Carbohydrate mouth rinsing – can you benefit from your carbohydrate drink if you don’t actually swallow it?

What is carbohydrate mouth rinsing?

It has been long understood that carbohydrate supplementation during endurance exercise is very useful in maximizing performance, particularly in events lasting over 90 minutes. Most carbohydrate supplements are used in the form of energy drinks, bars or chews. They optimise aerobic capacity by replenishing muscle glycogen stores, allowing glycolysis to generate energy for muscle contractions.¹

Interestingly, it has recently been suggested that it may be possible to get a benefit from carbohydrate even if it is not actually swallowed.² Carbohydrate mouth rinsing (CMR) refers to the process of swishing carbohydrate-containing solution around the mouth for a few seconds before expelling it.³ This article is going to investigate what evidence there is to support this, and what the potential mechanisms could be.

Where did the idea come from?

Carbohydrate stores in the body are enough to last and provide energy for about 90 minutes of exercise, after which it becomes a limiting factor of performance. However, it has been shown that carbohydrate supplementation can still improve performance even before our glycogen stores have been depleted.  

In 2004, a study by Carter et al.⁴ found that cycle ergometer time trial performance was no different when participants had glucose (a type of carbohydrate) infused into their blood, therefore avoiding contact with the mouth. This suggests that the performance enhancement via carbohydrate supplementation in events lasting under 90 minutes did not originate from an increased capacity to generate energy via glycolysis. 

A later study in 2004 by Carter et al.⁵ was the first to investigate whether the advantage could have originated from the carbohydrate’s presence in the oral cavity. Nine endurance cyclists, both male and female, were asked to complete a set amount of work in as little time as possible (i.e. a time trial). They rinsed a solution, either carbohydrate (maltodextrin) or water, around their mouth for a few seconds before spitting it out at regular intervals throughout the time trial. They found that the cyclists who rinsed with the carbohydrate solution completed the time trial about 3% faster than those who rinsed with water. Average power output during the time trial was also 3% greater in the carbohydrate group. 

It must be noted that this study had some drawbacks. Most importantly, four out of the seven participants could recognise which solution they were drinking, despite the carbohydrate drink being clear and non-sweet. This means it is possible that the differences in performance could have been a result of a placebo effect, and not because the carbohydrate rinse was actually advantageous. This means that it is very important to look at other studies before making a conclusion. 

What have other studies found?

When this investigation has been repeated by other researchers, there have been some mixed findings. It is likely that prior to an actual race/time trial, athletes will have eaten a large meal in order to maximise their carbohydrate stores and aid their aerobic capacity. Beelen et al.⁶ investigated this scenario and found that there was no performance advantage with CMR when the participants were in a fed state. This suggested that CMR may not be as useful in a real-life competition setting as first thought. 

In contrast, when using a similar protocol, another study found that time trial performance and mean power output was 7% and 6% greater (respectively) with CMR in a fed state. So perhaps there could be some advantage to gain from it.⁷

Findings from Lane et al.⁸ suggest some sort of intermediate. They found that while there was a greater increase in power output when CMR was done in a glycogen depleted state compared to a fed state (3.3% vs. 1.8% respectively), the greatest power output was achieved when participants used CMR in a fed state. Despite the effect of CMR being less in a fed state, it seems that it could be useful to achieve optimal performance.

What is the mechanism behind carbohydrate mouth rinsing?

Because the carbohydrate solution is not being ingested, the enhancements in cycling performance seen in the lab as a result of CMR must be due to neural activation. It has been put forward that there are receptors in the mouth, that when activated by carbohydrate, cause changes in emotional behaviour, reward systems and cognitive control.²

While using a similar protocol to the original CMR study, Chambers et al.⁹ monitored brain activity with an fMRI scan to locate active areas of the brain. They found greater brain activity in the striatum, anterior cingulate cortex and the orbitofrontal cortex. The striatum regulates goal-directed behaviour, so it seems that CMR allows individuals to push harder by reducing the perception of the workload.¹⁰ 

Are these findings applicable to real life?

Before concluding that CMR will help you win your next competition, it is crucial to appreciate the nature of the studies that have provided these findings. The majority of research relating to CMR has been lab-based, meaning that it is unknown how the findings would translate to uncontrolled settings. Just because CMR seems to be an effective way of improving cycling performance in the lab, doesn’t necessarily mean it would be useful in a real-life competition. Further field-based research is needed before we can draw any definitive conclusions, but it does appear to be a promising way of optimising performance.

References

1. Learsi SK, Ghiarone T, Silva-Cavalcante MD, et al. Cycling time trial performance is improved by carbohydrate ingestion during exercise regardless of a fed or fasted state. Scand J Med Sci Sports. 2019;29(5):651‐662. doi:10.1111/sms.13393
2. Brietzke C, Franco-Alvarenga PE, Coelho-Júnior HJ, Silveira R, Asano RY, Pires FO. Effects of Carbohydrate Mouth Rinse on Cycling Time Trial Performance: A Systematic Review and Meta-Analysis [published correction appears in Sports Med. 2019 Feb 22;:]. Sports Med. 2019;49(1):57‐66. doi:10.1007/s40279-018-1029-7
3. Baltazar-Martins G, Del Coso J. Carbohydrate Mouth Rinse Decreases Time to Complete a Simulated Cycling Time Trial. Front Nutr. 2019;6:65. Published 2019 May 15. doi:10.3389/fnut.2019.00065
4. Carter JM, Jeukendrup AE, Mann CH, Jones DA. The effect of glucose infusion on glucose kinetics during a 1-h time trial. Med Sci Sports Exerc. 2004;36(9):1543‐1550. doi:10.1249/01.mss.0000139892.69410.d8
5. Carter JM, Jeukendrup AE, Jones DA. The effect of carbohydrate mouth rinse on 1-h cycle time trial performance. Med Sci Sports Exerc. 2004;36(12):2107‐2111. doi:10.1249/01.mss.0000147585.65709.6f
6. Beelen M, Berghuis J, Bonaparte B, Ballak SB, Jeukendrup AE, van Loon LJ. Carbohydrate mouth rinsing in the fed state: lack of enhancement of time-trial performance. Int J Sport Nutr Exerc Metab. 2009;19(4):400‐409. doi:10.1123/ijsnem.19.4.400
7. Devenney S, Collins K, Shortall M. Effects of various concentrations of carbohydrate mouth rinse on cycling performance in a fed state. Eur J Sport Sci. 2016;16(8):1073‐1078. doi:10.1080/17461391.2016.1196735
8. Lane SC, Bird SR, Burke LM, Hawley JA. Effect of a carbohydrate mouth rinse on simulated cycling time-trial performance commenced in a fed or fasted state. Appl Physiol Nutr Metab. 2013;38(2):134‐139. doi:10.1139/apnm-2012-0300
9. Chambers ES, Bridge MW, Jones DA. Carbohydrate sensing in the human mouth: effects on exercise performance and brain activity. J Physiol. 2009;587(Pt 8):1779‐1794. doi:10.1113/jphysiol.2008.164285
10. Delgado MR. Reward-related responses in the human striatum. Ann N Y Acad Sci. 2007;1104:70‐88. doi:10.1196/annals.1390.002

Photo by Coen van den Broek on Unsplash