26 April 2011
In 1997 we did a research project that had a somewhat unexpected outcome. The purpose of the project was to find out whether drinking a carbohydrate solution during a 40 kilometer (25 mile) time trial would enhance performance. I was convinced that we were not going to see an effect. Surely the exercise was too short to see an effect?! The first drink was ingested at the start of the time trials and smaller amounts every 15 min thereafter. I calculated that perhaps 15 grams of carbohydrate could have been used as a fuel in the muscle during that time trial and did not expect that 15 grams (the energy in half a gel) would make any difference. Nineteen trained cyclists were recruited and they performed the time trial twice, once with a carbohydrate drink and once with a placebo drink that looked, tasted and smelled identical. To my big surprise the cyclists went over a minute faster with the carbohydrate drink!!
We had an exciting finding but I could not explain it. There were also other studies with similar results. We decided to investigate this a bit more and the first study we did was a study in which we had cyclist ride a 40 km time trial and this time we infused carbohydrate (glucose) directly into their blood. We used quite a high infusion rate, delivering a lot of energy. We compared this to the infusion of a salty water solution. The results were astounding: Despite delivering a lot of energy directly to the muscle, there was absolutely no difference in the performance! In the same study we also measured the uptake of glucose into the muscle and the utilisation of that glucose. It was found that the infused glucose was indeed taken up and utilised! Clearly, delivering energy could not be responsible for the performance improvement we observed earlier.
We then designed the next study. In this study we asked the cyclists to ride the same 40km time trial but this time they only rinsed their mouth with a carbohydrate solution. They did not swallow any of the drink. We compared this to rinsing with a placebo solution with the same taste. Remarkably, this time we found a 1 min improvement in performance again!
This was very exciting! We were starting to get an idea of a mechanism. The carbohydrate might have connected to certain sensors or receptors in the mouth that may have sent a signal to the brain.Somehow the brain must have been stimulated by this and allowed a greater power output and thus better performance. This is a phenomenon that most of us are actually quite familiar with. Imagine you have been out on a long ride without food, you have become hypoglycaemic, feel dizzy and weak. Someone then offers you a chocolate bar and you bite into it. Almost instantly you feel better! Your blood sugar has not changed, but you feel better.
The studies that followed used fMRI (functional Magnetic Resonance Imaging) to study the brain during a mouth rinse with carbohydrate. Indeed it was confirmed that certain areas in the brain were activated with a carbohydrate solution but not placebo. The centres activated included the pleasure centre in the brain and other areas similar to the areas activated with for example amphetamines and caffeine.
Several other studies at other Universities and different countries confirmed these findings and the general consensus is now that indeed carbohydrate mouth rinses can improve performance in some situations. It is likely particularly effective during 1 hour all out exercise but it has also been shown to be effective during a 30 min all out running bout. Recent studies have shown that muscle strength and very high intensity exercise is unaffected by a carbohydrate mouth rinse. Also, during more prolonged exercise when it is important to provide carbohydrate as a fuel it is unlikely that a mouth rinse would be very beneficial. Perhaps it can give a little boost temporarily but it would be advised in those conditions to ingest the carbohydrate.
So what does this mean in terms of recommendations? If your race/event is around 1 hour duration:
- Consider a mouth rinse with a carbohydrate solution
- The carbohydrate solution needs to be fairly concentrated 80-100 grams of carbohydrate per liter (34 Oz)
- Rinse at regular intervals (every 10-15 min)
- Rinse for at least 5 seconds every time
- If you can tolerate it, it is ok to consume the drink, if not you can spit it out
Asker Jeukendrup is a registered sport and exercise nutritionist. He is also a Fellow of American College of Sports Medicine and European College of Sport Sciences and a member of the Nutrition Society, Physiological Society, American Physiological Society, the New York Academy of Sciences and the American Diabetic Association. Asker is the Editor-in-Chief for the European Journal of Sport Science, member of the Advisory Editorial Board of the Journal of Sports Sciences, and served on the editorial board of the International Journal of Sports Medicine and Medicine and Science in Sport and Exercise. Asker is also the author of several books including High Performance Cycling and a Textbook on Sports Nutrition in collaboration with Prof Michael Gleeson. You can visit his website here.