International Journal of Sport Nutrition and Exercise Metabolism, 2014, 24, 605 -612 http://dx.doi.org/10.1123/ijsnem.2013-0097 © 2014 Human Kinetics, Inc
www.IJSNEM-Journal.com ORIGINAL RESEARCH
Combined Glucose Ingestion and Mouth Rinsing Improves Sprint Cycling Performance Edwin Chong, Kym J. Guelfi, and Paul A. Fournier This study investigated whether combined ingestion and mouth rinsing with a carbohydrate solution could improve maximal sprint cycling performance. Twelve competitive male cyclists ingested 100 ml of one of the following solutions 20 min before exercise in a randomized double-blinded counterbalanced order (a) 10% glucose solution, (b) 0.05% aspartame solution, (c) 9.0% maltodextrin solution, or (d) water as a control. Fifteen min after ingestion, repeated mouth rinsing was carried out with 11 × 15 ml bolus doses of the same solution at 30-s intervals. Each participant then performed a 45-s maximal sprint effort on a cycle ergometer. Peak power output was significantly higher in response to the glucose trial (1188 ± 166 W) compared with the water (1036 ± 177 W), aspartame (1088 ± 128 W) and maltodextrin (1024 ± 202W) trials by 14.7 ± 10.6, 9.2 ± 4.6 and 16.0 ± 6.0% respectively (p < .05). Mean power output during the sprint was significantly higher in the glucose trial compared with maltodextrin (p < .05) and also tended to be higher than the water trial (p = .075). Glucose and maltodextrin resulted in a similar increase in blood glucose, and the responses of blood lactate and pH to sprinting did not differ significantly between treatments (p > .05). These findings suggest that combining the ingestion of glucose with glucose mouth rinsing improves maximal sprint performance. This ergogenic effect is unlikely to be related to changes in blood glucose, sweetness, or energy sensing mechanisms in the gastrointestinal tract. Keywords: sprinting, carbohydrate, power output, mouthwash Carbohydrate ingestion before and during exercise can improve performance in prolonged aerobic events due, in part, to the maintenance of high rates of carbohydrate oxidation (Coyle et al., 1986), along with lower rates of muscle glycogen utilization to be spared for later use (Tsintzas et al., 1996). Carbohydrate ingestion has also been reported to be beneficial to intense aerobic events lasting 1 hr (Below et al., 1995; Jeukendrup et al., 1997), but not if carbohydrate is administered intravenously (Carter et al., 2004b), thus suggesting that the mechanism underlying the ergogenic effect of carbohydrate ingestion for intense aerobic exercise of short duration is independent of the gastrointestinal absorption and metabolism of carbohydrate, and may involve facilitation of the corticomotor output to both fresh and fatigued muscle (Gant et al., 2010). In support of this interpretation, rinsing the mouth with a carbohydrate solution without swallowing has been shown by most (Carter et al., 2004a; Chambers et al., 2009; Pottier et al., 2010; Rollo et al., 2010) but not all studies (Beelen et al., 2009; Whitham & McKinney, 2007) to improve 1-hr time trial performance, with the effect of stimulating other portions of the gastrointestinal tract still remaining to be tested. The authors are with the School of Sport Science, Exercise and Health, University of Western Australia, Crawley, Australia. Address author correspondence to Edwin Chong at [email protected].
Since sprint and endurance efforts affect muscle metabolism differently and have different mechanisms of fatigue, what still remains to be examined, is whether carbohydrate ingestion or mouth rinsing also improves maximal sprint performance. In this respect, Beaven and colleagues (2013) have recently shown that a single 5-second glucose mouth rinse without swallowing improves to a small extent mean power output (~39 W) in the initial sprint of a series of 5 × 6-s sprints compared with a noncaloric placebo, thus suggesting that carbohydrate receptors in the mouth mediate this effect. However, other findings show that a single 5-s carbohydrate mouth rinse does not affect more prolonged (30-s) maximal sprint cycling performance (Chong et al., 2011). Given that carbohydrate receptors are found in several portions of the gastrointestinal tract (e.g., oral cavity, esophagus, upper intestine; Bachmanov & Beauchamp, 2007; Bezencon et al., 2007), this raises the issue of whether the combined stimulation of the glucose receptors in the oral cavity and the remainder of the gastrointestinal tract together with a rise in blood glucose levels might cause a marked improvement in the performance of a maximal sprint effort. Therefore, the purpose of the current study was to investigate whether the combination of ingestion and mouth rinsing with a carbohydrate solution might improve maximal sprint performance, an important issue given that the effect of carbohydrate intake on sprint performance has never been tested before.
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Methods Participants Twelve competitive male cyclists (age: 30.9 ± 8.5 years, height: 182.6 ± 6.2 cm, mass: 79.7 ± 8.7 kg, peak rate of oxygen consumption, V·O2 peak: 57.8 ± 4.7 ml·kg-1·min-1, sum of eight skinfolds: 76.7 ± 32.1 mm) from cycling clubs in Western Australia were recruited for this study. Participants were informed of the testing procedures before their written informed consent was obtained. However, to minimize the possibility of a placebo effect, the participants were deceived about the true aims of the study and were initially told that the purpose of the study was to determine the effect of carbohydrate supplementation on blood pH and lactate responses to a maximal sprint effort. All participants were debriefed about the true purpose of the study after completing all trials. The procedures were approved by the human research ethics committee of The University of Western Australia.
Experimental Design Each participant attended the laboratory on five occasions, each separated by at least 1 week. The initial visit involved a familiarization session during which anthropometric data (height, body mass, and sum of eight skinfolds) and V·O2 peak were obtained. The V·O2 peak was determined using a graded exercise test performed to exhaustion on an air-braked front access cycle ergometer (Evolution Pty. Ltd., Adelaide), with the resistance to cycling increasing with the cycling rate as described previously (Fairchild et al., 2002). Following the V·O2 peak test, the participants were familiarized with the sprint protocol to be used in the experimental trials, with the seat height of the cycle ergometer adjusted to each participant’s preference and standardized for all testing sessions. A single familiarization trial has been reported to be sufficient to minimize learning effects in experienced cyclists (Hopkins et al., 2001). Before leaving the laboratory, the participants were provided with a logbook and asked to record their activity patterns for 48 hr before each experimental trial. Given that the ergogenic benefit of a carbohydrate mouth rinse may depend on nutritional status (Beelen et al., 2009), participants were also asked to record all food and drink intake in the log book, including the type of food, the amount consumed and the timing of the consumption for 48 hr before each trial. A copy of this information from the first trial was provided to each participant and they were asked to replicate this diet as closely as possible before each subsequent trial (Jeacocke & Burke, 2010). Compliance was confirmed upon arrival to the laboratory for each experimental trial both verbally and after inspection of food diaries by the investigator. Participants were also instructed to fast overnight (10 hr before each trial) and to avoid strenuous activity, alcohol, and caffeine in the 24-hr period preceding each trial. The four experimental trials were administered following a randomized, double-blind, counterbalanced design
and conducted in the morning after the overnight fast. Testing was conducted in the fasted state since the majority of previous research showing benefits of carbohydrate mouth rinsing has been conducted in this manner (Carter et al., 2004a; Chambers et al., 2009; Rollo et al., 2010). Each experimental trial was commenced with a 4-min light warm up at 40% of V·O2 peak on the cycle ergometer. This warm up was immediately followed by one practice start during which participants pedaled close to maximal intensity for 2 to 3 s followed by a 20-min rest. At the start of this 20-min rest period, each participant commenced the ingestion/ mouth rinsing protocol with either (a) 10% (w/v) glucose solution (Glucodin, Boots Healthcare, NSW, Australia), (b) 9.0% (w/v) maltodextrin solution (Mal; Polycose, Ross Laboratory, Columbus, OH) with energy content matching that of the glucose solution, (c) 0.05% (w/v) aspartame solution (Cadbury Schweppes, WA, Australia) or (d) water (Direct-Q 5 Ultrapure Water System, Millipore, MA, USA) as a tasteless and aroma-less control to mimic the somatosensory effects induced by the presence of a liquid in the mouth (de Araujo et al., 2003). Aspartame was used in the current study to match the sweetness of the 10% glucose solution (Schiffman et al., 1995) as it has been used as an artificial sweetener in previous studies (Chambers et al., 2009; Painelli et al., 2011; Pottier et al., 2010; Rollo et al., 2011). To double blind the participants and the investigator, all solutions were colorless with similar appearance. To ensure that the glucose-sensitive sensors affected by blood glucose level as well as those in the mouth and remainder of the upper gastrointestinal tract were stimulated by the carbohydrate solutions, the ingestion/mouth-rinsing protocol involved the ingestion of 100 ml of the assigned solution 20-min pre-sprint (based on pilot work from our laboratory showing that blood glucose levels reach maximal levels ~20 min after ingestion). Fifteen minutes after ingesting the solution, each participant was required to rinse their mouth with 15 ml of the same solution for 5 s before swallowing it and then repeating this procedure 10 more times at 30 s intervals over a 5-min period (165 ml of carbohydrate ingested in total and 55 s of mouth rinsing). A prolonged mouth rinsing protocol was adopted because of the evidence that longer duration mouth rinsing yields a greater improvement in exercise performance (Sinclair et al., 2013). An intermittent, rather than sustained, mouth rinsing protocol was adopted to avoid the discomfort and difficulty of performing uninterrupted prolonged mouth rinsing and to minimize maltodextrin breakdown by salivary amylase. Immediately after the last mouth rinse/ingestion, each participant performed a 45-s maximal sprint effort on the cycle ergometer. The ergometer was interfaced with a customized software program to measure flywheel velocity and power (Cyclemax, School of Sport Science, Exercise and Health, The University of Western Australia). This software allowed the measurement of peak power output (Ppeak), mean power output over 0 to 45 s (P0 to 45), 0 to 15 s (P0 to 15), 15 to 30 s (P15 to 30), and 30 to 45 s (P30 to 45) of the 45-s maximal sprint effort. The maximal sprint effort was initiated in a standing position, with the pre-
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ferred foot starting at the two o’clock position. Participants were instructed to cycle in an “all-out” manner for 45 s without pacing themselves. Capillary blood (125 μL) was sampled from a fingertip before the commencement of the ingestion/ rinsing protocol and at 0, 7, 15 and 30 min postexercise and analyzed for blood pH, lactate and glucose using an ABL 735 blood gas analyzer (Radiometer, Copenhagen, Denmark). At the same time-points, each participant gave a rating of perceived exertion (RPE; Borg, 1982) and indicated the intensity of nausea level on a 100-mm visual-analog scale anchored by the descriptors no nausea and extreme nausea at each end (Nausea Intensity Scale; NIS; Muth, Stern, Thayer, & Koch, 1996). The purpose of RPE and NIS was to determine if any of the solutions had an effect on the perception of exertion and the unpleasant feelings of nausea associated with a maximal sprint effort respectively. Since sweet taste is also known to induce a sensation of pleasure (Berridge, 2003), the participants were required to indicate their feelings of pleasure/ displeasure using the 10-point Likert Feeling Scale (FS; Hardy & Rejeski, 1989). Participants were also asked to rate the perceived sweetness of each solution on a scale of 1 to 10, with 1 indicating undetectable sweetness and 10 indicating maximal sweetness.
Statistical Analyses The effects of each solution on peak power and mean power during the 45-s sprint were compared using oneway repeated measures analysis of variance (ANOVA) followed by Bonferroni post hoc tests. All other variables
measured across time were compared using two-way repeated measure ANOVA (trial × time) followed by Bonferroni post hoc tests. All data were analyzed for normality and parametric statistics were applied accordingly. Statistical significance was set at p ≤ .05. All analyzes were performed using the Statistical Package for the Social Sciences Version 17.0 for Windows software (SPSS, Inc., Chicago, IL). Differences between trials were assessed using Cohen’s effect size (ES) statistic with modified descriptors (Hopkins, 2002a). To make inferences about the true value of an effect, the uncertainty in the effect was expressed as 90% confidence limits. All values, unless otherwise stated, are expressed as mean ± standard deviation (SD).
Results Sprint Cycling Performance Ingestion/rinsing of the glucose solution resulted in a significant improvement in Ppeak compared with the water trial (14.7 ± 10.6%; p = .031), the aspartame trial (9.2 ± 4.6%; p = .026) and the maltodextrin trial (16.0 ± 6.0%; p = .003; Table 1). There was no significant difference in Ppeak between aspartame, maltodextrin and water trials (p > .05, Table 1). With respect to P0 to 45, P0 to 15, P15 to 30 and P30 to 45, there was a strong trend for the glucose trial to be 8 to 14% higher compared with the water trial (p = .075, p = .058, p = .086, p = .103 over each time period, respectively). In contrast, P0 to 45, P0 to 15, P15 to 30 and P30 to 45 were similar between aspartame, maltodextrin and water trials (p > .05), with only small corresponding ES (Table 1).
Table 1 Peak Power (Ppeak) and Mean Power During a 45-s Maximal Sprint (P0–45) and Various Periods Within the Sprint (P0–15,15–30,30–45) Following Ingestion and Mouth-Rinsing With Glucose, Aspartame, Maltodextrin, or Water (n = 12; Mean ± SD)
Ppeak
P0–45
P0–15
P15–30
P30–45
Treatment water glucose aspartame maltodextrin water glucose aspartame maltodextrin water glucose aspartame maltodextrin water glucose aspartame maltodextrin water glucose aspartame maltodextrin
Power Output (Watts) 1036 ± 177 1188 ± 166 1088 ± 128 1024 ± 202 689 ± 159 768 ± 140 728 ± 110 687 ± 170 848 ± 175 968 ± 156 895 ± 111 843 ± 182 685 ± 49 759 ± 45 730 ± 38 680 ± 53 534 ± 43 578 ± 40 561 ± 37 537 ± 46
Comparison With Water Trial ±90% CL % ES; ±90% CL p value
Comparison with Glucose Trial ±90% CL % ES; ±90% CL p value
±131 ±89 ±93
14.7 5.0 -1.1
0.86; ±0.74 0.29; ±0.50 -0.07; ±0.52
.031* .160 .411
±83 ±86
9.2 16.0
0.78; ±0.64 0.81; ±0.42
.026† .003†
±91 ±60 ±70
11.4 5.6 -0.4
0.49; ±0.57 0.24; ±0.38 -0.02; ±0.44
.075 .136 .472
±56 ±57
5.5 11.8
0.36; ±0.50 0.48; ±0.34
.111 .013†
±127 ±85 ±90
14.2 5.6 -0.6
0.69; ±0.73 0.27; ±0.49 -0.03; ±0.52
.058 .167 .464
±75 ±73
8.2 14.8
0.65; ±0.68 0.69; ±0.40
.056 .005†
±91 ±60 ±71
10.9 6.7 -0.6
0.43; ±0.53 0.27; ±0.35 -0.03; ±0.41
.086 .099 .457
±56 ±60
4.0 11.6
0.22; ±0.43 0.42; ±0.32
.190 .019†
±58 ±43 ±51
8.1 4.9 0.4
0.29; ±0.39 0.18; ±0.29 0.02; ±0.34
.103 .150 .468
±43 ±42
3.0 7.6
0.13; ±0.34 0.25; ±0.26
.245 .053
*p < .05; indicates a significant difference from water trial †p < .05; indicates a significant difference from glucose trial €p < .05; indicates significant difference between glucose and maltodextrin vs. aspartame and water trials
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Of note, there was no effect of the order of trial administration on the Ppeak and P0 to 45 for the four different trials (p = .532 and p = .587, respectively) and the coefficient of variation for Ppeak and P0 to 45 between the familiarization and water trial were 4.6 and 4.0%, respectively.
Blood Metabolites and Perceptual Ratings Although blood pH, lactate and glucose concentrations changed significantly over time within trials in response to exercise (p < .001, Figure 1), no significant differences were observed in blood lactate concentrations and blood pH between trials (p > .05, Figure 1). On the other hand, there was a significant main effect of trial on blood glucose concentration (p < .001, Figure 1). Post hoc analyzes revealed a significantly higher blood glucose concentration in the glucose and maltodextrin trials compared with water at 0, 7, and 15 min postexercise (p < .01, Figure 1), with no significant differences between the glucose and maltodextrin trials at any of these time points (p > .05). RPE, NIS, and FS scores were similar between all trials (p > .05, Figure 1). In response to exercise, there was a significant main effect for time for RPE, NIS and FS (p < .001, Figure 1), with RPE and NIS scores increasing, and FS decreasing, to a similar extent between trials. RPE returned to preexercise levels within 15 min postexercise in all trials (p > .05, Figure 1). Likewise, NIS returned to baseline in the aspartame and water trials by 15 min postexercise, while NIS in the glucose and maltodextrin trials returned to baseline level within 30 min postexercise (p > .05, Figure 1). The FS scores for water returned to baseline 15 min after exercise, and after 30 min for glucose, aspartame and maltodextrin (p > .05, Figure 1). Sweetness taste scores did not differ between glucose (6 ± 1) and aspartame (7 ± 1; p > .05), both being significantly higher than maltodextrin (4 ± 1; p < .001) which in turn was higher than water (2 ± 1; p < .05).
Discussion This study shows for the first time that combining the ingestion of a 10% glucose solution with its use as a mouth rinse significantly improves peak power output during a 45-s maximal sprint cycling effort compared with water, aspartame or maltodextrin solutions. Since the ingestion/rinsing of isoenergetic maltodextrin or sweetness-matched aspartame had no effect on any of the indicators of sprint performance examined here, the ergogenic effect of glucose mouth rinsing/ingestion is unlikely to be mediated by changes in blood glucose levels, a placebo effect, sweetness or energy sensing mechanisms in the gastrointestinal tract. To the best of our knowledge, this is the first study to examine the effect of combining glucose ingestion and mouth rinse on sprint performance and to show that the ingestion/rinsing of a glucose solution improves peak power output. The glucose treatment significantly
improved Ppeak by 9 to 16% compared with the water, aspartame, and maltodextrin trials. Importantly, the greater Ppeak at the start of the sprint did not compromise performance in the latter stages of the sprint since the trend for mean power output (P0 to 45) was also favorable in the glucose trial compared with the water, aspartame and maltodextrin trials. Given that the smallest worthwhile enhancement for Ppeak and P0 to 45 were 1.4% and 1.2% respectively (calculated as 0.3 of the coefficient of variation in performance; Hopkins, 2002b), the improvement in Ppeak reported here is of sufficient magnitude to be considered meaningful to the competitive cyclists tested. However, it is important to stress that the research design of this study does not allow one to distinguish between the effect of the initial ingestion, mouth rinse, or subsequent ingestion on sprint performance. It also remains to be determined whether the ergogenic effect observed in the current study is attributable to the combined stimulus of ingestion plus repeated mouth-rinsing, or whether glucose ingestion alone would be sufficient to improve Ppeak since glucose ingestion in itself entails some mouth rinsing, albeit for a shorter duration than that tested here. This is an area for future research. The mechanisms underlying the ergogenic benefit of the glucose ingestion/rinsing protocol tested here on Ppeak are unlikely to involve a placebo effect. This is, in part, because the participants were deceived about the true purpose of the study by being initially informed that the primary aim was to determine the effect of different carbohydrate supplementations on the response of blood pH and lactate to a maximal sprint. As the participants were expected to perceive the difference in taste between some of the solutions (e.g., glucose and water solution), such a deception had to be performed to minimize the possibility of a placebo effect between treatments. In addition, the risk of a placebo effect was further reduced by the use of the artificial sweetener, aspartame, at a level reported by the participants in this study to match the sweetness of the glucose solution. Furthermore, the finding that Ppeak was significantly higher in the glucose trial compared with the aspartame trial, and that all performance variables in the aspartame and water trials were similar, suggests that there was no/minimal placebo effect in this study. Of note, as is the case with other research concerned with the effect of glucose ingestion or mouth rinsing on exercise performance (Beelen et al., 2009; Carter et al., 2004a; Chambers et al., 2009; Pottier et al., 2010; Rollo et al., 2010; Whitham & McKinney, 2007), the current study did not include a control condition where no solution was given as it would have increased the number of experimental trials to five and perhaps compromised participant compliance and more importantly increased the possibility of a “nocebo” effect, a condition that may potentially widen the performance gap between the control and intervention conditions (Benedetti et al., 2007). Finally, to further increase the reliability of our findings, a cross-over and double blinded design was adopted (Clark et al., 2000), competitive cyclists were recruited, and a familiarization trial was employed (Hopkins et al., 2001).
Figure 1 — Effect of ingesting and mouth-rinsing various solutions (glucose, aspartame, maltodextrin, and water) on the response of (A) blood lactate, (B) blood pH, (C) blood glucose, (D) perceived nausea, (E) feeling scale and (F) ratings of perceived exertion before and after a 45-s maximal sprint effort. * indicates significant difference from preexercise in all trials (p < .05); † indicates significant difference from preexercise in glucose, aspartame and maltodextrin trials (p < .05); ‡ indicates significant difference from preexercise in glucose and maltodextrin trials (p < .05). € indicates significant difference between glucose and maltodextrin vs. aspartame and water trials (p < .05). Values are expressed as mean ± SD (n = 12).
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The improved Ppeak in the glucose trial is unlikely to be mediated by the rise in blood glucose level alone and the effect that this might have on the central nervous system and skeletal muscle metabolism. This interpretation is based on the observation that Ppeak did not improve in the maltodextrin trial when compared with water despite a similar increase in blood glucose level to that observed in the glucose trial. It is noteworthy that the benefits of carbohydrate ingestion on intense aerobic exercise lasting 1 hr have also been reported to be independent of the gastrointestinal absorption of glucose and associated rise in blood glucose level (Carter et al., 2004b). The absence of significant differences between the maltodextrin and water trials also suggests that both the sweetness and energy content of maltodextrin are insufficient to result in different sprint performance between these two trials. If we accept that the performance benefits are mediated by rapid brain signaling, our interpretation is consistent with the observation that maltodextrin affects the brain in a manner different from glucose (Chambers et al., 2009). This latter interpretation is consistent with the observation that the maltodextrin used here (Polycose), which is known to have a mild sweet taste that is distinct from that of glucose (Feigin et al., 1987), affects the brain differently compared with glucose (Chambers et al., 2009). Indeed, glucose, but not maltodextrin, activates the dorsal regions of the anterior cingulate cortex, with this region of the brain believed to mediate the behavioral and autonomic responses to rewarding stimuli (Rolls, 2007). Whether these different brain responses explain the absence of any effect of maltodextrin on sprint performance remain to be determined. The benefits of combining glucose ingestion and rinsing on Ppeak raise the issue of the exact site along the gastrointestinal tract carrying the glucose sensing cells mediating the ergogenic effect of glucose. Although this study was not designed to address this issue, one may argue that if the similar rise in blood glucose between the maltodextrin and glucose trials were to reflect a comparable increase in glucose levels in the upper intestine, where most maltodextrose is normally broken down to glucose, this predicted similar increase in intestinal glucose levels would imply that the effect of glucose ingestion on Ppeak is not mediated by the sole activation of glucose sensing cells in the upper intestine since maltodextrin is without any significant effect on performance. Among the other gastrointestinal sites likely to be involved, the glucose sensing cells in the throat/esophagus as well as in the oral cavity are possible candidates. A role for the glucose sensing cells in the oral cavity is supported by recent findings of Beaven and colleagues (2013) which show that a 5-s glucose mouth rinse affects sprint performance, albeit to a far lesser extent than reported here. However, it is important to note that this interpretation is challenged by both the fact that their exercise protocol (Beaven et al., 2013) differed from ours and by recent findings that glucose mouth rinsing alone has no ergogenic effect on maximal sprint performance (Chong et al., 2011). Nevertheless, our findings do not exclude the possibility
that both the serial rinsing with glucose and the increased duration of the mouth rinse protocol adopted here compared with that in Chong and colleagues (2011) might have been sufficient for the glucose sensing cells in the oral cavity to play some role in mediating the ergogenic effect of our glucose mouth rinsing/ingestion protocol. If this were to be the case, our findings would also imply that the stimulation of glucose sensing cells in the oral cavity is not responsive to caloric signals alone, given the lack of effect of the isocaloric maltodextrin solution on performance. Irrespective of the location of the gastrointestinal glucose sensing cells mediating the effect of glucose on sprint performance, our results suggest that the improvement in Ppeak with the glucose solution does not involve aspartame-sensitive glucose sensing mechanisms. This is suggested by the absence of significant differences between the aspartame and water trials on any of the indicators of sprint performance examined here. Others examining the effect of glucose ingestion on endurance performance lasting 1 hr have also concluded that glucose improves performance via mechanisms insensitive to artificial sweeteners (Chambers et al., 2009; Jeukendrup et al., 1997), which is not surprising given that glucose and aspartame trigger different receptor signaling pathways in the gut (Margolskee et al., 2007) and initiate different brain responses regardless of sweetness (Chambers et al., 2009). One issue arising from our findings is the identity of the mechanisms whereby glucose sensing in the gastrointestinal tract exerts its effect on the brain. As suggested by others, glucose may improve endurance performance by altering the perception of effort during exercise which allows participants to exercise at a higher intensity (Carter et al., 2004a; Pottier et al., 2010). Evidence that this may also be the case in the current study is the observation that RPE did not differ between glucose and water trials despite the higher Ppeak in the glucose trial, thus suggesting that the perception of effort, motivation, and/or arousal level of the participants might have been enhanced in the glucose trial. In support of this view, glucose ingestion has been shown to facilitate corticomotor output to both fresh and fatigued muscles (Gant et al., 2010), and functional magnetic resonance imaging shows that glucose ingestion affects the brain activity (O’Doherty et al., 2001; Smeets et al., 2005) of the orbitofrontal cortex and amygdales, both areas being associated with motivation and reward (O’Doherty et al., 2001). However, it is important to stress that since these functional studies on the brain were performed under experimental conditions that differed from those adopted in the current study (e.g., different glucose dose and exposure time), the relevance of their findings to our results must be taken with caution.
Conclusion In conclusion, the current study shows for the first time that the combined ingestion and repeated mouth rins-
Glucose Ingestion and Sprint Cycling Performance 611
ing with a glucose solution significantly improves peak power during a maximal sprint effort. These findings are important as almost all of the studies performed to date on carbohydrate supplementation have focused on endurance exercise performance rather than sprint activities. For this reason, the ergogenic effects of glucose ingestion/rinsing on a range of short duration events should be further investigated to corroborate our findings and help further explore the mechanisms involved. Future research should also examine whether the present findings are influenced by the nutritional status of the individual (i.e., fed versus fasted), and also whether the benefits extend to related activities such as repeated sprint performance. Obviously, the protocol described here was not designed to be practical for athletes (e.g., repeated mouth rinsing), but rather to test the theoretical notion that there are conditions where carbohydrate can affect sprint performance. Therefore, future research should examine whether glucose ingestion alone or in combination with a shorter mouth-rinsing period would be just as beneficial in improving performance. Finally, our findings suggest that the mechanisms underlying the ergogenic effect of glucose ingestion/rinsing are unlikely to involve changes in blood glucose levels, but more likely to involve the stimulation of aspartame-insensitive glucose sensing cells in the gastrointestinal tract, including the oral cavity. However, the identity and location of these glucose sensors involved remain to be determined. Acknowledgment This study was undertaken while Edwin Chong was in receipt of the Endeavour International Postgraduate Research Scheme Scholarship funded by the Australian Government and Scholarship for International Research Fees from The University of Western Australia. The funding for this study came solely from The University of Western Australia. The authors would also like to acknowledge the support of Perth Integrated Cycling Group and thank the cyclists for their participation and enthusiasm.
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effect of carbohydrate mouth rinse on maximal strength and strength endurance. European Journal of Applied Physiology, 111, 2381–2386. PubMed doi:10.1007/ s00421-011-1865-8 Pottier, A., Bouckaert, J., Gilis, W., Roels, T., & Derave, W. (2010). Mouth rinse but not ingestion of a carbohydrate solution improves 1-h cycle time trial performance. Scandinavian Journal of Medicine & Science in Sports, 20, 105–111. PubMed doi:10.1111/j.1600-0838.2008.00868.x Rolls, E.T. (2007). Sensory processing in the brain related to the control of food intake. The Proceedings of the Nutrition Society, 66, 96–112. PubMed doi:10.1017/ S0029665107005332 Rollo, I., Cole, M., Miller, R., & Williams, C. (2010). Influence of mouth rinsing a carbohydrate solution on 1-h running performance. Medicine and Science in Sports and Exercise, 42, 798–804. PubMed doi:10.1249/ MSS.0b013e3181bac6e4 Rollo, I., Williams, C., & Nevill, M. (2011). Influence of ingesting versus mouth rinsing a carbohydrate solution during a 1-h run. Medicine and Science in Sports and Exercise, 43(3), 468–475. PubMed doi:10.1249/ MSS.0b013e3181f1cda3 Schiffman, S.S., Booth, B.J., Losee, M.L., Pecore, S.D., & Warwick, Z.S. (1995). Bitterness of sweeteners as a function of concentration. Brain Research Bulletin, 36(5), 505–513. PubMed doi:10.1016/0361-9230(94)00225-P Sinclair, J., Bottoms, L., Flynn, C., Bradley, E., Alexander, G., McCullagh, S., . . . Hurst, H.T. (2014). The effect of different durations of carbohydrate mouth rinse on cycling performance. European Journal of Sport Science. PubMed Smeets, P.A.M., de Graaf, C., Stafleu, A., van Osch, M.J.P., & van der Grond, J. (2005). Functional magnetic resonance imaging of human hypothalamic responses to sweet taste and calories. The American Journal of Clinical Nutrition, 82, 1011–1016. PubMed Tsintzas, O.K., Williams, C., Boobis, L., & Greenhaff, P. (1996). Carbohydrate ingestion and single muscle fiber glycogen metabolism during prolonged running in man. Journal of Applied Physiology, 81, 801–809. PubMed Whitham, M., & McKinney, J. (2007). Effect of a carbohydrate mouthwash on running time-trial performance. Journal of Sports Sciences, 25, 1385–1392. PubMed doi:10.1080/02640410601113676
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www.IJSNEM-Journal.com ORIGINAL RESEARCH
Combined Glucose Ingestion and Mouth Rinsing Improves Sprint Cycling Performance Edwin Chong, Kym J. Guelfi, and Paul A. Fournier This study investigated whether combined ingestion and mouth rinsing with a carbohydrate solution could improve maximal sprint cycling performance. Twelve competitive male cyclists ingested 100 ml of one of the following solutions 20 min before exercise in a randomized double-blinded counterbalanced order (a) 10% glucose solution, (b) 0.05% aspartame solution, (c) 9.0% maltodextrin solution, or (d) water as a control. Fifteen min after ingestion, repeated mouth rinsing was carried out with 11 × 15 ml bolus doses of the same solution at 30-s intervals. Each participant then performed a 45-s maximal sprint effort on a cycle ergometer. Peak power output was significantly higher in response to the glucose trial (1188 ± 166 W) compared with the water (1036 ± 177 W), aspartame (1088 ± 128 W) and maltodextrin (1024 ± 202W) trials by 14.7 ± 10.6, 9.2 ± 4.6 and 16.0 ± 6.0% respectively (p < .05). Mean power output during the sprint was significantly higher in the glucose trial compared with maltodextrin (p < .05) and also tended to be higher than the water trial (p = .075). Glucose and maltodextrin resulted in a similar increase in blood glucose, and the responses of blood lactate and pH to sprinting did not differ significantly between treatments (p > .05). These findings suggest that combining the ingestion of glucose with glucose mouth rinsing improves maximal sprint performance. This ergogenic effect is unlikely to be related to changes in blood glucose, sweetness, or energy sensing mechanisms in the gastrointestinal tract. Keywords: sprinting, carbohydrate, power output, mouthwash Carbohydrate ingestion before and during exercise can improve performance in prolonged aerobic events due, in part, to the maintenance of high rates of carbohydrate oxidation (Coyle et al., 1986), along with lower rates of muscle glycogen utilization to be spared for later use (Tsintzas et al., 1996). Carbohydrate ingestion has also been reported to be beneficial to intense aerobic events lasting 1 hr (Below et al., 1995; Jeukendrup et al., 1997), but not if carbohydrate is administered intravenously (Carter et al., 2004b), thus suggesting that the mechanism underlying the ergogenic effect of carbohydrate ingestion for intense aerobic exercise of short duration is independent of the gastrointestinal absorption and metabolism of carbohydrate, and may involve facilitation of the corticomotor output to both fresh and fatigued muscle (Gant et al., 2010). In support of this interpretation, rinsing the mouth with a carbohydrate solution without swallowing has been shown by most (Carter et al., 2004a; Chambers et al., 2009; Pottier et al., 2010; Rollo et al., 2010) but not all studies (Beelen et al., 2009; Whitham & McKinney, 2007) to improve 1-hr time trial performance, with the effect of stimulating other portions of the gastrointestinal tract still remaining to be tested. The authors are with the School of Sport Science, Exercise and Health, University of Western Australia, Crawley, Australia. Address author correspondence to Edwin Chong at [email protected].
Since sprint and endurance efforts affect muscle metabolism differently and have different mechanisms of fatigue, what still remains to be examined, is whether carbohydrate ingestion or mouth rinsing also improves maximal sprint performance. In this respect, Beaven and colleagues (2013) have recently shown that a single 5-second glucose mouth rinse without swallowing improves to a small extent mean power output (~39 W) in the initial sprint of a series of 5 × 6-s sprints compared with a noncaloric placebo, thus suggesting that carbohydrate receptors in the mouth mediate this effect. However, other findings show that a single 5-s carbohydrate mouth rinse does not affect more prolonged (30-s) maximal sprint cycling performance (Chong et al., 2011). Given that carbohydrate receptors are found in several portions of the gastrointestinal tract (e.g., oral cavity, esophagus, upper intestine; Bachmanov & Beauchamp, 2007; Bezencon et al., 2007), this raises the issue of whether the combined stimulation of the glucose receptors in the oral cavity and the remainder of the gastrointestinal tract together with a rise in blood glucose levels might cause a marked improvement in the performance of a maximal sprint effort. Therefore, the purpose of the current study was to investigate whether the combination of ingestion and mouth rinsing with a carbohydrate solution might improve maximal sprint performance, an important issue given that the effect of carbohydrate intake on sprint performance has never been tested before.
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Methods Participants Twelve competitive male cyclists (age: 30.9 ± 8.5 years, height: 182.6 ± 6.2 cm, mass: 79.7 ± 8.7 kg, peak rate of oxygen consumption, V·O2 peak: 57.8 ± 4.7 ml·kg-1·min-1, sum of eight skinfolds: 76.7 ± 32.1 mm) from cycling clubs in Western Australia were recruited for this study. Participants were informed of the testing procedures before their written informed consent was obtained. However, to minimize the possibility of a placebo effect, the participants were deceived about the true aims of the study and were initially told that the purpose of the study was to determine the effect of carbohydrate supplementation on blood pH and lactate responses to a maximal sprint effort. All participants were debriefed about the true purpose of the study after completing all trials. The procedures were approved by the human research ethics committee of The University of Western Australia.
Experimental Design Each participant attended the laboratory on five occasions, each separated by at least 1 week. The initial visit involved a familiarization session during which anthropometric data (height, body mass, and sum of eight skinfolds) and V·O2 peak were obtained. The V·O2 peak was determined using a graded exercise test performed to exhaustion on an air-braked front access cycle ergometer (Evolution Pty. Ltd., Adelaide), with the resistance to cycling increasing with the cycling rate as described previously (Fairchild et al., 2002). Following the V·O2 peak test, the participants were familiarized with the sprint protocol to be used in the experimental trials, with the seat height of the cycle ergometer adjusted to each participant’s preference and standardized for all testing sessions. A single familiarization trial has been reported to be sufficient to minimize learning effects in experienced cyclists (Hopkins et al., 2001). Before leaving the laboratory, the participants were provided with a logbook and asked to record their activity patterns for 48 hr before each experimental trial. Given that the ergogenic benefit of a carbohydrate mouth rinse may depend on nutritional status (Beelen et al., 2009), participants were also asked to record all food and drink intake in the log book, including the type of food, the amount consumed and the timing of the consumption for 48 hr before each trial. A copy of this information from the first trial was provided to each participant and they were asked to replicate this diet as closely as possible before each subsequent trial (Jeacocke & Burke, 2010). Compliance was confirmed upon arrival to the laboratory for each experimental trial both verbally and after inspection of food diaries by the investigator. Participants were also instructed to fast overnight (10 hr before each trial) and to avoid strenuous activity, alcohol, and caffeine in the 24-hr period preceding each trial. The four experimental trials were administered following a randomized, double-blind, counterbalanced design
and conducted in the morning after the overnight fast. Testing was conducted in the fasted state since the majority of previous research showing benefits of carbohydrate mouth rinsing has been conducted in this manner (Carter et al., 2004a; Chambers et al., 2009; Rollo et al., 2010). Each experimental trial was commenced with a 4-min light warm up at 40% of V·O2 peak on the cycle ergometer. This warm up was immediately followed by one practice start during which participants pedaled close to maximal intensity for 2 to 3 s followed by a 20-min rest. At the start of this 20-min rest period, each participant commenced the ingestion/ mouth rinsing protocol with either (a) 10% (w/v) glucose solution (Glucodin, Boots Healthcare, NSW, Australia), (b) 9.0% (w/v) maltodextrin solution (Mal; Polycose, Ross Laboratory, Columbus, OH) with energy content matching that of the glucose solution, (c) 0.05% (w/v) aspartame solution (Cadbury Schweppes, WA, Australia) or (d) water (Direct-Q 5 Ultrapure Water System, Millipore, MA, USA) as a tasteless and aroma-less control to mimic the somatosensory effects induced by the presence of a liquid in the mouth (de Araujo et al., 2003). Aspartame was used in the current study to match the sweetness of the 10% glucose solution (Schiffman et al., 1995) as it has been used as an artificial sweetener in previous studies (Chambers et al., 2009; Painelli et al., 2011; Pottier et al., 2010; Rollo et al., 2011). To double blind the participants and the investigator, all solutions were colorless with similar appearance. To ensure that the glucose-sensitive sensors affected by blood glucose level as well as those in the mouth and remainder of the upper gastrointestinal tract were stimulated by the carbohydrate solutions, the ingestion/mouth-rinsing protocol involved the ingestion of 100 ml of the assigned solution 20-min pre-sprint (based on pilot work from our laboratory showing that blood glucose levels reach maximal levels ~20 min after ingestion). Fifteen minutes after ingesting the solution, each participant was required to rinse their mouth with 15 ml of the same solution for 5 s before swallowing it and then repeating this procedure 10 more times at 30 s intervals over a 5-min period (165 ml of carbohydrate ingested in total and 55 s of mouth rinsing). A prolonged mouth rinsing protocol was adopted because of the evidence that longer duration mouth rinsing yields a greater improvement in exercise performance (Sinclair et al., 2013). An intermittent, rather than sustained, mouth rinsing protocol was adopted to avoid the discomfort and difficulty of performing uninterrupted prolonged mouth rinsing and to minimize maltodextrin breakdown by salivary amylase. Immediately after the last mouth rinse/ingestion, each participant performed a 45-s maximal sprint effort on the cycle ergometer. The ergometer was interfaced with a customized software program to measure flywheel velocity and power (Cyclemax, School of Sport Science, Exercise and Health, The University of Western Australia). This software allowed the measurement of peak power output (Ppeak), mean power output over 0 to 45 s (P0 to 45), 0 to 15 s (P0 to 15), 15 to 30 s (P15 to 30), and 30 to 45 s (P30 to 45) of the 45-s maximal sprint effort. The maximal sprint effort was initiated in a standing position, with the pre-
Glucose Ingestion and Sprint Cycling Performance 607
ferred foot starting at the two o’clock position. Participants were instructed to cycle in an “all-out” manner for 45 s without pacing themselves. Capillary blood (125 μL) was sampled from a fingertip before the commencement of the ingestion/ rinsing protocol and at 0, 7, 15 and 30 min postexercise and analyzed for blood pH, lactate and glucose using an ABL 735 blood gas analyzer (Radiometer, Copenhagen, Denmark). At the same time-points, each participant gave a rating of perceived exertion (RPE; Borg, 1982) and indicated the intensity of nausea level on a 100-mm visual-analog scale anchored by the descriptors no nausea and extreme nausea at each end (Nausea Intensity Scale; NIS; Muth, Stern, Thayer, & Koch, 1996). The purpose of RPE and NIS was to determine if any of the solutions had an effect on the perception of exertion and the unpleasant feelings of nausea associated with a maximal sprint effort respectively. Since sweet taste is also known to induce a sensation of pleasure (Berridge, 2003), the participants were required to indicate their feelings of pleasure/ displeasure using the 10-point Likert Feeling Scale (FS; Hardy & Rejeski, 1989). Participants were also asked to rate the perceived sweetness of each solution on a scale of 1 to 10, with 1 indicating undetectable sweetness and 10 indicating maximal sweetness.
Statistical Analyses The effects of each solution on peak power and mean power during the 45-s sprint were compared using oneway repeated measures analysis of variance (ANOVA) followed by Bonferroni post hoc tests. All other variables
measured across time were compared using two-way repeated measure ANOVA (trial × time) followed by Bonferroni post hoc tests. All data were analyzed for normality and parametric statistics were applied accordingly. Statistical significance was set at p ≤ .05. All analyzes were performed using the Statistical Package for the Social Sciences Version 17.0 for Windows software (SPSS, Inc., Chicago, IL). Differences between trials were assessed using Cohen’s effect size (ES) statistic with modified descriptors (Hopkins, 2002a). To make inferences about the true value of an effect, the uncertainty in the effect was expressed as 90% confidence limits. All values, unless otherwise stated, are expressed as mean ± standard deviation (SD).
Results Sprint Cycling Performance Ingestion/rinsing of the glucose solution resulted in a significant improvement in Ppeak compared with the water trial (14.7 ± 10.6%; p = .031), the aspartame trial (9.2 ± 4.6%; p = .026) and the maltodextrin trial (16.0 ± 6.0%; p = .003; Table 1). There was no significant difference in Ppeak between aspartame, maltodextrin and water trials (p > .05, Table 1). With respect to P0 to 45, P0 to 15, P15 to 30 and P30 to 45, there was a strong trend for the glucose trial to be 8 to 14% higher compared with the water trial (p = .075, p = .058, p = .086, p = .103 over each time period, respectively). In contrast, P0 to 45, P0 to 15, P15 to 30 and P30 to 45 were similar between aspartame, maltodextrin and water trials (p > .05), with only small corresponding ES (Table 1).
Table 1 Peak Power (Ppeak) and Mean Power During a 45-s Maximal Sprint (P0–45) and Various Periods Within the Sprint (P0–15,15–30,30–45) Following Ingestion and Mouth-Rinsing With Glucose, Aspartame, Maltodextrin, or Water (n = 12; Mean ± SD)
Ppeak
P0–45
P0–15
P15–30
P30–45
Treatment water glucose aspartame maltodextrin water glucose aspartame maltodextrin water glucose aspartame maltodextrin water glucose aspartame maltodextrin water glucose aspartame maltodextrin
Power Output (Watts) 1036 ± 177 1188 ± 166 1088 ± 128 1024 ± 202 689 ± 159 768 ± 140 728 ± 110 687 ± 170 848 ± 175 968 ± 156 895 ± 111 843 ± 182 685 ± 49 759 ± 45 730 ± 38 680 ± 53 534 ± 43 578 ± 40 561 ± 37 537 ± 46
Comparison With Water Trial ±90% CL % ES; ±90% CL p value
Comparison with Glucose Trial ±90% CL % ES; ±90% CL p value
±131 ±89 ±93
14.7 5.0 -1.1
0.86; ±0.74 0.29; ±0.50 -0.07; ±0.52
.031* .160 .411
±83 ±86
9.2 16.0
0.78; ±0.64 0.81; ±0.42
.026† .003†
±91 ±60 ±70
11.4 5.6 -0.4
0.49; ±0.57 0.24; ±0.38 -0.02; ±0.44
.075 .136 .472
±56 ±57
5.5 11.8
0.36; ±0.50 0.48; ±0.34
.111 .013†
±127 ±85 ±90
14.2 5.6 -0.6
0.69; ±0.73 0.27; ±0.49 -0.03; ±0.52
.058 .167 .464
±75 ±73
8.2 14.8
0.65; ±0.68 0.69; ±0.40
.056 .005†
±91 ±60 ±71
10.9 6.7 -0.6
0.43; ±0.53 0.27; ±0.35 -0.03; ±0.41
.086 .099 .457
±56 ±60
4.0 11.6
0.22; ±0.43 0.42; ±0.32
.190 .019†
±58 ±43 ±51
8.1 4.9 0.4
0.29; ±0.39 0.18; ±0.29 0.02; ±0.34
.103 .150 .468
±43 ±42
3.0 7.6
0.13; ±0.34 0.25; ±0.26
.245 .053
*p < .05; indicates a significant difference from water trial †p < .05; indicates a significant difference from glucose trial €p < .05; indicates significant difference between glucose and maltodextrin vs. aspartame and water trials
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Of note, there was no effect of the order of trial administration on the Ppeak and P0 to 45 for the four different trials (p = .532 and p = .587, respectively) and the coefficient of variation for Ppeak and P0 to 45 between the familiarization and water trial were 4.6 and 4.0%, respectively.
Blood Metabolites and Perceptual Ratings Although blood pH, lactate and glucose concentrations changed significantly over time within trials in response to exercise (p < .001, Figure 1), no significant differences were observed in blood lactate concentrations and blood pH between trials (p > .05, Figure 1). On the other hand, there was a significant main effect of trial on blood glucose concentration (p < .001, Figure 1). Post hoc analyzes revealed a significantly higher blood glucose concentration in the glucose and maltodextrin trials compared with water at 0, 7, and 15 min postexercise (p < .01, Figure 1), with no significant differences between the glucose and maltodextrin trials at any of these time points (p > .05). RPE, NIS, and FS scores were similar between all trials (p > .05, Figure 1). In response to exercise, there was a significant main effect for time for RPE, NIS and FS (p < .001, Figure 1), with RPE and NIS scores increasing, and FS decreasing, to a similar extent between trials. RPE returned to preexercise levels within 15 min postexercise in all trials (p > .05, Figure 1). Likewise, NIS returned to baseline in the aspartame and water trials by 15 min postexercise, while NIS in the glucose and maltodextrin trials returned to baseline level within 30 min postexercise (p > .05, Figure 1). The FS scores for water returned to baseline 15 min after exercise, and after 30 min for glucose, aspartame and maltodextrin (p > .05, Figure 1). Sweetness taste scores did not differ between glucose (6 ± 1) and aspartame (7 ± 1; p > .05), both being significantly higher than maltodextrin (4 ± 1; p < .001) which in turn was higher than water (2 ± 1; p < .05).
Discussion This study shows for the first time that combining the ingestion of a 10% glucose solution with its use as a mouth rinse significantly improves peak power output during a 45-s maximal sprint cycling effort compared with water, aspartame or maltodextrin solutions. Since the ingestion/rinsing of isoenergetic maltodextrin or sweetness-matched aspartame had no effect on any of the indicators of sprint performance examined here, the ergogenic effect of glucose mouth rinsing/ingestion is unlikely to be mediated by changes in blood glucose levels, a placebo effect, sweetness or energy sensing mechanisms in the gastrointestinal tract. To the best of our knowledge, this is the first study to examine the effect of combining glucose ingestion and mouth rinse on sprint performance and to show that the ingestion/rinsing of a glucose solution improves peak power output. The glucose treatment significantly
improved Ppeak by 9 to 16% compared with the water, aspartame, and maltodextrin trials. Importantly, the greater Ppeak at the start of the sprint did not compromise performance in the latter stages of the sprint since the trend for mean power output (P0 to 45) was also favorable in the glucose trial compared with the water, aspartame and maltodextrin trials. Given that the smallest worthwhile enhancement for Ppeak and P0 to 45 were 1.4% and 1.2% respectively (calculated as 0.3 of the coefficient of variation in performance; Hopkins, 2002b), the improvement in Ppeak reported here is of sufficient magnitude to be considered meaningful to the competitive cyclists tested. However, it is important to stress that the research design of this study does not allow one to distinguish between the effect of the initial ingestion, mouth rinse, or subsequent ingestion on sprint performance. It also remains to be determined whether the ergogenic effect observed in the current study is attributable to the combined stimulus of ingestion plus repeated mouth-rinsing, or whether glucose ingestion alone would be sufficient to improve Ppeak since glucose ingestion in itself entails some mouth rinsing, albeit for a shorter duration than that tested here. This is an area for future research. The mechanisms underlying the ergogenic benefit of the glucose ingestion/rinsing protocol tested here on Ppeak are unlikely to involve a placebo effect. This is, in part, because the participants were deceived about the true purpose of the study by being initially informed that the primary aim was to determine the effect of different carbohydrate supplementations on the response of blood pH and lactate to a maximal sprint. As the participants were expected to perceive the difference in taste between some of the solutions (e.g., glucose and water solution), such a deception had to be performed to minimize the possibility of a placebo effect between treatments. In addition, the risk of a placebo effect was further reduced by the use of the artificial sweetener, aspartame, at a level reported by the participants in this study to match the sweetness of the glucose solution. Furthermore, the finding that Ppeak was significantly higher in the glucose trial compared with the aspartame trial, and that all performance variables in the aspartame and water trials were similar, suggests that there was no/minimal placebo effect in this study. Of note, as is the case with other research concerned with the effect of glucose ingestion or mouth rinsing on exercise performance (Beelen et al., 2009; Carter et al., 2004a; Chambers et al., 2009; Pottier et al., 2010; Rollo et al., 2010; Whitham & McKinney, 2007), the current study did not include a control condition where no solution was given as it would have increased the number of experimental trials to five and perhaps compromised participant compliance and more importantly increased the possibility of a “nocebo” effect, a condition that may potentially widen the performance gap between the control and intervention conditions (Benedetti et al., 2007). Finally, to further increase the reliability of our findings, a cross-over and double blinded design was adopted (Clark et al., 2000), competitive cyclists were recruited, and a familiarization trial was employed (Hopkins et al., 2001).
Figure 1 — Effect of ingesting and mouth-rinsing various solutions (glucose, aspartame, maltodextrin, and water) on the response of (A) blood lactate, (B) blood pH, (C) blood glucose, (D) perceived nausea, (E) feeling scale and (F) ratings of perceived exertion before and after a 45-s maximal sprint effort. * indicates significant difference from preexercise in all trials (p < .05); † indicates significant difference from preexercise in glucose, aspartame and maltodextrin trials (p < .05); ‡ indicates significant difference from preexercise in glucose and maltodextrin trials (p < .05). € indicates significant difference between glucose and maltodextrin vs. aspartame and water trials (p < .05). Values are expressed as mean ± SD (n = 12).
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The improved Ppeak in the glucose trial is unlikely to be mediated by the rise in blood glucose level alone and the effect that this might have on the central nervous system and skeletal muscle metabolism. This interpretation is based on the observation that Ppeak did not improve in the maltodextrin trial when compared with water despite a similar increase in blood glucose level to that observed in the glucose trial. It is noteworthy that the benefits of carbohydrate ingestion on intense aerobic exercise lasting 1 hr have also been reported to be independent of the gastrointestinal absorption of glucose and associated rise in blood glucose level (Carter et al., 2004b). The absence of significant differences between the maltodextrin and water trials also suggests that both the sweetness and energy content of maltodextrin are insufficient to result in different sprint performance between these two trials. If we accept that the performance benefits are mediated by rapid brain signaling, our interpretation is consistent with the observation that maltodextrin affects the brain in a manner different from glucose (Chambers et al., 2009). This latter interpretation is consistent with the observation that the maltodextrin used here (Polycose), which is known to have a mild sweet taste that is distinct from that of glucose (Feigin et al., 1987), affects the brain differently compared with glucose (Chambers et al., 2009). Indeed, glucose, but not maltodextrin, activates the dorsal regions of the anterior cingulate cortex, with this region of the brain believed to mediate the behavioral and autonomic responses to rewarding stimuli (Rolls, 2007). Whether these different brain responses explain the absence of any effect of maltodextrin on sprint performance remain to be determined. The benefits of combining glucose ingestion and rinsing on Ppeak raise the issue of the exact site along the gastrointestinal tract carrying the glucose sensing cells mediating the ergogenic effect of glucose. Although this study was not designed to address this issue, one may argue that if the similar rise in blood glucose between the maltodextrin and glucose trials were to reflect a comparable increase in glucose levels in the upper intestine, where most maltodextrose is normally broken down to glucose, this predicted similar increase in intestinal glucose levels would imply that the effect of glucose ingestion on Ppeak is not mediated by the sole activation of glucose sensing cells in the upper intestine since maltodextrin is without any significant effect on performance. Among the other gastrointestinal sites likely to be involved, the glucose sensing cells in the throat/esophagus as well as in the oral cavity are possible candidates. A role for the glucose sensing cells in the oral cavity is supported by recent findings of Beaven and colleagues (2013) which show that a 5-s glucose mouth rinse affects sprint performance, albeit to a far lesser extent than reported here. However, it is important to note that this interpretation is challenged by both the fact that their exercise protocol (Beaven et al., 2013) differed from ours and by recent findings that glucose mouth rinsing alone has no ergogenic effect on maximal sprint performance (Chong et al., 2011). Nevertheless, our findings do not exclude the possibility
that both the serial rinsing with glucose and the increased duration of the mouth rinse protocol adopted here compared with that in Chong and colleagues (2011) might have been sufficient for the glucose sensing cells in the oral cavity to play some role in mediating the ergogenic effect of our glucose mouth rinsing/ingestion protocol. If this were to be the case, our findings would also imply that the stimulation of glucose sensing cells in the oral cavity is not responsive to caloric signals alone, given the lack of effect of the isocaloric maltodextrin solution on performance. Irrespective of the location of the gastrointestinal glucose sensing cells mediating the effect of glucose on sprint performance, our results suggest that the improvement in Ppeak with the glucose solution does not involve aspartame-sensitive glucose sensing mechanisms. This is suggested by the absence of significant differences between the aspartame and water trials on any of the indicators of sprint performance examined here. Others examining the effect of glucose ingestion on endurance performance lasting 1 hr have also concluded that glucose improves performance via mechanisms insensitive to artificial sweeteners (Chambers et al., 2009; Jeukendrup et al., 1997), which is not surprising given that glucose and aspartame trigger different receptor signaling pathways in the gut (Margolskee et al., 2007) and initiate different brain responses regardless of sweetness (Chambers et al., 2009). One issue arising from our findings is the identity of the mechanisms whereby glucose sensing in the gastrointestinal tract exerts its effect on the brain. As suggested by others, glucose may improve endurance performance by altering the perception of effort during exercise which allows participants to exercise at a higher intensity (Carter et al., 2004a; Pottier et al., 2010). Evidence that this may also be the case in the current study is the observation that RPE did not differ between glucose and water trials despite the higher Ppeak in the glucose trial, thus suggesting that the perception of effort, motivation, and/or arousal level of the participants might have been enhanced in the glucose trial. In support of this view, glucose ingestion has been shown to facilitate corticomotor output to both fresh and fatigued muscles (Gant et al., 2010), and functional magnetic resonance imaging shows that glucose ingestion affects the brain activity (O’Doherty et al., 2001; Smeets et al., 2005) of the orbitofrontal cortex and amygdales, both areas being associated with motivation and reward (O’Doherty et al., 2001). However, it is important to stress that since these functional studies on the brain were performed under experimental conditions that differed from those adopted in the current study (e.g., different glucose dose and exposure time), the relevance of their findings to our results must be taken with caution.
Conclusion In conclusion, the current study shows for the first time that the combined ingestion and repeated mouth rins-
Glucose Ingestion and Sprint Cycling Performance 611
ing with a glucose solution significantly improves peak power during a maximal sprint effort. These findings are important as almost all of the studies performed to date on carbohydrate supplementation have focused on endurance exercise performance rather than sprint activities. For this reason, the ergogenic effects of glucose ingestion/rinsing on a range of short duration events should be further investigated to corroborate our findings and help further explore the mechanisms involved. Future research should also examine whether the present findings are influenced by the nutritional status of the individual (i.e., fed versus fasted), and also whether the benefits extend to related activities such as repeated sprint performance. Obviously, the protocol described here was not designed to be practical for athletes (e.g., repeated mouth rinsing), but rather to test the theoretical notion that there are conditions where carbohydrate can affect sprint performance. Therefore, future research should examine whether glucose ingestion alone or in combination with a shorter mouth-rinsing period would be just as beneficial in improving performance. Finally, our findings suggest that the mechanisms underlying the ergogenic effect of glucose ingestion/rinsing are unlikely to involve changes in blood glucose levels, but more likely to involve the stimulation of aspartame-insensitive glucose sensing cells in the gastrointestinal tract, including the oral cavity. However, the identity and location of these glucose sensors involved remain to be determined. Acknowledgment This study was undertaken while Edwin Chong was in receipt of the Endeavour International Postgraduate Research Scheme Scholarship funded by the Australian Government and Scholarship for International Research Fees from The University of Western Australia. The funding for this study came solely from The University of Western Australia. The authors would also like to acknowledge the support of Perth Integrated Cycling Group and thank the cyclists for their participation and enthusiasm.
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