The Impact of a Sucrose Mouth-rinse in the Fasted and Fed-state on Time Trial Performance

It has been well established that carbohydrate (CHO) ingestion during prolonged, moderate- to high-intensity endurance-type exercise can delay the onset of fatigue and enhance exercise performance. Interestingly, several studies have reported that CHO ingestion can also improve performance during more intense exercise of short duration-less than 45-60 min. However, there is no apparent metabolic explanation for this improvement, because endogenous CHO stores should not limit exercise performance during short duration exercise tasks. To examine exogenous and endogenous glucose kinetics during high-intensity cycling exercise, Carter and colleagues tested 6 endurance athletes for 1 h at 75% Wmax while infusing either 20% glucose or 0.9% saline. Despite greater plasma glucose availability in the glucose-infusion trial, total CHO oxidation rates did not differ between treatments. Moreover, no performance benefits were observed after intravenous glucose administration. Consequently, the authors suggested that CHO ingestion during high-intensity exercise of short duration might exert its ergogenic effect by acting through the central nervous system, possibly mediated by glucose receptors in the mouth or gastrointestinal tract.

To test this hypothesis, Carter and colleagues investigated the impact of a carbohydrate mouth-rinse solution on 1-h time trial performance. The use of a mouth-rinse treatment, in which a CHO solution is spat out without swallowing, was chosen to remove any influence of the gut or exogenous CHO oxidation on performance. Participants were reported to cycle faster after a mouth rinse with a 6.4% maltodextrin solution at every 12.5% of the trial completed compared with a placebo rinse. The authors concluded that CHO mouth rinsing improves time trial performance and that the mechanism responsible might be an increase in central drive or motivation mediated by glucose receptors in the mouth. However in the former study, subjects were tested in a fasted state, which is impractical compared to how athletes normally prepare for training and competition.

In follow-up to the former study, we tested the performance effect of a carbohydrate mouth rinse 2 h after subjects ingested a CHO rich meal. From an evolutionary viewpoint, it can be speculated that the potential stimulating effect of glucose in the mouth might be of considerable impact under conditions when liver glycogen stores might be compromised. In agreement with this theory, there was no performance improvement with a CHO mouth rinse during time trial performance 2 h following the ingestion of a CHO -rich meal. However, as there are now studies demonstrating results both for and against any ergogenic effect of a CHO mouth rinse during short duration time trials it remains to be determined whether CHO mouth rinse improves performance during a 1 h time trial in the fasted state only. Therefore, the present study aims to investigate any potential ergogenic effect of a CHO mouth rinse during a 1 h time trial performance in both the fasted and fed state.