No need to swallow?  //  Rinse your mouth out with sports drinks – it beats water…!

23 Nov 2008 Posted by
Once again, apologies for the absence – our posting frequency has fallen right off the charts, and we’re down to an embarassing one per week! The end of the year tends to bring with it a log-jam of work, as all the outstanding work from the previous 10 months is suddenly squeezed into the final two months of the year! Combined with “post-fatigue”, I’m tempted to say that like Olympic athletes, we’re in the off-season period!

Also, very little seems to be happening in the world of sport, unless you count cricket and rugby, but there are already about a billion blogs and articles being written about those “small” sports, so I thought it best to leave them alone!

Instead, I came across a very interesting study this past week, which is actually a repeat of a study that was first published in 2004, in which simply rinsing the mouth with a glucose-containing drink (like Gatorade for those in the USA, Powerade/Energade for SA) improved performance during a time-trial lasting about one hour. You can see the abstract for the orginal 2004 study here, and the latest one here.

The 2004 study – rinse with a carbohydrate drink, not water

In the 2004 study, cyclists had to complete a given amount of work in as fast a time as possible. This is important to note, because as we’ve discussed previously in our series on fatigue, the MODE of performance trial can influence the conclusions drawn quite substantially. In both these studies, the cyclists were able to freely select their work-rate and slow down or speed up in response to the swishing around of the carbohydrate drink or the water.

The figure below shows the performance part of the research study, and summarises its results:


So, the rinsing with the malto-dextrin solution improvement performance by almost 3%, quite independently of the usual metabolic effects of glucose on performance. That is, our usual thinking would be that glucose is superior because it is rapidly metabolised, converted to energy, and delays the onset of fatigue. This study clearly challenges that, for a number of reasons.

First, the performance trial is quite short – the theory that glucose ingestion has a profound effect on performance is particularly true for long-duration exercise, where the depletion of liver or muscle glycogen may be possible. That said, there is some evidence that glucose ingestion improves performance even during much shorter exercise trials, which is difficult to explain because only a small part of your energy comes from that ingested glucose when you do shorter, high intensity training – that was one of the reasons this study was done in the first place.

Also, in this study, the performance of the cyclists was better on CHO from the very first quarter of the trial. So when performance is broken down, it turns out that you go faster within the first few seconds, and that clearly has nothing to do with metabolic effects.

Another important point is that the malto-dextrin solution was chosen because it is colourless and non-sweet, which means it’s less likely to be obvious to the subjects when they’re on it – this would otherwise introduce the possibility of a large placebo effect. It turns out that some subjects did pick up the difference: Four of the nine cyclists guessed they were on the CHO-trial, based on taste and viscosity. Of these four, three improved, one did worse. I guess the placebo effect can never be ruled out, but it does seem to have been controlled as much as possible in the study.

Increased central drive – feedback from the mouth, output to the muscles?

So what then is the explanation? Before that, a couple of cautionary words. Firstly, this study did NOT show that rinsing the mouth is better than swallowing the solution. It simply compared rinsing with CHO to rinsing with H2O. It is not, therefore, an excuse not to drink glucose-containing drinks during exercise! (the second study discusses this a bit more).

However, what it does do is help us understand how shorter exercise might be improved by carbohydrates. The authors of this study speculated that there are receptors in the mouth, which are stimulated by the CHO-drink (but not by water), and then trigger centres in the brain that then increase the central drive for exercise.

Those who’ve followed our site for a while now will recall our Fatigue Series, in which we described how it is the brain that regulates performance, by controlling muscle activation in response to various cues (like heat, oxygen availability etc.). This CHO-finding would fit in with that kind of thinking – the brain receives information from the mouth that glucose is available, and it allows an increase in muscle activation.

The ultimate outcome of this is that the perceived exertion is regulated, and performance is a function of that regulation. In other words, athletes pace themselves according to how they feel (this is obvious, I’m sure), and the rinsing with CHO changes that perception, which allows them to cycle faster, and thus achieve the same perception at a higher power output (again, read the Fatigue Series for more discussion on this)

If you think about it, this is really the ONLY way to explain how power output and hence performance are improved when you swish a drink around in your mouth. It cannot be metabolic, only sensory. And sensation (or perception) is, as you may recall, the key component of this regulatory system where the brain monitors perceived exertion and performance. Think also of the last time you went for a run or ride in very hot conditions, and what the first drink you had afterwards felt like – it’s instantly pleasurable, and that’s because of feedback to the brain, a different kind of physiology from the classic thinking that something in the body has gone “wrong”.

The second study: Repeating the finding

This one study could be dismissed as a spurious finding, but it has just been repeated, in a study that has yet to be published but is available online. It followed an almost identical method to the first, so we won’t bore you with the details, but will point out that it differs in that this second study also looked at ingestion of CHO and H2O.

The figure below summarizes the design and main finding:


So, a repeat of the finding that rinsing with the CHO-solution improves performance compared to rinsing with water (this time by almost 4%). There was not, however, a difference between ingesting the same CHO-mixture and drinking it, and nor was there a difference between rinsing and ingesting.

A couple of interesting points about this study – they did not use a malto-dextrin solution, but rather a commercially available sports drink, which means the sweetness/taste issue could be a factor. Quite why drinking the solution did not have an effect is difficult to know – it would have to pass the mouth and presumably trigger the same sensations as if it was rinsed. That remains an unknown.

However, the key point, a repeat of study 1, is that the subjects rode at the same Perception of Effort (or RPE), for a higher power output in the CHO-rinsing trial. Once again, this would suggest that the brain picks up the “reduction” in effort as a result of the glucose in the mouth, and the cyclist rides harder, with the result that the perception increases to the desired level at a higher overall performance level. It supports the notion that the sensation and perception are the regulators of performance.

A third study in the “sandwich” – no effect in runners

Finally, I must make mention that a study in 2007, between these two, did the same thing in runners and found NO DIFFERENCE – that is, rinsing with malto-dextrin did not improve performance in a 45-minute running trial. Is it a running vs. cycling difference? Or something related to habitual diet, a placebo effect? I suspect it may be related to the intensity, and the “cluster” of signals that determine how the athlete paces themselves. Running and cycling differ with regards to the eccentric loading on the muscles and joints, and I suspect this might be a key difference, negating the effect of the signals from the mouth. Difficult to know.

Perhaps a discussion for another day!

We’ll hopefully post sooner than next weekend!