The carbohydrate mouth rinse and performance

By Asker Jeukendrup PhD
In 2002 three people met in an office at the University of Birmingham in the UK, to discuss the results of a study that was difficult to explain. The study had shown improvements in 40 km cycling time trial performance with the ingestion of a carbohydrate electrolyte drink versus water. This result was unexpected because carbohydrate availability was not thought to be limiting exercise of about 1 hour duration. The three people were Professor David Jones, the Head of School of Sport and Exercise Science at the University of Birmingham at that time, James Carter who just started his PhD student and myself. A series of studies were discussed to help explain the results of the time trial study.

We were going to infuse carbohydrate to see if carbohydrate availability was an important factor (it turned out later that it wasn’t because infusion of glucose during a time trial had no effect on performance) or whether there was some sort of oral carbohydrate sensing, that improved performance independent of carbohydrate delivery as a fuel. This idea resulted in the design of the first mouth rinse study.

James Carter went on to perform the study and showed that you can rinse your mouth, without swallowing any of the drink and still be faster! Here is a brief overview of the studies on the carbohydrate mouth rinse phenomenon so far.

This study was followed up by Ed Chambers, also a PhD student at the University of Birmingham at the time, who showed that a carbohydrate mouth rinse resulted in activation of certain areas of the brain, most notably the pleasure and rewards centres of the brain. So, it looked like a mouth rinse with a carbohydrate solution was sensed in the mouth, signals were sent to the brain and this somehow improved performance without any carbohydrate absorption, increase in blood glucose or carbohydrate oxidation. This finding is in line with an experience that most of us have had. We have all developed symptoms of hypoglycemia after a long walk, run or bike ride and felt weak and dizzy. All it takes to feel better is a bite in a candy or chocolate bar. Instantly we feel better, the tiredness is gone, the dizziness disappears and it is possible to continue exercise for a little longer. The effect may be short-lived, but it suggests that there is a neural signal that makes us feel better. It means that there must be connections from the mouth directly to the brain.

In James Carter’s study, cyclists were asked to perform 40km time trials, whilst rinsing their mouth with a carbohydrate solution (without swallowing any of it) or rinsing their mouth with a placebo solution. The carbohydrate used in that first study was a non-sweet maltodextrin solution, containing carbohydrate but virtually tasteless. The rinsing protocol was standardized: 5 seconds every 12.5% of the time trial, before spitting the drink out into a bowl. The effects on performance were the same as the previous study where carbohydrates were ingested.

Professor Romain Meeusen from the Free University in Brussels, Belgium, an expert in exercise and the brain, used to say: “exercise starts and ends in the brain”. Indeed, muscle contraction is initiated by a signal from motor neurons in the brain. During strenuous exercise many signals from the muscle muscle, joints, lungs, skin and core temperature receptors are sent back to the brain. Over time, these signals will be perceived as unpleasant and consciously or unconsciously this will lead to an inhibition of the motor output. This is what we call ‘fatigue’. Athletes tend to regulate their physical activity to keep their levels of discomfort within acceptable limits. It is not clear exactly which pathways are involved in this inhibitory activity, but it seems plausible that the signals arising from the carbohydrate receptors in the mouth are counteracting some of these negative signals. Perhaps the sensors are telling the brain that: “you have nothing to worry about, because energy is on its way!”.

The number of studies using mouth rinses has increased exponentially the last few years and at this point we have around 40 studies investigating the effects on exercise performance during endurance activities (lasting roughly between 30 min and 1:15h). As you can see in this figure trends are positive.

There are a few studies that found only small differences or no difference, but the majority of studies seems to suggest significant effects on performance. In this figure the darker blue columns are studies in which time to exhaustion was used as an outcome measure and it is well known that these tests exaggerate the actual performance outcomes, so not surprising to see that those studies show larger effects than the time trial studies. There are now also studies during shorter duration exercise, resistance exercise, intermittent exercise, skill-based performances and cognitive performance. From a practical point of view there are a number of important questions that we may be able to answer based on the available studies we have: