Spit or swallow? Carb mouth rinse and performance

It is well accepted that carbohydrate drinks can enhance performance. But research suggests that actually swallowing your favourite sports drink might not always be necessary. Bizarre as it may sound, you can rinse your mouth, spit the drink out on the floor and still be faster! Here is a brief overview of the studies on the carbohydrate mouth rinse phenomenon so far.

Almost 2 decades ago, we reported that carbohydrate feeding can also improve performance during shorter duration exercise of higher intensity (1). We studied cyclists who performed a 40-km time-trial with or without carbohydrate and, on average, they were 1 minute faster with carbohydrate. This was a large and unexpected effect for which we did not have an explanation at the time.

During exercise of one hour or less duration, you don’t run out of carbohydrate and your blood sugar concentration does not drop, so why would ingesting carbohydrate during exercise have an effect? Also, by the time carbohydrate is being utilized in the muscle to any significant degree, the exercise is over.

One explanation could be that the carbohydrate somehow influences the brain. For example, there is evidence that taste influences mood and it may also influence the perception of effort. An interesting observation provides support for a central nervous system effect. When you are hypoglycemic after a long ride or run without food and you are feeling weak and dizzy, all you have to do is bite in a candy bar to feel better. Almost instantly the feelings of weakness and dizziness are reduced, and you feel better long before the carbohydrate has reached the blood circulation and the brain. This means that there must be connections from the mouth directly to the brain.

This means that there must be connections from the mouth directly to the brain

Under direction of James Carter - who was doing his PhD at the University of Birmingham (UK) - in the next study, cyclists were asked to repeat the 40km time trial but only rinse their mouth with a carbohydrate solution without swallowing any of it (2). The carbohydrate used in this study was a non-sweet maltodextrin solution (a tasteless carbohydrate). The rinsing protocol was standardised. Subjects would rinse their mouth for 5 seconds with the drink and then spit the drink out into a bowl. They were not allowed to swallow any of the drink.

The results were remarkable: performance was improved with the carbohydrate mouth rinse compared with placebo and the magnitude of the effect was the same as the effect we had seen in the early study with carbohydrate ingestion! They were about 1 minute faster, even though none of the carbohydrate had actually entered the body (no carbohydrate is absorbed in the mouth). Perhaps the carbohydrate in the mouth rinse had connected with a receptor in the mouth that subsequently sent a signal to the brain. This signal probably informed the brain that food was on its way and this reduced the perception of effort, making the exercise task easier.

Performance was improved with the carbohydrate mouth rinse compared with placebo

Since then numerous carbohydrate mouth rinse studies have been performed and performance has been measured. The results of studies investigating endurance performance of 30-75 min duration have been summarised in the figure above. Although some studies did not find differences (at least not statistically), the pendulum seems to swing in a positive direction and most studies show positive effects of a carbohydrate mouth rinse.

In future blogs I will discuss some studies in more detail and we will discuss how the carbohydrate mouth rinse works, why not all studies show positive effects and many practical questions will be addressed as well. For now, the main conclusion is that carbohydrate whether it is ingested or just used as a mouth rinse can improve all-out exercise performance of approximately 1h duration. Several reviews on the topic have been written (3,4,5,6).