J Physiol 587.11 (2009) pp 2425–2426 2425

JOURNAL CLUB

Go rinse your mouth: a novelway to improve enduranceperformance?

Nicolas PlaceInstitut des Sciences du Mouvement et de laMedecine du Sport, Universite de Geneve,Geneva, Switzerland

Email: nicolas.place@unige.ch

Who would have ever believed that justrinsing one’s mouth with a sports energydrink and not swallowing but spitting it outwould improve endurance performance?Not me, at least not until reading the excitingstudy of Chambers et al. (2009), whichconsiderably challenges our current under-standing of exercise physiology. And theevidence of the mechanisms underlying thisunexpected finding is puzzling.

Peak performances in endurance sportsare realized by individuals who are ableto limit the decrease in speed or poweroutput (i.e. fatigue) while competing. Thefactors limiting endurance performanceof prolonged exercise, i.e. the causes offatigue, remain incompletely understood.Generally, fatigue is explained in termsof limits to oxygen transport capacityand/or muscle metabolic capacity and/orthe development of central and peri-pheral fatigue. Numerous experiments haveestablished that intramuscular processesare impaired during repeated muscleactivation. For example, intact single fibresrepeatedly stimulated show a decreasedforce production intimately related toalterations in Ca2+ handling, which canexplain the reduction in muscle forceproduction associated with prolongedexercise in humans under certain conditions(Allen et al. 2008). Although fatigue candevelop within muscle fibres, motor unitactivity is centrally modulated. Indeed, thedescending neural drive from the brainto the exercising muscles determines thestrategy in terms of motor unit recruitmentand de-recruitment during prolongedexercise, and therefore the force-generatingcapacity of the muscle (Kayser, 2003). Thus,the development of central fatigue wouldbe set above a safe level via integrationof multiple neural and humoral inputs to

maintain body homeostasis during repeatedor sustained muscle contractions. If thesafety margin is threatened, the functionaloutcome would be a reduced neural drive,leading to decreased muscle activity andthus a lowering of exercise intensity or taskfailure depending on the details of the task.

In a recent issue of The Journal of Physio-logy, Chambers et al. (2009) assessed theeffect of mouth rinsing with carbohydratesolutions on a cycle time trial of ∼1 hduration. Chambers et al. (2009) usedfunctional magnetic resonance imaging ina second set of experiments to identifybrain areas eventually activated by oralcarbohydrates. Trained cyclists performedtime trials simulated on a cycle ergometer(separated by at least 3 days) in afasted (> 6 h) state, where they had tocomplete a fixed workload (correspondingto 1 h at 75% maximal work) whencomplemented with glucose, maltodextrinor placebo (saccharin) solutions. Anartificial sweetener was used to reducesensory clues between solutions. Sub-jects were asked every 12.5% of thetime trial to rinse the solution in themouth for ∼10 s but not to swallowanything. Intriguingly, the results showedthat a 6.4% carbohydrate solution producedan astonishing improvement of 2–3% asindicated by the average power output,when compared to the placebo solution.This could equate to more than 1 mindifference in the longer time trials in theTour de France. The rate of perceivedexertion (RPE) was not different betweenconditions, which led the authors to suggestthat oral exposure to carbohydrate reducesthe perception of exertion for a givenworkload allowing them to increase poweroutput during the time trial. The authorshypothesized that the caloric contentdetected by ‘unidentified receptors’ mightmediate a neural response. Neuroimagingmeasurements then were performed whilethe subjects at rest rinsed their mouthin the same manner as they did whilecycling, showing that oral carbohydratesacted via supraspinal pathways duringexercise. Brain activation was similar withglucose and maltodextrin solutions, andbrain regions were activated, including theanterior cingulated cortex (ACC), that werenot activated with placebo. Previous reportsusing hypnosis during cycling exercise

have shown that ACC activity is closelyrelated to RPE; ACC blood flow decreasedwhen subjects cycled on a cycle ergometerunder hypnosis while perceiving a down-hill descent compared to a flat, level grade(Williamson et al. 2001). Furthermore,results from Marcora et al. (2009) showthat mental fatigue induced by a cognitivetask prior to exercise significantly reducesthe time to exhaustion when cycling at aworkload corresponding to 80% of peakpower output. The alteration in enduranceperformance was attributed to the higherRPE during exercise performed afterinduction of mental fatigue: a possibleimplication of the ACC, although this wasnot quantified, was suspected (Marcora et al.2009). The novel findings of Chambers et al.(2009) move us forward. They show, for thefirst time, that central fatigue developmentduring prolonged exercise may be post-poned or attenuated by activating brainregions that were possibly inactive orinhibited. Nevertheless, these results raiseseveral new questions: what are the receptorsthat convey sensory information from themouth to the brain? Would the sameresults have been observed in a competitionsituation, i.e. in a fed state? Would itbe possible to observe improvements inperformance in highly trained athletes?Indeed, one can calculate that performanceover a half-marathon (∼1 h duration) notrealized in the heat would be improved ifthe athlete does not swallow the solution bypreventing (i) any, even minor, weight gainand (ii) risks of gastrointestinal problems,frequently observed in running. Thus, itappears important to attempt to addresssome of these questions with the use of oralcarbohydrates combined with non-invasivemethods such as surface electromyographyand transcranial magnetic stimulation ofthe motor cortex.

In summary, Chambers et al. (2009) showan unexpected pathway exists to counter-act central fatigue development. Althoughcentral fatigue is difficult to quantify, thispioneer study again underlines the factthat the brain must always be consideredwhen studying fatigue. Further studies areneeded to extend these data and identifythe underlying mechanisms explaining thenon-metabolic, central positive impact oforal carbohydrates in order to optimizeendurance performance. Puzzled I may be,

C© 2009 The Author. Journal compilation C© 2009 The Physiological Society DOI: 10.1113/jphysiol.2009.172874

2426 Journal Club J Physiol 587.11

but I am also excited about the prospect offuture findings.

References

Allen DG, Lamb GD & Westerblad H (2008).Skeletal muscle fatigue: cellular mechanisms.Physiol Rev 88, 287–332.

Chambers ES, Bridge MW & Jones DA (2009).Carbohydrate sensing in the human mouth:effects on exercise performance and brainactivity. J Physiol 587, 1779–1794.

Kayser B (2003). Exercise starts and ends in thebrain. Eur J Appl Physiol 90, 411–419.

Marcora SM, Staiano W & Manning V (2009).Mental fatigue impairs physical performancein humans. J Appl Physiol 106, 857–864.

Williamson JW, McColl R, Mathews D, MitchellJH, Raven PB & Morgan WP (2001). Hypnoticmanipulation of effort sense during dynamicexercise: cardiovascular responses and brainactivation. J Appl Physiol 90, 1392–1399.

Acknowledgements

I would like to thank Professor B. Kayser andDr J. D. Bruton for reading this manuscript andtheir constructive comments.

C© 2009 The Author. Journal compilation C© 2009 The Physiological Society