effect of mouth-rinsing carbohydrate solutions on endurance performance

Effect of Mouth-Rinsing CarbohydrateSolutions on Endurance PerformanceIan Rollo and Clyde Williams

School of Sport and Exercise and Health Sciences, Loughborough University, Loughborough, UK

Contents

Abstract. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4491. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4502. Evidence: Performance Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 451

2.1 Cycle Time Trials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4512.2 Running Time Trials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 453

3. Performance Protocols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4544. Ratings of Perceived Exertion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4555. Pre-Exercise Nutritional Status . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4576. Mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4577. Practical Implications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4598. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 459

Abstract Ingesting carbohydrate-electrolyte solutions during exercise has been re-ported to benefit self-paced time-trial performance. The mechanism respon-sible for this ergogenic effect is unclear. For example, during short duration(£1 hour), intense (>70%maximal oxygen consumption) exercise, euglycaemia israrely challenged and adequate muscle glycogen remains at the cessation ofexercise. The absence of a clear metabolic explanation has led authors tospeculate that ingesting carbohydrate solutions during exercise may have a‘non-metabolic’ or ‘central effect’ on endurance performance. This hypothe-sis has been explored by studies investigating the performance responsesof subjects when carbohydrate solutions are mouth rinsed during exercise.The solution is expectorated before ingestion, thus removing the provision ofcarbohydrate to the peripheral circulation. Studies using this method havereported that simply having carbohydrate in the mouth is associated withimprovements in endurance performance. However, the performance re-sponse appears to be dependent upon the pre-exercise nutritional status of thesubject. Furthermore, the ability to identify a central effect of a carbohydratemouth rinse maybe affected by the protocol used to assess its impact on per-formance. Studies using functional MRI and transcranial stimulation haveprovided evidence that carbohydrate in the mouth stimulates reward centresin the brain and increases corticomotor excitability, respectively. However,further research is needed to determine whether the central effects of mouth-rinsing carbohydrates, which have been seen at rest and during fatiguingexercise, are responsible for improved endurance performance.

REVIEWARTICLESports Med 2011; 41 (6): 449-461

0112-1642/11/0006-0449/$49.95/0

ª 2011 Adis Data Information BV. All rights reserved.

1. Introduction

The endogenous stores of carbohydrate arefinite. Prolonged fixed-intensity exercise to fatigueis associated with the depletion of muscle glycogenand/or hypoglycaemia. Thus, it is widely accept-ed that providing carbohydrate during exercisecan improve endurance capacity by preventinghypoglycaemia and can provide a ready fuel forthe working muscles (for reviews see Coyle[1] andTsintzas and Williams[2]). A common methodof providing carbohydrate during exercise is inthe form of carbohydrate-electrolyte solutions. Pro-viding carbohydrate in the form of a carbohydrate-electrolyte solution supplies fuel as well as providingfluid and electrolytes that are lost as a conse-quence of sweating. Ingesting fluid during ex-ercise has been reported to reduce cardiovascularstress and hyperthermia associated with exercise-induced dehydration.[3] In addition, fluid ingestionhas been reported to have a profound metaboliceffect during exercise. For example, ingesting fluidalone has been reported to improve endurancecapacity[4] by reducing the utilization of muscleglycogen.[5] Therefore, it is not surprising thatmost laboratory studies have examined the influ-ences of ingesting carbohydrate-electrolyte solu-tions on endurance capacity (>1 hour) rather thanendurance performance (i.e. time trials). This isbecause the associated increases in core temper-ature and heart rate during short-duration ex-ercise are not as pronounced as exercise of greaterdurations (>1 hour).[3,6] In addition, hypoglycaemiaand severe depletion of muscle glycogen have notbeen reported following short periods (£1 hour)of intense (>70% maximal oxygen consumption[.VO2max]) exercise.

[7]

The metabolic effect of carbohydrate inges-tion appears to differ depending upon the modeof exercise. For example, blood glucose con-centrations during prolonged treadmill running donot decrease to the same extent as with prolongedcycling.[2] It is important to note that the majorityof studies investigating the impact of carbohydrateingestion on endurance performance have used cy-cling rather than running as the mode of exercise.

McConell et al.[8] reported that during high-intensity cycling, only a small percentage (26%)

of the total carbohydrate ingested actually entersthe peripheral circulation during exercise. In ad-dition, ingesting glucose has been reported tohave no effect on carbohydrate oxidation, musclemetabolism or performance when cycling to fa-tigue at approximately 80%

.VO2max.

[8] Further-more, when glucose was infused directly into thecirculation (60 g/h), the rate of muscle glycogenoxidation was unaffected. Exogenous carbohy-drate was reported to contribute to only 9 g of the54 g of carbohydrate oxidized in the final quarterof a 1-hour cycling time trial.[9] However, inrunning, the ingestion of 50 g of carbohydrate ina 5.5% solution has been reported to result in a28% sparing of glycogen in the vastus lateralismuscle during a 60-minute treadmill run. Theingestion of the carbohydrate solution resulted ina 42% sparing of glycogen in the type I musclefibres, with type II muscle fibres unaffected. Theamount of glycogen spared was directly related tothe magnitude of serum insulin increase withinthe first 20 minutes of exercise.[10] Nevertheless,adequate concentrations of glycogen remained inthe muscle following the 60-minute treadmill runat 70%

.VO2max in both the carbohydrate and pla-

cebo trials. For a comprehensive review on muscleglycogen metabolism during both running and cy-cling exercise, consult Tsintzas and Williams.[2] Toour knowledge, no studies have measured muscleglycogen concentrations in response to mouth rins-ing with a carbohydrate solution.

Despite the absence of a clear metabolic ra-tionale, both fluid and carbohydrate ingestion havebeen reported to independently improve time-trial performance.[6] Below et al.[6] asked subjectsto cycle at a constant intensity (80%

.VO2max) for

50 minutes followed by a 10-minute time trial, inwhich the task was to complete a fixed amount ofwork as quickly as possible. Providing both fluidand carbohydrate improved time-trial perfor-mance by approximately 6%. Furthermore, thebeneficial independent effects of fluid and carbo-hydrate ingestion on performance were reportedto be additive. The improvements in performancewith fluid ingestion was attributed to maintaininga higher cardiac output and attenuating the in-creases in core temperature and heart rate, whichwere observedwhen no fluidwas ingested.However,

450 Rollo & Williams

ª 2011 Adis Data Information BV. All rights reserved. Sports Med 2011; 41 (6)

there was no evidence that carbohydrate inges-tion influenced either core temperature or heartrate. Furthermore, ingesting carbohydrate didnot appear to have a significant effect on bloodglucose concentrations or carbohydrate oxida-tion. Thus, an explanation by which carbohy-drate improved performance in this study wasreported to be ‘unclear’.[6]

It is important to note that not all studieshave reported a benefit following the inges-tion of fluid[11] or carbohydrate[12,13] on time-trialperformance. Nevertheless, there is substantialevidence showing that ingesting appropriatecarbohydrate-electrolyte solutions during exercisecan improve endurance performance of approxi-mately 1 hour in duration. Benefits to perfor-mance have been reported in both cycling[14-17]

and running.[18,19] However, a mechanism to ex-plain this improvement in performance remains tobe established.

Intriguingly, the absence of a clear metabolicbenefit when subjects ingest carbohydrate has ledauthors to speculate that carbohydrate may in-fluence ‘central’ or ‘non-metabolic’ pathwaysduring exercise. To this end, this review will in-clude a consideration of those studies that haveinvestigated the potential ‘central’ effect of car-bohydrate on performance. Studies testing thishypothesis have removed the provision of glucoseor fluid to the peripheral circulation by requiringtheir subjects to simply mouth rinse the carbo-hydrate solution without ingestion. Thus, thisreview will focus on the performance response tosimply having carbohydrate in the mouth andpotential mechanisms by which this may exertan ergogenic effect. The impact of pre-exercisenutritional status, mode of exercising testing,concentration and type of carbohydrate in therinsed solution will be discussed separately. Theimportance of the method and protocols usedto detect a possible ergogenic effect of mouth-rinsing carbohydrate-electrolyte solutions willalso be considered. The literature cited in thisreview was retrieved using online search data-bases (i.e. PubMed and SportDiscus�). Key searchterms used included ‘carbohydrate’, ‘mouth rinse’,‘performance’, ‘oral’, ‘central’ and ‘exercise (run-ning and cycling)’.

2. Evidence: Performance Studies

To our knowledge, only six studies have in-vestigated the influence of mouth rinsing with acarbohydrate solution on endurance performance.The purpose of this section is to review the studiesin the order in which they were published. Fur-thermore, the impact that mouth rinsing with acarbohydrate solution has on performance, hasonly been investigated using cycling and running.Therefore, the original studies completed in cy-cling will be reviewed first followed by the studiesthat used running as a mode of exercise.

2.1 Cycle Time Trials

Carter et al.[20] were the first to provide evi-dence that mouth rinsing with a carbohydratesolution during exercise could improve cycletime-trial performances of approximately 1 hourin duration. In this study, seven male and twofemale cyclists completed two experimental trials,where the task was to complete a fixed amount ofwork (914 – 40 kJ) as quickly as possible. In thetwo trials, which were separated by 1 week, thesubjects mouth rinsed with either a 6.4% malto-dextrin solution or water (25mL) at every 12.5% ofthe time trial completed. The solutions wererinsed in the mouth for approximately 5 secondsbefore being expectorated. The trials were com-pleted following a 4-hour postprandial period.However, the exact composition of the pre-exercisemeal was not stated. The mean power output wassignificantly greater when mouth rinsing withcarbohydrate than with water (259– 16W vs 252–16W, respectively). Of note, an increase in poweroutput was observed during the first three-quartersof the time trial. Eight of the nine cyclists im-proved their performance during the carbohy-drate trial. Thus, time to complete the fixed amountof work was reduced on average by 2.9% whencyclists mouth rinsed with carbohydrate ratherthanwater (59.57– 1.50minutes vs 61.37– 1.56min-utes, respectively). There was no difference inheart rate (172– 1beats/min and 171– 1beats/min)or ratings of perceived exertion (RPE) [16– 1] whenmouth rinsing with carbohydrate or water, respec-tively. Unfortunately, the volume of expectorate

Endurance Performance and Mouth-Rinsing Carbohydrate Solutions 451


was not measured and there was no blood sam-pling during exercise because the investigators didnot want to disrupt the time-trial performance.Thus, whether or not any carbohydrate was inad-vertently ingested could not be established.

Pottier et al.[21] investigated performance re-sponses of 12 male cyclists to the same trial con-ditions reported by Carter et al.[20] In this study,each cyclist was required to complete the timetrial (975 – 85 kJ) on four occasions separated by48 hours. The four experimental conditions in-volved either mouth rinsing with a placebo orcarbohydrate-electrolyte solution, or ingestinga placebo or carbohydrate-electrolyte solution.The total quantity of solution rinsed/ingestedwas 14mL/kg body mass. The subjects received2mL/kg body mass of the solution before the5-minute warm up (100W). The subjects then re-ceived 1.5mL/kg body mass immediately beforeand at every 12.5% of the time trial completed. Dur-ing the mouth-rinse trials, subjects mouth rinsedthe solution for 5 seconds before the solution wasexpectorated. The carbohydrate-electrolyte solu-tion was a commercially available sports drink.The placebo solution was identical in formulationexcept that it contained no carbohydrate. Thecyclists completed the time trial significantly faster(3.7%) when mouth rinsing with the carbohydrate-electrolyte solution (61.7 – 5.1 minutes) than withmouth rinsing the placebo (64.1 – 6.5 minutes).However, ingesting the carbohydrate-electrolytesolution was reported not to improve performance(63.2 – 6.9 minutes) over the ingestion of the pla-cebo solution (62.5 – 6.9 minutes). Surprisingly,mouth rinsing with the carbohydrate-electrolytesolution resulted in a greater improvement inperformance than when ingesting the same solu-tion. The authors suggest that performance wasimproved due to the presence of carbohydrate inthe oral cavity. However, they speculate that thisperformance benefit may be lost due to the shortoral transit time when the carbohydrate-electrolytesolution is ingested.[21] There were no clear differ-ences in the physiological variables (blood lactate,blood glucose, heart rate) or RPE recorded duringexercise. However, an interesting observation isthat when ingesting or mouth rinsing with thecarbohydrate-electrolyte solution, subjects began

their exercise with higher blood glucose con-centrations than subjects in either placebo trial.In this study, subjects were requested to consumea carbohydrate-rich meal 3 hours before the testand consume a carbohydrate-rich diet (400 gcarbohydrate) the day before the trial. Unfortu-nately, the actual values and subject compliance tothese dietary requests are not reported. It is im-portant to note that different day-to-day dietarypreparation for the time trials would have largeeffects on performance.[17] For example, theeffect of mouth rinsing with a carbohydrate so-lution was investigated in cyclists who had con-sumed a standardized breakfast 2 hours beforecompleting a time trial.[22] The breakfast provid-ed before exercise contained 2.4 g of carbohy-drate per kg of the subjects body mass. Ingestingsimilar quantities of carbohydrate 3 hours beforeexercise has been reported to increase muscleglycogen by 11–15%.[23,24] Fourteen maleendurance-trained cyclists completed the sametime trial as that used in previous cycling perfor-mance studies.[20,21] Identical to Carter et al.,[20]

cyclists mouth rinsed with a 6.4% maltodextrinsolution or water immediately before and every12.5% of the time trial completed. Eight of 14cyclists completed the time trial faster whenmouth rinsing with the maltodextrin solutionthan with water. However, in this study, the per-formance time (68.14 – 1.14 minutes vs 67.52 –1.00 minutes) and average power output (265 –5W vs 266 – 5W) did not differ between the car-bohydrate or water trials, respectively. Therewere no differences in heart rate reported be-tween trials. Unfortunately, no blood samples orexpired air was collected during exercise.

Chambers et al.[25] reported the results of twoseparate cycling time-trial performance studiesthat used the same protocols as described by Carteret al.[20] In both studies, the time trials were com-pleted following an overnight fast, with each trialseparated by at least 3 days. Subjects mouthrinsed with either a carbohydrate or placebo so-lution immediately before and every 12.5% of thetime trial completed. In these studies, the testsolutions were mouth rinsed for approximately10 seconds (double the duration of previousstudies) before being expectorated into a bowl.



The placebo solutions used in both studies werewater that was artificially sweetened with a non-calorific concentrate, aspartame and saccharin. Inthe first performance study, eight male cyclistsmouth rinsed either a placebo or 6.4% glucosesolution. Seven of eight cyclists completed thetime trial (914 – 29 kJ) faster when mouth rinsingwith the carbohydrate than the placebo solution.The mean times to complete the time trial were60.4 – 3.7 minutes and 61.6 – 3.8 minutes, re-spectively. Thus, the average improvement was2.0 – 1.5% when mouth rinsing with the carbo-hydrate solution. There were no differences inmean RPE (16.0 – 1.8 vs 16.0 – 1.6) for the car-bohydrate and placebo trials. Mean heart ratewas elevated on the carbohydrate trial (180– 3 vs177– 4beats/min); however, there were no signif-icant differences during the two trials. In thesecond study, six male and two female cyclistsmouth rinsed with either a placebo or a 6.4%maltodextrin solution. As in the first study, sevenof eight subjects completed the carbohydrate trialfaster than the placebo trial. Mean times to com-plete the time trial (837 – 68 kJ) were 62.6 – 4.7minutes and 64.6 – 4.9 minutes for the carbohy-drate and placebo trials, respectively. The meanincrease in power output being 3.1 – 1.7%. De-spite values being lower than those in the firststudy, no differences were reported in the RPE(15– 2 vs 15– 2) or heart rate response duringeither the carbohydrate or placebo trials, re-spectively. It is important to note that subjectswere unable to identify the carbohydrate solutionin either study. To interpret the significance of theperformance benefits reported in cycling is diffi-cult because investigators commonly fail to setthreshold values for ‘worthwhile’ improvements intrial performances. The implications of performancedifferences within the known day-to-day variationof the testing methods are discussed in section 3.

2.2 Running Time Trials

Whitham and McKinney[26] were the first toexamine the influence of mouth rinsing with acarbohydrate solution on running performance.In their study, running performance was assessedusing a manually controlled treadmill; the subjects

were seven healthy males. Their previous runningexperience was reported to be ‡10 km althoughthe runners’

.VO2max values (57.8– 2.7mL/kg/min)

were below those that are associated withendurance-trained athletes.[27] Following a 4-hourpostprandial period, the subjects completed a 15-minute warm-up run at 65%

.VO2max, followed by

a 45-minute time trial. The aim of the time trialwas to achieve the greatest distance possible in theset time. At the beginning of exercise and at every6 minutes during the time trial, runners were is-sued with a 500mL bottle containing either 200mLof 6% carbohydrate (97% polysaccharide, 2%disaccharide, 1% glucose) solution or placebo.Unsweetened lemon juice was added to both thecarbohydrate and placebo solutions in an attemptto make the taste of the solutions indistinguish-able. The runners were instructed to mouth rinsewith a mouthful of the solution for 5 secondsbefore expectorating into a bowl. No differencewas reported in respiratory exchange ratios, oxy-gen consumption, heart rate or the RPE betweenthe carbohydrate and placebo trials. Furtheranalyses of the results could not be undertakenbecause the authors did not report numericalvalues for their study. Blood glucose was report-ed to increase as a result of the time trial but,again, no values are reported. Overall distancecovered in 45 minutes was not significantly dif-ferent when the runners mouth rinsed with eitherthe 6% carbohydrate (9333 – 988m) or a flavour-matched placebo solution (9309– 993m).[26] Withregard to performance, the authors recognizethat the running tests requiring a manual controlof pace may not be optimal for detecting a po-tentially sub-conscious ‘central’ effect of carbo-hydrate mouth rinse.

Rollo et al.[28] investigated mouth rinsing witha carbohydrate-electrolyte solution on runningperformance using an automated treadmill thatallowed runners to change their running speedwithout manual input.[29] The subjects were tenendurance-trained male runners who completedtwo trials separated by 1 week. After an overnightfast of 13–15 hours, the runners completedeach 1-hour time trial during which they wereinstructed to cover as much distance as possiblein the set time. Solutions were supplied to the



runners in plastic volumetric syringes. In randomorder, runners either mouth rinsed with 25mL of6.4% carbohydrate-electrolyte solution (glucosesyrup and maltodextrin) or placebo solution im-mediately before and at 15, 30 and 45 minutes.The placebo solution was matched in formula-tion to the carbohydrate-electrolyte solutionexcept that it contained no carbohydrate. Thesolution was mouth rinsed for 5 seconds beforebeing expectorated into a pre-weighed container.No differences were found in respiratory ex-change ratios (~0.90) or carbohydrate oxidationbetween trials (~3.0 g/min). The total distancecovered during the carbohydrate trial (14298 –685m) was significantly greater than thatachieved when runners mouth rinsed with theplacebo (14086 – 732m), representing 1.5% of thetotal distance covered. There were no differencesin the RPE reported by the runners. Previousmouth-rinse studies did not record the volume ofexpectorated solution; therefore, it is not knownwhether any of the rinsed solution was ingestedduring the mouth-rinse procedure. However, inthis study, there was clear evidence that subjectswere able to mouth rinse and expectorate the testsolution without ingestion during exercise (vol-ume of expectorate equal to or greater than vol-ume rinsed). Albeit, it is important to note thatsaliva in the mouth would have contributed to thevolume of expectorate; thus, small quantities ofthe solution may have been ingested. Neverthe-less, blood glucose concentrations were no dif-ferent before or after the 1-hour run when mouthrinsing with the carbohydrate-electrolyte or pla-cebo solution (4.3 – 0.3mmol/L).

3. Performance Protocols

Studies investigating the effect of mouth rin-sing with carbohydrate solutions have assessedendurance performance (i.e. time trials). To ourknowledge, no studies have investigated the effectof mouth rinsing with a carbohydrate solution onendurance capacity (time to fatigue). The maindifference between endurance performance andendurance capacity protocols is that exercise in-tensity is self-selected by the subject during thetime trials. It appears that subjects completing

time trials have a greater ability to accuratelyreplicate their performance in comparison tocompeting time-to-exhaustion tests.[30-32] How-ever, this is not the case in all studies, particularlywhen adequate habituation trials have beencompleted.[33,34]

It is important to consider that a placebo effectmay have a large influence, especially when inves-tigating small changes in exercise performance.For example, in a study that asked cyclists tocomplete a 40-km time trial, simply informingthem that they were ingesting carbohydrate wasreported to enhance their performance by ap-proximately 4%.[35] In those studies that reporta benefit of carbohydrate mouth rinse, Carteret al.[20] reported that four of their nine subjectswere able to identify the trial that used carbohy-drate, despite the maltodextrin solution beingcolourless and unsweetened. Pottier et al.[21] ex-amined whether or not subjects could detect thecarbohydrate solution by organizing an indepen-dent test. In a triangular sensory test, only 8 of34 subjects were able to identify the carbohydrate-electrolyte solution. Chambers et al.[25] used arti-ficial sweeteners in the test solutions and reportedthat none of the subjects were able to identify thecarbohydrate solution. In the running study thatreported a benefit of mouth rinsing with carbo-hydrate, Rollo et al.[28] reported that only two often runners were able to distinguish between thecarbohydrate and placebo solution. The solutiondiffered from the maltodextrin solution used inthe previous running mouth-rinse study reportedby Whitham and McKinney.[26] However, thesolutions were suitably indistinguishable with-out the addition of any potentially undesirablemasking agents such as bitter lemon juice, whichwas added to both test solutions in the study byWhitham and McKinney.[26]

All the cycling studies investigating mouthrinsing with carbohydrate have used the same per-formance test. This test, which requires cyclists tocomplete a set amount of external mechanicalwork as quickly as possible (approximately 1 hourin duration), has been reported to have a coeffi-cient of variation (CV) of 3.35%.[31] Of note,Pottier et al.[21] is the only cycling study to reportan improvement in performance greater than that



of the known variation of performing the cyclingtime trial (i.e. 3.7%) [table I]. Asking runners tocover as much distance as possible during 1 hourof treadmill running has a CV of 1.4%.[29] There-fore, in running, the reported benefit of mouthrinsing with a carbohydrate solution (1.5%) wasbeyond the day-to-day variation of the testingmethod. The CVs of the performance tests werederived between subjects, which is appropriate forrepeated-measures data. Nevertheless, authors ofperformance studies should be encouraged to re-port and justify the smallest worthwhile changesfor the performance test used. It is important tonote that the use of inferential statistics, whichcategorizes performance differences into gradedmagnitudes, such as positive, trivial and negative,provides a more comprehensive analysis of per-formance data.[37]

In performance studies, it is interesting to notethat different pacing strategies are adopted bysubjects in cycling and in running time trials.Observations from running studies show that run-ners typically maintain their self-selected runningspeed for the majority of the trial and sprint to-wards the end of the 1 hour.[26,29] In cycling stu-dies, power output gradually declines during thefirst three-quarters of the time trial before beingincreased towards the end of the set amount ofexternal work.[16,20,21,25] These observations sug-gest that care must be taken when making com-parisons between cycling and running studies.The distinct difference being that mouth rinsingwith a carbohydrate solution appears to improverunning performance by increasing self-selectedspeed, whereas, in cycling, the benefit to perfor-mance appears to be achieved by reducing thedecline in power output during the time trial.

4. Ratings of Perceived Exertion

Studies that report a performance benefit frommouth rinsing with a carbohydrate solution, com-monly report no difference in the subjects’ per-ception of effort, despite working at higher workloads (table I). A similar phenomenon has alsobeen reported in studies that have investigatedthe perceptual and performance response to in-gesting caffeine.[38] These observations are of

interest because they are consistent with reportsthat subjects ‘feel better’ and report lower RPEwhile ingesting a carbohydrate-electrolyte solu-tion during prolonged exercise.[39-43] Loweringthe perception of effort during exercise may be aviable mechanism by which mouth rinsing with acarbohydrate solution could improve performance.Speculatively, if mouth rinsing with a carbohydratesolution lowers the perception of effort at a givenworkload, the subjectmay increase their self-selectedexercise intensity to match their ‘anticipated’ per-ception of effort. Thus, performance would be im-proved by the increases in power output or runningspeed that are self-selected during exercise.

Using conventional time-trial performanceprotocols may not be the optimal method to usein order to test this hypothesis. This is becausetime-trial tests require the subject to perform attheir maximal effort in order to achieve their bestperformance. Thus, if a subject was feeling ‘bad’,this may not be reflected in their self-selection ofexercise intensity due to the overriding motiva-tion to perform well. Therefore, instead of a timetrial, Rollo et al.[44] adopted a less demandingprocedure to allow more focus on the psycho-logical response to mouth rinsing with a carbo-hydrate-electrolyte solution. In this study, sub-jects were simply asked to select their exerciseintensity according to ‘how they felt’, whilstmouth rinsing 25mL of either a 6% carbohy-drate-electrolyte (glucose syrup and malto-dextrin) or placebo solution. Following an over-night fast (12–13 hours), ten recreational male run-ners completed a 10-minute warm up, followed bya 30-minute self-selected run. The trials werecompleted on an automated treadmill that al-lowed changes in running speed without manualinput. The subjects mouth rinsed with the testsolution for 5 seconds immediately prior to andthen at 3, 6 and 9.5-minutes during the warm-up run, and at 5-minute intervals during the30-minute run. In order for the runners to self-select the same range of running speeds, they wereasked to select a pace that represented a ratingof 15 (hard) on the Borg RPE scale.[45] Mouthrinsing the carbohydrate-electrolyte solution sig-nificantly altered the self-selection of running speedduring the first 5 minutes of the 30-minute run



Table I. Summary table of studies completed investigating the influence of mouth rinsing carbohydrate (CHO) solutions on endurance performance

Study (y) No. of

subjects

and sex

.VO2max

(mL/kg/min)

[mean –SD]

Mode Time trial

[mean –SD]Fasting

duration

Beverage CHO

(%)

No. of mouth

rinses (duration

[sec])

HR

(beats/min)

[mean –SD]

RPE

[mean –SD]

Result

[mean –SD]% Diff.

Carter et al.[9]

(2004)

7 M, 2 F 63.2 – 2.7 Cycle 914 – 40 kJ 4 h Maltodextrin 6.4 8 (5) 172 –1 16– 1 59.57 – 1.50mina 2.9

Water 0 171 –1 16– 1 61.37 – 1.56min

Pottier et al.[21]

(2010)

12 M 61.7 – 3.1 Cycle 975 – 85 kJ 3 h Sucrose

(5.4 g)/glucose(0.46 g)

6 8 (5) 161 –12 15.4 –1.4 61.7 – 5.1mina 3.7b

Placebo 0 157 –12 15.5 –1.7 64.1 – 6.5min

Beelen et al.[22]

(2009)

14 M NR Cycle 1053 – 48 kJ 2 h Maltodextrin 6.4 8 (5) 169 –2 16.4 –0.3 68.14 – 1.14min NA

Water 0 168 –2 16.7 –0.3 67.52 – 1.00min

Chambers

et al.[25] (2009)

8 M 60.8 – 4.1 Cycle 914 – 29 kJ Overnight Glucose 6.4 8 (10) 180 –3 16– 1.8 60.4 – 3.7mina 2.0 – 1.5

Placebo 0 177 –4 16– 1.6 61.6 – 3.8min

6 M, 2 F 57.8 – 3.2 Cycle 837 – 68 kJ Overnight Maltodextrin 6.4 8 (10) 181 –10 15– 1.8 62.6 – 4.7mina 3.1 – 1.7

Placebo 0 180 –10(peak HR)

15– 1.5 64.6 – 4.9min

Whitham and

McKinney[26]

(2007)

7 M 57.8 – 2.7 Run 45min 4 h Maltodextrin 6 10 (5) NR NR 9333 –988 km NA

Placebo 0 ~160 –20 NR 9309 –993 km

Rollo et al.[36]

(2010)

10 M 63.9 – 4.3 Run 1 h 13–15h Glucose/maltodextrin

6.4 4 (5) 163 –13 14– 1 14298 –685 kma 1.5b

Placebo 0 163 –12 14– 1 14086 –732 kma Indicates reported significant difference between CHO and placebo trials.

b % Diff. indicates performances differences beyond day-to-day variation in testing method.

F = female; HR= heart rate; M =male; NA =not applicable; NR = not reported; RPE = ratings of perceived exertion;.VO2max =maximal oxygen consumption; % Diff. =percentage

performance differences.

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&William

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Med2011;41(6)

compared with mouth rinsing the placebo(12.5 – 0.1 km/h vs 12.1 – 1.1 km/h, respectively).Total distance covered during the 30-minute runwas greater during the carbohydrate trial (6584 –520m) than the placebo trial (6469 – 515m). Inaddition, the increase in speed at the beginning ofexercise corresponded with runners experiencingelevated feelings of pleasure as assessed by the‘feeling scale’.[46]

Future studies could benefit from administer-ing psychological scales that have been validatedduring exercise. This is because psychologicalscales, such as those used by Backhouse et al.,[39]

may offer additional insights into the possiblecentral effects of mouth rinsing with carbohy-drate solutions. For example, the RPE scale[45]

provides information on the intensity of the per-ceived exertion but it does not help describe ‘how’the runners feel during exercise.[46] Furthermore,whether a subject is feeling ‘good or bad’ (pleasure/displeasure) or feels ‘energized’ (i.e. an activatedstate) during exercise is also relevant because it islikely that this may have a significant impact onperformance.[39,47] Nevertheless, endurance perfor-mance and endurance capacity will be determinedby an array of physiological and psychologicalfactors that will influence motivation of the in-dividual. Thus, it is important to note that mouthrinsing with a carbohydrate solution is probablyjust one of many sensory inputs that may affectthe perception of exertion during exercise.

5. Pre-Exercise Nutritional Status

Many athletes avoid eating immediately beforeearly morning training or competition. However,given the choice, most athletes would prefer tohave ameal a few hours prior to competing ratherthan to fast before exercise. Consuming a pre-exercise meal in combination with ingestingcarbohydrate-electrolyte solutions during ex-ercise has been reported to improve endurancecapacity compared with when either of these in-terventions is adopted alone. These findings havebeen reported in both cycling[48] and running[49]

studies. In contrast, a common variable that ex-ists between time-trial investigations that report aperformance benefit with the ingestion of carbo-

hydrate-electrolyte solutions[7,15,17,19,50] and thosethat do not[13,36,51] is the pre-exercise carbohy-drate status of the subject. For example, studiesthat have attempted to ‘optimize’ endogenousstores of glycogen prior to exercise, often reportno further improvements in performance wheningesting carbohydrate-electrolyte solutions dur-ing exercise.

Interestingly, Chambers et al.[25] recognizethat the central response to detecting carbohy-drate in the mouth could be altered by the physi-ological status of the body (i.e. in a fasted or fedstate). The effect of hunger and satiety on the cen-tral response to taste has recently been investi-gated by Haase et al.[52] In this study, functionalMRI was used to investigate the blood-oxygen-level dependent signal change to taste stimuli in18 healthy subjects. The subjects were presentedwith a variety of taste stimuli, including saccha-rin, sucrose and sodium chloride solutions, ineither a hungry or satiated state. Globally, brainactivation in the hunger condition producedmore robust activation to pure taste stimuli re-lative to water. In addition, tasting sucrose wasreported to result in the most robust activationcompared with all the other taste stimuli.[52]

As previously mentioned, Beelen et al.[22] re-ported that performance benefits associated withmouth rinsing with a carbohydrate solutionare not evident following the ingestion of a pre-exercise meal.[22] Accordingly, as reported byWhitham and McKinney,[26] the 4-hour post-prandial period may also account for the absenceof an effect in runners; however, the evidence isinconclusive. It is too early to speculate that thepotential benefit of mouth rinsing with a carbo-hydrate solution could be lost following a car-bohydrate-rich pre-exercise meal. This is becauseboth Carter et al.[20] and Pottier et al.[21] reportedsignificant performance benefits from mouthrinsing with a carbohydrate solution in subjectswho started exercise following a 4- and 3-hourfast, respectively.

6. Mechanisms

The first mouth rinse study performed byCarter et al.[20] was devised following two key



observations from investigations into cycling time-trial performance with carbohydrate supplemen-tation. The time trials, approximately 1 hour induration, required cyclists to complete an in-dividualized amount of work as fast as possible.First, it was reported that ingesting a carbohy-drate-electrolyte drink was associated with im-proved 1-hour endurance performance (2.3%) incomparison with ingesting a placebo solution.[16]

Second, it was found that infusing glucose di-rectly into the peripheral circulation (60 g/h) hadno impact on performance in comparison with in-fusing a non-glucose-containing saline.[9] Thus,despite increasing glucose availability to the work-ing muscle, the rate of muscle glycogen oxidationwas unaffected. The obvious difference betweenthe ingestion and infusion study was the route ofentry of carbohydrate into the body (i.e. infusingglucose bypassed the gastrointestinal tract).

It is well known that the digestion of carbo-hydrate begins in the mouth where the tonguebegins the analysis of food, determining whetherit is nutritive (i.e. sugar) and should be ingested,or is potentially harmful and therefore should beexpectorated.[53] The ability of the body to re-cognize incoming energy and also potentiallytoxic substances would clearly be an evolutionaryadvantage. Sweet stimuli (e.g. glucose, sucrose,fructose and artificial sweeteners) are detectedby taste receptor cells (G-protein-coupled recep-tor proteins T1R2 and T1R3) on the tongue.[54]

These receptor cells release a neurotransmitter(a-gustducin) that is detected by primary afferentnerve fibre terminals, which send information tothe brainstem. The central processing of sweettaste activates feeding circuits as well as brainreward systems that promote sweet appetite.[54]

Interestingly, studies have reported that flavour-ing or sweetening solutions can substantially in-crease the voluntary intake of fluid during bothexercise and recovery.[55,56]

Receptors on the tongue also extract informa-tion about the texture and temperature of food.This processing prepares the gastrointestinalsystem for compounds in the mouth by causingthe organism to salivate, masticate, swallow orexpel, as well as to release insulin and other pep-tides.[53] In humans, simply tasting food in the

oral cavity can stimulate the release of insulinfrom the pancreas, known as cephalic phase in-sulin release. For example, under fasting condi-tions, both nutritive (sucrose) and non-nutritivesweetener (saccharin) solutions have been reportedto stimulate cephalic phase insulin release, whenmouth rinsed for 45 seconds and expectoratedwithout ingestion.[57] However, this study wasperformed in resting subjects and mouth rinsingwith a carbohydrate solution for 45 secondswould be an impractical recommendation duringexercise.

From studies performed on rodents, it washypothesized that two separate groups of carbo-hydrate receptors exist in the mouth. Specifically,one group of receptor has been proposed to respondto ‘sweetness’ and the other to ‘polysaccharide’.This hypothesis is based upon observations that,given a free choice, rodents preferred maltodex-trin over sucrose, maltose, glucose and fructose atlow concentrations; sucrose was only preferred athigh concentrations.[58] Other species have alsobeen reported to possess ‘polysaccharide’ recep-tors in the mouth.[59] However, whether thesereceptors are present in humans is unknown.

Interestingly, in humans, it has been reportedthat both glucose (sweet) and maltodextrin (non-sweet) in the mouth activate regions in the brain,such as the insula/frontal operculum, orbito-frontal cortex and striatum, which are associatedwith reward.[25]

Furthermore, no central activation of possiblereward centres was reported when subjects mouthrinsed with an artificially sweetened solution.These findings suggest that there are separatereceptors that respond independently to ‘sweet-ness’ and carbohydrate.[25] The regions of thebrain that are activated by carbohydrate in themouth are also associated with reward and arebelieved to mediate behavioural responses torewarding stimuli.[60] To this end, a potentialrationale for mouth rinsing with carbohydratesolutions maybe that carbohydrate provides arewarding stimulus. This, in turn, impacts on be-haviour; for example, the self-selection of greaterexercise intensities during time-trial performances.Unfortunately, however, thus far the concentra-tions of the glucose and maltodextrin solutions



used in the functional MRI studies have beenmore concentrated (18%) than those solutionsthat have been reported to improve exercise per-formance (6–6.4%). Nevertheless, they providean important insight into the response of thebrain to the presence of carbohydrate in the oralcavity.

A recent study has provided evidence for a linkbetween carbohydrate in the mouth and skeletalmuscle function.[61] In this study, transcranialmagnetic stimulation of the primary motor cor-tex was used to investigate the effect of mouthrinsing with unsweetened carbohydrate on corti-comotor excitability and voluntary force produc-tion. The exercise protocol required 17 participantsto perform fatiguing isometric elbow flexions for30 minutes. The authors report that the ampli-tude of the motor evoked potential from the rightfirst dorsal interosseous increased by 9% whilstmouth rinsing carbohydrate, when the muscle wasvoluntarily activated.[61] However, it is importantto note that these observations were obtainedduring exercise that targeted a relatively smallgroup of arm muscles that had previously beenexercised to fatigue. To date, no studies have in-vestigated whether simply mouth rinsing with acarbohydrate solution can restore or maintainperformance following fatiguing ‘whole body’exercise such as running or cycling.

7. Practical Implications

Theoretically, mouth rinsing with a carbohy-drate solution without ingestion during exercisecould have several practical applications. For ex-ample, individuals who experience gastrointes-tinal complaints while ingesting sports drinksduring exercise may use mouth rinsing as a methodto seek a performance benefit.[62] However, thus far,it appears that mouth rinsing with a carbohydratesolution does not improve gastrointestinal com-fort in comparison with ingestion of appropriatevolumes of fluid during exercise.[19,28] Apart fromexercise performance, mouth rinsing with a car-bohydrate solution could cause individuals toself-select higher intensities during exercise with-out the ingestion of additional energy. Therefore,individuals who are attempting to lose weight

may benefit from a greater energy deficit achievedduring exercise in comparison with the ingestionof a carbohydrate solution, or even water. Fur-thermore, it would appear that this benefit couldbe gained without any increase in the perceptionof effort during exercise. However, it is importantto note that the only study to investigate theself-selected response to exercising at a set RPEwhilst mouth rinsing with a carbohydrate solu-tion was performed using recreationally activeyoung men.[44] Further studies are required toinvestigate if mouth rinsing with a carbohydratesolution could provide an ergogenic stimulus foran obese population embarking on an exerciseprogramme.

8. Conclusions

There are relatively few studies that have in-vestigated the effect of mouth rinsing with car-bohydrate solutions on endurance performance.The majority of these studies have been con-ducted in cycling. The available evidence suggeststhat mouth rinsing with carbohydrate solutionsroutinely during exercise can have a beneficialeffect on endurance performance of approxi-mately 1 hour in duration in fasted subjects. Al-though not all studies have reported a benefitfrom mouth rinsing with a carbohydrate solution,it is important to note that, thus far, no studieshave reported any adverse or negative effects onperformance.

The mechanisms to explain the ergogeniceffect of mouth rinsing with carbohydrate solu-tions during exercise include the (i) activationof reward centres in the brain; (ii) lowering ofthe perception of effort during exercise; and(iii) increase of corticomotor excitability. Whenmouth rinsing with carbohydrate solutions, thesemechanisms could theoretically lead to subjectsself-selecting an exercise intensity beyond thatselected when mouth rinsing with water or aplacebo solution. Future research may extendthese observations to include how the nutritionalstatus of the subject, as well as the concentrationof the carbohydrate solution along with the fre-quency of mouth rinses, influences the CNS andexercise performance.



Acknowledgements

No sources of funding were used to assist in the prepara-tion of this review. The authors have no conflicts of interestthat are directly relevant to the content of this review.

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Correspondence: Dr Ian Rollo, School of Sport, Exercise andHealth Sciences, Loughborough University, Ashby Road,Loughborough, Leicestershire, LE11 3TU, UK.E-mail: [email protected]



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