Carbohydrate (CHO) is the primary substrate for high-intensity exercise (1), and CHO intake has a positive effect on endurance performance (2). The impact of CHO supplementation on metabolism, and subsequent performance, is multifactorial and depends on length and intensity of exercise, CHO intake rate, subject training status and even the type of exercise (3,4). Glycogen stores can become limiting during high-intensity/prolonged duration exercise (≥2 h), and the current consensus statement recommendations are for the consumption of ∼30 to 60 g CHO·h−1 (of ∼4%–8% CHO solution) during an endurance event (5). Importantly, it has been consistently shown that 30% to 50% of endurance athletes experience some level of gastrointestinal (GI) issues during endurance exercise (6). Additionally, GI problems have been shown to be increased and more likely to occur in athletes with a history of GI problems (7), with CHO intake itself (8) and as exercise time increases (9). Therefore, prolonged and intense endurance exercise with CHO intake provide the ideal context to induce not only an ergogenic effect, but also the potential conditions for GI upset.
It is proposed that CHO supplementation during exercise contributes to performance in several ways: 1) direct contribution of CHO energy via exogenous CHO oxidation and/or 2) mental/cognitive stimulation of the central nervous system probably due to a stimulation of the pleasure and reward centers of the brain by CHO exposure to the oral cavity (for review, see Stellingwerff and Cox (2)). Carter et al. (10) were the first to demonstrate that 1-h time trial (TT) performance could be improved by 2.9% by simply rinsing the oral cavity with a CHO solution and no ingestion. This study was mechanistically supported several years later by functional magnetic resonance imaging data showing that CHO mouth rinse stimulates the pleasure and reward centers of the brain (11). Since this work, there have been a multitude of CHO mouthwash studies (>15), with most studies (>10) demonstrating that CHO mouth rinse can improve performance (∼1 h), especially when subjects are in the fasted state (for review, see Jeukendrup (12)). During prolonged exercise (>2 h), with or without CHO supplementation, CHO depletion will occur, with more muscle glycogen depletion occurring in favor of preserving hepatic glucose levels (13). However, the amount of muscle versus liver glycogen depletion is complex and depends on the exercise intensity and exogenous CHO intake rates. Accordingly, Ataide-Silva et al. (14) demonstrated the largest performance benefits of CHO mouthwash when subjects were both muscle and liver glycogen depleted, compared with fed or just liver glycogen depleted (via an overnight fast). During performance with depleted glycogen, there is evidence that muscle activation and power changes reflect a fatigued state more so than during nondepleted conditions (14,15). Taken together, this suggests that CHO mouth rinse may improve performance by modifying the perception and/or motor response to fatigue, which may be augmented with low endogenous CHO availability (liver and muscle glycogen), and thus allow athletes to work at higher-power outputs compared with placebo (PLA) conditions. However, CHO mouth rinsing to help late endurance performance (>2 h) is in its infancy, and to our knowledge, only one study has examined the effect CHO mouth rinse on late endurance performance with combined measures of muscle activity and performance (16).
To properly evaluate the effectiveness of CHO mouth rinse on performance, it is necessary to replicate the nutritional conditions before and during actual endurance race scenarios. To our knowledge, no study to date has looked at the effect of a CHO mouth rinse during late endurance exercise in conditions of high ecological validity. These conditions would include a typical CHO-rich standardized precompetition snack and CHO intake (FED) during prolonged endurance exercise leading into an intense TT (similar to the last 30 min of a marathon or cycling race when potential GI upset will be the greatest). Therefore, this study aims to determine if CHO mouth rinse during an approximately ∼30 min of cycling TT, after prolonged exercise (2 h), will enhance performance as compared with PLA (artificial sweetener). We hypothesized that CHO mouth rinse will improve power output and TT performance compared with PLA.
Ten well-trained male cyclists (age, 29 ± 9 yr; body mass, 79.1 ± 9.9 kg; stature, 179.9 ± 8.5 cm; maximal oxygen uptake [V˙O2max], 64.5 ± 6.5 mL·kg−1·min−1; maximal power output [Wmax], 405 ± 38 W) were recruited and gave verbal and written consent under Human Ethics at the University of Victoria. All subjects were regional to national level category 1 or 2 with Cycling British Columbia.
General experimental design
This study featured a double-blind, randomized, PLA-controlled crossover design, in which subjects made four visits to the laboratory (21.9°C ± 0.5°C, 34.1% ± 10.4% relative humidity) with all cycling testing carried out using the same electronically braked ergometer (Velotron Pro, RacerMate Inc, Seattle, WA). During visit 1, the subject performed a graded test to exhaustion to determine their Wmax and V˙O2max. During visit 2, the subject performed an augmented practice session to familiarize the experimental protocol and the TT and to confirm the approximately 60% V˙O2max power output (208 ± 20 W) achieved during the 1-h steady-state (SS) session. Visits 3 and 4 were experimental trials, featuring 2 h at 60% V˙O2max followed by a TT in which a set amount of work (total work = 0.75 Wmax × 1800 s = 552 ± 48 kJ) (17) had to be performed as quick as possible over approximately 30 min (Fig. 1).
Body mass was measured using a standard scale (Avery Berkel HL120; Avery Weigh-Tronix Inc, Fairmont, MN) before the start of SS and upon completion of TT, where percent body mass loss was calculated as the difference between starting mass and mass upon completion of TT. Subjects received 1500 mL of liquid during each session and were not allowed water ad libitum. All urine samples were measured for urine specific gravity (USG) using an Atago PAL-10S Refractometer (Atago, Bellevue, WA).
The two randomized experimental interventions, all following a standardized CHO pretrial snack 2 h prior (details below), were as follows:
- 30 g CHO · h−1 during SS + CHO wash during TT (FEDWASH)
- 30 g CHO · h−1 during SS + PLA wash during TT (FEDPLA)
During the SS and TT, HR (Polar T31, Polar Electro) and RPE (Borgscale(6–20)) were taken according to the schedule outlined in Figure 1. During the SS ventilation, oxygen uptake (V˙O2) and carbon dioxide production were recorded (TrueOne 2400 metabolic cart; Parvo Medics, Sandy, UT) according to the schedule outlined in Figure 1. Using indirect calorimetry calculations, energy expenditure, fat, and CHO oxidation rates during the SS were estimated. Blood glucose (Contour®; Next EZ, Parsippany, NJ) and lactate (Lactate Pro, Akray, Japan) were taken during the SS and immediately after the TT according to the schedule outlined in Figure 1. Upon laboratory arrival, each participant was prepared for EMG electrode placement by shaving hair and cleaning the skin with 70% alcohol. EMG electrodes (Delsys Trigno, Natick, MA) were then placed on anatomical landmarked sites over the soleus (SOL), medial gastrocnemius (MG), tibialis anterior, vastus lateralis, rectus femoris, vastus medialis, biceps femoris, and gluteus maximus (18). All measurements for electrode placement were recorded so that on subsequent trials all electrodes could be positioned in the same place. EMG was recorded at 2000 Hz through a 16-bit analog/digital converter (NI 6034E; National Instruments, Austin, TX). EMG signals resolved into EMG intensities using 10 wavelets (19), which act as a band-pass filter (11–432 Hz). The median frequency (MDF) and total intensity of the EMG signal was compared between conditions (20 pedal revolutions) and normalized to the first hour of the SS for each individual muscle. Total EMG intensity is a positive envelope quantifying the EMG and is equivalent to twice the square of the root-mean-square of the EMG (19). Urine samples were collected before the start of SS. The experimental trials were separated by at least 7 d, and each subject conducted their sessions at the same time (±1 h) to minimize the effect of diurnal variation.
Maximal workload capacity
After a 5-min warm-up at 150 W, the test started at a workload of 180 W and was then increased by 30 W every 3 min until the subject could no longer maintain the required power output. Ventilation, V˙O2, and carbon dioxide production were recorded continuously (TrueOne 2400 metabolic cart; Parvo Medics). V˙O2max was determined by the largest 60 s mean V˙O2 value. Wmax was determined by: Wmax = Wout + ((t/180) × 0.30), Wout is the workload of the last completed stage, and t is the time in seconds spent in the final stage (17). Heart rate, RPE, and lactate were collected/recorded at the completion of each workload.
Preexperimental testing and preparation
Participants were instructed to maintain regular training for the duration of the study and to abstain from any intense exercise, caffeine and alcohol in the previous 24 h before each session. Participants recorded their food and liquid consumption for 24 h before each experimental trial and repeated these diets for each subsequent trial. All subjects received a standardized pretrial snack that was to be consumed 2 h before the start of the experimental sessions (158 g CHO, 18 g protein), to be in line with current position stand recommendations on preevent fueling (2.1 ± 0.3 g·kg−1 BM CHO, 0.24 ± 0.03 g·kg−1 BM protein) (5). Subjects were instructed to collect first urine void of the morning.
There was no verbal encouragement and subjects were void of any distractions by being situated behind a barrier. The only visual feedback the subjects received was the percent work complete. At every 20% of work completed, the subjects reported RPE and then received either CHO or PLA mouth rinse for 5 to 10 s, before expelling the entire amount (Fig. 1), whereas HR was recorded continuously. Lactate and glucose were measured immediately after the TT. After each trial, subjects were asked to rate how they felt during the SS and TT portions on a scale of 1 to 5 (1, horrible to 5, amazing) and were also probed as to whether they could identify which trial/solution they received to ensure blinding. After the last experimental session, subjects were asked 2 additional questions: 1) Which session do you think you had the best performance? 2) In the future would you self-select a CHO mouth rinse during a TT or during the last 30 min of a road race? Urine sample was collected after completion of the TT and before any additional liquids were consumed.
Mouth rinse protocol/SS solution
Each sample consisted of 25 mL of either 8% maltodextrin (GLOBE® 10 DE Maltodextrin; Ingredion, Canada) solution or taste matched PLA which was weighed before and after mouth rinsing (Mettler PC 400, Columbus, OH) to account for any solution that may have been ingested. Both the WASH and PLA solutions (distilled water) were supplemented with 0.2% artificial sweetener (Crystal Light, Kraft Food, USA) to make the solutions indistinguishable. The mouth rinse solutions were coded by a nonaffiliated researcher to ensure double-blinding. During SS, subjects were given 30 g CHO·h−1 in a 4% custom CHO solution (0.32 g per 100 mL Sucrose, 1.09 g per 100 mL maltodextrin, 0.2 g per 100 mL artificial sweetener, distilled water). This was equivalent to 1500 mL fluid during the 2-h SS, and 250 mL was consumed every 20 min (after removal of metabolic mouthpiece) (Fig. 1).
For measures of time to completion and power, a two-way ANOVA with repeated measures was used to examine main effects of 1) trial (FEDWASH, FEDPLA), 2) percentage of TT completed (time), and 3) trial–time interactions. Additionally, a priori planned orthogonal contrasts were made between the second (20%–40%) and fourth (60%–80%) intervals and between the third (40%–60%) and fifth (80%–100%) intervals to signify changes in performance between mid and late components of the TT respectively. For median EMG frequency and total EMG intensity, paired t-tests were used to compare the difference between the beginning (first 20%) and end (last 20%) of the TT for each condition. Additionally, magnitude-based inference analysis was used to examine the influence of the CHO mouth rinse on time to completion, within conditions and paired t-tests (Sidak correction, P = 0.0169) were used to compare between conditions (20). Log-transformed data were used to estimate the effect of CHO mouth rinse as the difference in mean percentage (with 90% confidence intervals) between FEDPLA and FEDWASH trials. A value of 1% was used as the smallest worthwhile change in time to completion of a cycling TT (21), with further magnitude-based inferences (22) used to provide clinical insights into performance outcome effects representing a true change (benefit, harm, or trivial). Chances of benefit or harm were determined as follows: 1% to 5%, very unlikely; 5% to 25% unlikely; 25% to 75%, possible; 75% to 95%, likely; 95% to 99%, very likely; >99%, almost certain (20). All data are presented as mean ± SD and significance is set at P ≤ 0.05, unless otherwise stated. All statistics were performed via Statistica (Version 13).
All SS data are presented in Table 1. All exercise-induced metabolic, respiratory responses throughout the 2-h SS exercise were as expected, and there were no significant differences between or within conditions for any variable (Table 1).
There was a significant improvement in the time to complete the final 20% (interval, 80%–100%) versus the interval of 40% to 60% (Fig. 2) during the FEDWASH trial (P = 0.01), whereas FEDPLA showed no significant changes in interval time (P > 0.05) (Fig. 2). For time to complete the TT, there was no significant difference between FEDWASH and FEDPLA (P = 0.51) (Fig. 3). Although not significant, subjects completed the FEDWASH TT 1.7% (90% confidence interval, +6.4% to −3.2%; Cohen ES, 0.21) faster compared with FEDPLA treatment (35 s), with qualitative probabilities demonstrating a 60% and 23% chance of a likely positive or trivial outcome, respectfully (Fig. 3). The individual differences and group mean in time to complete TT across both trials are shown in Figure 3.
For EMG frequency analysis between the beginning (first 20%) and the end (final 20%) of the TT, there was a significant increase in MDF for SOL only during FEDWASH (P = 0.006; Fig. 4A). Conversely, there was a significant decrease in MDF of MG only during FEDPLA (P < 0.001; Fig. 4B). The remaining lower-extremity muscles measured did not show significant changes for MDF or total EMG intensity; therefore, only the MDF results from the SOL and MG muscles are presented. There was no significance between trials differences for TT cadence, HR, and RPE but typical temporal changes for each parameter throughout the TT (Table 2). There were no trial differences for mouthwash rinse time (7.2 ± 1.0 s), rinse volume (24.56 ± 0.4 mL), and average spit volume (24.14 ± 0.6 mL). Post-TT lactate (mmol·L−1) was 7.5 ± 3.7 and 6.8 ± 2.3; glucose (mmol·L−1) was 5.4 ± 1.2 and 5.6 ± 0.9 for FEDPLA, and FEDWASH, respectively.
Pretrial/posttrial hydration status and fluid intakes
The total SS fluid consumed (mL) was 1500 ± 1.4 and 1500 ± 1.8 for FEDPLA and FEDWASH, respectively. There was a significant decrease in body mass for FEDPLA (P < 0.001) and FEDWASH (P < 0.001). There were no differences in hydration status between trials assessed via USG or percent loss in body mass (USG: average morning void 1.018 ± 0.007 and pretrial 1.009 ± 0.004).
Perception and blinding questionnaires
From the 20 trials, subjects indicated they could distinguish a taste difference 10 times. However, subjects identified the correct solution only 15% of the time indicating blinding was effective. There was no significant difference on how the subjects felt on the five-point feeling scale during SS (3.6 ± 0.5 and 3.5 ± 0.9 for FEDPLA and FEDWASH, respectively). There was a very strong trend on how subjects felt during the TT between the FEDWASH and FEDPLA trials (P = 0.051) (feeling scale were 2.6 ± 1.2 and 3.2 ± 1.1 for FEDPLA and FEDWASH, respectively).
Overall, the results of this study support a small positive performance benefit of CHO mouth rinse during the late stages of high-intensity cycling TT, after prolonged submaximal cycling in a nutritionally ecologically valid condition. Specifically, we observed that when a CHO mouth rinse was used, both time to completion and power (during different 20% portions) did not change throughout as opposed to a noticeable decrement in performance when a CHO mouth rinse was not used. On average, this resulted in a 1.7% (35 s) faster TT for FEDWASH compared with FEDPLA treatment, with qualitative probabilities demonstrating a 60% and 23% chance of a likely positive or trivial outcome, respectfully. It is important to note that the performance improvement was not observed in all participants, suggesting that the response may be dependent on the individual. This is the first study to observe differences in measures of muscle activity associated with CHO mouth rinse from multiple leg muscles. These results mechanistically support a modification in muscle activation in lower leg muscles elicited by CHO mouth rinse. The positive performance effect coupled with the change in muscle activity support a potential reduction of neuromuscular fatigue with CHO mouth rinse during a TT after a prolonged endurance task.
TT performance effects of CHO mouthwash after prolonged submaximal exercise
The maintenance of a consistent time to completion and power with the use of CHO mouth rinse during different portions of a TT observed in the present study is similar to other studies examining shorter durations of exercise (<1 h) (11,15). For example, Chambers et al. (11) showed that power was better maintained during the CHO mouth rinse compared with PLA, particularly in the late stages of the exercise, and there was no difference in their subjects RPE. Our findings suggest that although the CHO mouth rinse in FEDWASH did not result in a statistical (P > 0.05) increase in power and decrease in time to completion as compared to FEDPLA, it does not result in the same decrement in performance during the last half of the TT observed when CHO mouth rinse is not used as in the PLA condition. The current results would suggest that during prolonged activity (>1 h), CHO mouth rinse may partially mitigate the fatiguing effect of an endurance bout for some athletes (6 of 10 athletes of the current study). As an example of the impact that an intervention might have on the minute differences in elite sport, a 35-s improvement in performance is the difference between gold and 7th place in the approximately 45-min women’s 2016 Olympic cycling TT (23) and the difference between gold and 15th place in the approximately 27-min men’s 2016 Olympic 10,000-m event. These results are consistent with previous studies examining CHO mouth rinse on performance and are especially relevant to endurance sport because this study used a protocol that incorporates the longest, most ecologically valid exercise paradigm to date to investigate the effects of CHO mouth rinse on performance.
Ecological validity considerations of CHO mouthwash studies—in field application
When interpreting the utility of a CHO mouth rinse for endurance sport it is important to consider the experimental protocols used to examine the effects of CHO mouth rinse and the degree to which these study protocols replicate the real world of elite performance endurance sport. Investigations of the performance effects of CHO mouth rinse have mostly been focused on shorter-duration activities (<1 h) where endogenous CHO is not a limiting factor (10,24). Prolonged exercise requires the nearly continuous ingestion of CHO to allow for sustained and high CHO oxidation rates to enhance performance (2). However, CHO intake has been known to cause GI problems in some athletes during later stages of a race (7). Because glycogen starts to deplete during prolonged exercise, the positive effect of CHO intake significantly increases performance (r = 0.356; P = 0.004; (2)) and the potential benefit of a CHO mouth rinse may have added benefit (2).
Athletes who experience GI upset related to CHO ingestion may benefit from the use of CHO mouth rinse to limit the amount of CHO that is required to be ingested during the later stages of an event and also during very high intensity exercise (e.g., interval workouts) when blood shunting away from the GI tract can cause GI issues (25). Beyond Ataide-Silva et al. (14), this is only the second study to add to the current knowledge of the effect of CHO mouth rinse on high-intensity performance towards the end of prolonged exercise (≥2 h). This is particularly important to some athletes participating in endurance sport as for example, commonly the last 30 min of a marathon or cycling race is when potential GI upset related to ingestion of CHO will be the greatest and ultimately the most limiting to performance (9).
Previously, Luden et al. (16) reported that CHO mouth rinse used during a ∼3 min TT after 3 h of exercise with no CHO ingestion resulted in a 3.8% improvement in performance. Although it is important to establish a potential improvement during late endurance exercise in a state where nutritional depletion and neuromuscular fatigue is prevalent, the level of nutritional depletion utilized by Luden et al. (16) is not representative of real-world conditions, as some ingestion of CHO is required during prolonged exercise situations (5). In the current study, we tested endurance trained athletes following typical race conditions or prolonged training sessions. During these sessions, athletes would commonly consume a meal approximately 2 h before competition, followed by consumption of CHO during early to mid stages of exercise and potential use of the CHO mouth rinse during the later stage of exercise to mimic the last sustained effort (push) to the finish line. To our knowledge, no other study to date has looked at the effect of a CHO mouth rinse during the late stages of endurance activity in conditions of high ecological validity that include a typical CHO-rich standardized pretrial snack and CHO intake during SS prolonged exercise (2 h), followed by an approximately 30-min TT. Therefore, these results are applicable to athletes in sports with demands of prolonged racing (>1 h) which culminates in a final maximal exertion such as a sustained period (∼30 min) of high-intensity exercise followed by a sprint to the finish line.
It has been shown that CHO stimulate receptors in the mouth that activate areas of the brain associated with motor control and reward (11). It has been suggested that CHO mouth rinse may be more effective when performed in a postabsorptive overnight fasted state (when the oral receptors have a greater sensitivity to CHO) (14,15,26). Fasting is thought to sensitize CHO receptors and therefore result in greater central drive to improve performance in response to CHO (15). Additionally, the effect of a CHO mouth rinse on performance has been reported to be greater where muscle glycogen in diminished (14). Therefore, the majority of previous studies have incorporated a fasting protocol and were designed to fatigue/deplete athletes in an attempt to maximize the potential observed benefit of a CHO mouth rinse (11,14,15). The present results may also support the beneficial effect of CHO mouth rinse during a late endurance bout when muscle glycogen stores are diminished as the effect is only noticed during the later portions of the protocol as the TT component may start with reduced muscle glycogen.
Muscle fatigue response
Examination of the MDF content of the EMG signals suggests that fiber type/motor unit recruitment may differ during a fatiguing bout of exercise performed with or without CHO mouth rinse. The decreasing MDF of the MG EMG signal during FEDPLA may demonstrate fatiguing muscle activity with decreased recruitment of type II fibers and an increased representation of type I fibers active within the muscle (14). Although not significant, a similar decrease in MDF is visually evident in SOL EMG signal during the FEDPLA condition when looking at the last 20% of the TT (Fig. 4A). These findings of muscle activation may suggest the potential of modification of central neural drive related to fatigue in the triceps surae muscle group during FEDPLA. This potential evidence of muscle fatigue and concomitant decrease in power output is thought to be an internal neural protective strategy that results from reduced CHO availability (14). The increase in MDF of the SOL EMG signal and the maintenance of MDF of the MG EMG signal during the FEDWASH condition suggests that both an increase and maintenance of recruitment of type II fibers may be present to reduce negative effects of neuromuscular fatigue to maintain constant cycling power output (27). In summary, these findings show that fatigue of the plantarflexor muscles may only be present during FEDPLA, whereas during FEDWASH, this fatigue is not evident, possibly supporting the role of CHO mouth rinse in delaying the onset of type II motor unit derecruitment to maintain performance (28).
However, this beneficial performance effect observed during FEDWASH may only be short lived as greater recruitment of type II fibers would result not only in a greater ability to maintain power output but also greater chance of failure due to fatigue as a result of glycogen depletion and the greater reliance of type II fibers on anaerobic metabolism (29,30). Evidence of this short term centrally mediated increase in performance with CHO mouth rinse, with a later erosion in power outputs/performance, has been observed during cycling sprint performance (30). Additionally, this short-term benefit is similar to that observed during an acute performance bout where CHO mouth rinse reduced the attenuation in maximal leg extension force in a fatigued state, but later bouts were not benefited by CHO mouth rinse (31). Thus, the effect of CHO mouth rinse may be to prolong onset of fatigue to some muscles while allowing other muscles to recruit more motor units to enable an acute enhancement of power output and performance. These results are also very intriguing considering the different physiological fiber compositions (32) and biomechanical roles of the triceps surae muscles observed in our study. That is, SOL has a greater percentage of Type I fibers compared to MG (32) and is a single joint muscle preferentially recruited for high resistances, whereas MG is a multijoint muscle preferentially recruited for actions that require high shortening velocities (33). Taken together the differing results in MG and SOL during the study conditions could support complex muscle recruitment strategies dependent on CHO availability.
Under conditions of high ecological validity, the current study suggests CHO mouth rinse appears to have a marginal impact on TT performance after a prolonged endurance task in a fed state for some participants. This performance effect occurs concurrently with evidence of changes in muscle recruitment potentially associated with fatigue. In real-world competitions, this may have a meaningful impact on performance outcomes. These findings are encouraging for endurance athletes and may be particularly meaningful to those who demonstrate GI intolerance of CHO ingestion at later stages of the race.
This study was designed by M. J., M. K., and T. S. Data were collected and analyzed by M. J. Data interpretation and article preparation were undertaken by M. J., M. K., B. S. and T. S. All authors approved the final version of the article. This study was financially supported by MITACS accelerate fellowship and the Canadian Sport Institute-Pacific. The authors wish to thank all the participants for volunteering for the study. The authors have no conflict of interest and made every attempt to present the results of the study clearly, honestly, and without fabrication, falsification, or inappropriate data manipulation. The authors also acknowledge that results of the present study do not constitute endorsement by ACSM.
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