprinting that affect gastric emptying and the mechanism that mediate this response remains to be determined.
In both the present study and in the study involving five-a-side soccer (16), the total fluid volume emptied into the intestine during the second 15-min period of exercise tended to be greater than that emptied during the first 15-min period. In the LIST trial, this could be partially explained as a volume effect (14) as there was a larger volume of fluid in the stomach at the start of the second period because the second drink bolus was added to the gastric contents remaining from the first exercise period. However, in the five-a-side soccer study, this could not be the reason as the gastric contents remaining at the end of the first period of play were completely removed before the second bolus of the test drink was instilled into the stomach. The finding that there was a tendency for the total volume of fluid emptied to be greater during the second period of exercise suggests that there might be a learning effect that occurs as individuals become accustomed to exercising with a relatively large fluid volume in their stomach. If this is true then the learning effect must be quite short acting since in the five-a-side soccer study no difference was detectable between the rates of gastric emptying in the familiarization trial and the actual study soccer match trial (P = 0.93). The familiarization trial consisted of a five-a-side soccer game with fluid ingestion and gastric aspiration at the appropriate times, and it was carried out within 2 wk of the start of the study. At present, the reason for the emptying of greater volumes of carbohydrate drinks during the second 15 min of intermittent high-intensity exercise is not apparent.
Increasing the energy density and/or the osmolality of a drink slows the gastric emptying rate of the drink, with energy density exerting the greater effect (6,14,18,31). Most studies have found that solutions with a carbohydrate concentration of 6% or above empty more slowly than do energy-free drinks when ingested at rest or during low-to-moderate intensity constant output exercise (14,18,31). In the present study, the volume of the carbohydrate-free placebo drink that was emptied over 30 min was greater than that of the 6.4% carbohydrate drink during the walking trials. Because of the faster emptying rates of low-energy drinks, ingestion of water or dilute carbohydrate solutions has been recommended as being potentially more effective than sports drinks in situations, such as during soccer matches, where gastric emptying might be slowed (19,29). In one constant output cycling study, water was found to empty marginally faster than a 7.2% carbohydrate drink during exercise at about 55% and 75% of V̇O2max (28). However, during the LIST trials in the present study both the carbohydrate-free and carbohydrate-containing drinks emptied at the same slow rate, which indicates that exercise has a greater effect than carbohydrate content at this intensity. This present study is the first to conclusively show that variable-intensity exercise, which simulates the activity pattern of soccer, slows the rate of gastric emptying of energy-free drinks such as water to the same extent as that of carbohydrate-containing sports drinks. Given the potential benefits of exogenous carbohydrate intake in delaying the fatigue process and also in promoting intestinal water absorption (18,19), there appears little reason to advocate drinking water rather than a suitable sports drink while playing a soccer match.
Subjects in the present study were overnight fasted before each trial, if individuals eat a meal 1–4 h before playing a game, the effect of nutrients in the stomach and intestine may well influence the emptying of fluids ingested during exercise. The result is likely to be a further slowing in the rate of gastric emptying, but the outcome will depend on the amount and type of nutrient consumed and the length of time after ingestion that the exercise starts. The findings of the current study would suggest, however, that there would be no difference between the emptying rate of water and a 6.4% carbohydrate beverage.
It has been previously shown that ingestion of a carbohydrate-electrolyte solution, similar to that used in the present study, improves endurance capacity after variable-intensity running (25). In the study of Nicholas and colleagues (25), there was a trend for higher blood glucose concentrations after ingestion of the carbohydrate-electrolyte solution compared with that when a noncarbohydrate placebo solution was drunk; during the exercise, differences were found only at the 30-min time point of the LIST and at the point of exhaustion. In the recovery period, however, a more sustained increased blood glucose concentration and higher serum insulin levels were seen on the carbohydrate trial compared with the placebo trial. The pattern of circulating glucose and insulin response seen during the intermittent high-intensity running trials is consistent with the suggestion that gastric emptying of the carbohydrate-electrolyte solution was markedly slowed during the exercise and that the gastric contents were rapidly emptied into the intestine after the exercise stopped. The model of LIST exercise and drinking in the present study is the same as that in the study of Nicholas and colleagues (25). The present study has shown that about 36% of the 587 mL of ingested carbohydrate-electrolyte sports drink was emptied from the stomach over the initial 30 min of the LIST, so only about 13–14 g of exogenous glucose was available for intestinal absorption and therefore able to be oxidized. Although this rate of glucose delivery is less than the maximum rate of exogenous glucose oxidation of 1 g·min−1 (13), it is close to the 0.5–1.0 g·min−1 carbohydrate intake proposed as being sufficient to delay fatigue by maintaining blood glucose oxidation (8). Other studies have suggested that, at least during prolonged constant output cycling exercise, ingestion of dilute carbohydrate solutions can increase exercise capacity in excess of that likely to have been produced by oxidation of the exogenous glucose supplied (20).
Even though exercise-induced slowing of gastric emptying occurs during the LIST test, sufficient exogenous carbohydrate was available to maintain blood glucose oxidation and enhance exercise capacity. In addition, there was a tendency for the rate of gastric emptying during the second 15-min test period to be faster than that in the first 15-min period. This suggests that the continued drinking of the carbohydrate-electrolyte solution at the same rate during the 75 min of the LIST trial in the study of Nicholas and colleagues (25) would have promoted faster gastric emptying throughout the trial and resulted in sufficient circulating blood glucose or even muscle glycogen sparing (24) to account for the enhanced exercise capacity demonstrated at the end of the LIST trial.
The volume of water emptied into the small intestine during the LIST trial in the present study was similar for both the placebo and carbohydrate-electrolyte drinks. The actual volumes were less than the calculated sweat loss incurred by the subjects and at the end of the 30 min of the LIST trial subjects were dehydrated on average by approximately 0.8% of their body mass. In the previous LIST trial studies of Nicholas and colleagues (23,25), body mass losses, which were equated to sweat rates, were calculated from body mass changes minus the total volume of drink ingested during the exercise. However, as some of the ingested drink must have been absorbed and integrated with the body water pool, the level of dehydration of their subjects at the end of the exercise was not the same as the calculated body mass losses indicated. In those particular studies, the volume of drink emptied into the intestine was unknown. If we assume that the emptying rate of the sports drink ingested in those studies was similar to that of the sports drink used in the present study over 30 min, then the actual level of dehydration in the previous studies of Nicholas and colleagues (23,25) would approximate to about 2–2.3% rather than the 2.7–3% of body mass that might be inferred from their calculated sweat rates. Dehydration by as little as 1.8% of body mass has been shown to reduce high-intensity performance (32), and decrements in exercise performance and capacity may be progressive as dehydration levels increase (17). In addition, frank dehydration at a level of about 3.5–4% body mass loss is associated with slowing of gastric emptying and a higher frequency of gastrointestinal distress in exercising individuals (28). It is clear from the present study that similar volumes of energy-free placebo and the carbohydrate sports drink were emptied into the small intestine during the LIST trial. As there were no differences in plasma volume changes and heart rate values between the placebo and carbohydrate drink trials in the LIST study of Nicholas and coworkers (25), the improvement in exercise capacity seen in their study after ingestion of the carbohydrate drink was likely to be due to the exogenous energy supplied rather than to improved fluid balance.
Clearly, specific recommendations for fluid ingestion during variable-intensity exercise, such as soccer, must be drawn with care from studies that have used exercise with a constant power output. Small amounts of high-intensity sprinting can slow the gastric emptying rate of ingested drinks. However, the carbohydrate content of most sports drinks, intended for use during exercise, does not appear to inhibit gastric emptying further and can increase exercise capacity. There appears to be no reason to recommend ingesting water rather than a suitable sports drink during variable-intensity exercise, other than if it is the individual’s preference.
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