Although the exact mechanisms are still not completely understood, it has been known for some time that carbohydrate ingestion during exercise can increase exercise capacity and improve exercise performance (for reviews see Jeukendrup [12, 15]). In general, during exercise longer than 2 h, carbohydrate feeding will prevent hypoglycemia, will maintain high rates of carbohydrate oxidation, and increase endurance capacity compared with placebo ingestion. It was initially believed that the duration of exercise had to be at least 2 h for carbohydrates to have an effect.

However, more recently, it has become clear that carbohydrate ingestion during exercise can improve exercise performance even during shorter duration, higher intensity exercise (for example, approximately 1 h at 75 % of maximal oxygen uptake; V O 2max ). The mechanism behind these performance improvements is completely different. In fact, it was demonstrated that when glucose was infused into the systemic circulation, this glucose was taken up at high rates but no performance effect was found [16]. This provides evidence that increasing glucose availability, as a substrate to the working muscle, has no effect during this type of activity. Interestingly, however, when individuals rinsed their mouths with a carbohydrate solution it resulted in performance improvements [11] that were very similar to the improvements seen with carbohydrate ingestion. There are now numerous studies that, on balance, demonstrate that this effect is real. Those studies are reviewed in several recent papers [10–14]. This would suggest that the beneficial effects of carbohydrate feeding during exercise are not confined to its conventional metabolic advantage, but may also contribute to a more positive afferent signal capable of modifying motor output [17]. These effects are specific to carbohydrates and are independent of taste [18].

It is known that whenever food or drink is placed in the mouth, taste receptor cells are stimulated and provide the first analysis of potentially ingestible food [19–21]. Taste receptor cells exist in groups of 50–100 in the taste buds, which are distributed across different papillae of the tongue, soft palate, and epiglottis [22]. Electrical activity initiated by a taste cue is transmitted to gustatory neurons (cranial nerves VII, IX, and X) that innervate the taste buds [23, 24]. This information converges on the nucleus of the solitary tract in the medulla, and is subsequently relayed by the ventral posterior medial nucleus of the thalamus to the primary taste cortex, located in the anterior insula and adjoining frontal operculum, and the putative secondary taste cortex located in the orbitofrontal cortex [19]. The primary taste cortex and orbitofrontal cortex have projections to regions of the brain, such as the dorsolateral prefrontal cortex, anterior cingulate cortex, and ventral striatum, which are thought to provide the link between gustatory pathways and the appropriate emotional, cognitive, and behavioral response [25, 26]. The fact that many of these higher brain regions have been reported to be activated by oral carbohydrates and not non-nutritive sweeteners [18, 27, 28] may provide a mechanistic explanation for the positive effects of a carbohydrate mouth rinse on exercise performance.

However, the receptors in the oral cavity that mediate these effects relating to performance have not yet been identified, and the exact roles of the various brain areas are not clearly understood. The taste receptor cells that are involved are not actually detecting taste but rather carbohydrate or energy.

Further research is warranted to understand fully the separate taste transduction pathways for various types of carbohydrates and how these differ between mammalian species, particularly in humans. However, it has been convincingly demonstrated that carbohydrate is detected in the oral cavity by unidentified receptors, and that this can be linked to improvements in exercise performance (for a review see Jeukendrup and Chambers [11]). The new guidelines suggested here take these findings into account (Fig. 1).

Fig. 1 The new carbohydrate intake guidelines. Carbohydrate intake recommendations during exercise depend on the duration of exercise. In general, carbohydrate intake recommendations increase with increasing duration. The type of carbohydrate may also vary as well as recommendations for nutritional training. These recommendations are for well trained athletes. Aspiring athletes may need to adjust these recommendations downwards Full size image

Practical Implications of the Mouth Rinse Studies

These results suggest that it is not necessary to ingest large amounts of carbohydrate during exercise lasting approximately 30 min to 1 h and that a mouth rinse with carbohydrate may be sufficient to obtain a performance benefit (Fig. 1). In most conditions, the performance effects with the mouth rinse were similar to ingesting the carbohydrate drink, so there does not seem to be a disadvantage of consuming the drink, although occasionally athletes may complain of gastrointestinal distress when consuming larger amounts. When the exercise is more prolonged (2 h or more), carbohydrate becomes a very important fuel, and to prevent a decrease in performance it is essential to ingest carbohydrate. As discussed in the following two sections, larger amounts of carbohydrate may be required for more prolonged exercise.