This is an excerpt from Sport Nutrition-3rd Edition by Asker Jeukendrup & Michael Gleeson.
One hour of sustained or intermittent high-intensity exercise is not limited by the availability of muscle glycogen stores given adequate nutritional preparation. Therefore, evidence of enhanced performance when carbohydrate is consumed during a variety of such exercise protocols has been perplexing. Findings of a lack of improvement in the performance of a 1-hour cycling time trial protocol with glucose infusion but benefits from carbohydrate ingestion (Carter, Jeukendrup, and Jones 2004) created an intriguing hypothesis that the central nervous system might sense the presence of carbohydrate via receptors in the mouth and oral space, thereby promoting an enhanced sense of well-being and improved pacing. This theory was subsequently confirmed by observations that simply rinsing the mouth with a carbohydrate solution can also enhance performance of the cycling bout (Carter, Jeukendrup, and Jones 2004). A number of studies have now investigated this phenomenon, including several in which brain imaging technology (fMRI) tracked changes in various areas of the brain related to carbohydrate mouth sensing (Chambers, Bridges, and Jones 2009). In these studies, both sweet and nonsweet carbohydrates were shown to activate regions in the brain associated with reward and motor control. There is robust evidence that in situations when a high power output is required over durations of about 45 to 75 minutes, mouth rinsing or ingesting very small amounts of carbohydrate play a nonmetabolic role in enhancing performance by about 2% to 3% (figure 6.6). Not all studies have reported this effect though, possibly because a carbohydrate-rich preevent meal is associated with a dampening of the effect (Jeukendrup and Chambers 2013a).
Mouth rinse studies were initiated to study the mechanisms, not to develop a new strategy whereby athletes rinse their mouths with a carbohydrate solution and then spit it out. Ingesting the carbohydrate solution works just as well. There may be a few situations in which a mouth rinse can be practical, such as when an athlete cannot ingest any carbohydrate because of stomach problems or when energy intake needs to be restricted. Whether the central nervous system effects of glucose feeding are mediated by sensory detection of glucose or perception of sweetness is not known, but studies with placebo solutions containing artificial sweeteners with identical taste to glucose solutions suggest that sweetness is not the key factor (Jeukendrup 2013a, 2014). Brain imaging studies also show that increased brain activity is specific to carbohydrates.
Fig 6.6 Overview of carbohydrate mouth rinse studies. The gray bars indicate time to exhaustion tests as opposed to time trials. The magnitude of performance change is influenced by the type of performance test (i.e., time to exhaustion gives exaggerated changes; time trials provide more realistic estimates of the effects as they are typically more representative of real sporting events). ns = nonsignificant; TTE = time to exhaustion.
© Asker Jeukendrup. www.mysportscience.com