Jeukendrup, Asker E. MSc, PhD, FACSM
Traditionally carbohydrate has been viewed as a substrate for fuel metabolism during exercise, and this has been shown to result in improved endurance capacity (25). During prolonged exercise, carbohydrate feeding can help to maintain plasma glucose concentration and prevent hypoglycemia; it can maintain high rates of carbohydrate oxidation, spare hepatic glycogen, and in some cases delay muscle glycogen depletion. However performance benefits also have been observed during exercise lasting approximately 1 h. During exercise of this duration, hypoglycemia does not develop, blood glucose concentrations do not decrease (and may even increase), and glycogen depletion is not believed to be a performance-limiting factor. So during this type of exercise, the performance effects unlikely are to be explained by metabolic factors but likely are to involve the central nervous system (CNS).
In 2004, a landmark study by our group (10) showed that a simple carbohydrate mouth rinse (without ingesting the carbohydrate) resulted in similar performance benefits, providing indirect evidence for a “central effect.” It was proposed that carbohydrate is detected by receptors in the oral cavity and that afferent neural signals directly to the brain are responsible for observed performance improvements.
Since then, several studies have confirmed these findings, but there also are some contrasting findings (Table 1). The purpose of this short report is to review critically the evidence for the oral carbohydrate mouth rinse effect. We will discuss the available studies and establish the likelihood that the findings are a real effect or a placebo effect. In addition, we will touch on the practical implications of these observations.
Carbohydrate Ingestion and Performance
Although the effects of carbohydrate on prolonged exercise performance (>2 h) have been established since the 1980s (25), the observation that carbohydrate feeding also can improve performance during shorter duration exercise of higher intensity is relatively novel (23).
In a study by Jeukendrup and McLaughlin (23), cyclists performed a 40-km time trial with or without the ingestion of a carbohydrate-electrolyte solution, and they were approximately 1 min faster with the carbohydrate feeding: a performance improvement of 2.3%. This was a large and unexpected effect for which there was no explanation at the time. In addition, it takes time before carbohydrate is absorbed, transported to, and used by the muscle, and it can be calculated that only a small percentage of the carbohydrate ingested during a 1-h time trial actually can be used as a fuel. It was estimated that the amount of exogenous carbohydrate that was oxidized during a “40-km time trial” was around 15 g (23). This amount equates roughly to 0.25 g·min−1 or 1 kcal·min−1. During this exercise, the cyclists were expending more than 20 kcal·min−1, and most of this would have been from carbohydrate sources. The ingested 0.25 g·min−1 most likely would have been used at the expense of endogenous carbohydrate. The exogenous carbohydrate contribution was thought to be too small to provide additional fuel and result in the relatively large beneficial effect that was observed. The reason for the observed ergogenic effect was therefore unclear.
This observation did not occur in isolation, and in fact, it confirmed some earlier work. One of the earliest studies to show an effect of carbohydrate during exercise of 1-h duration was an investigation by Neufer et al. (34). Subjects cycled for 45 min at 77% V˙O2max, followed by 15 min in which they had to complete as much work as possible. It was found that performance was improved by 10% when 45 g of carbohydrate was ingested immediately before exercise compared to placebo. Anantaraman et al. (1) studied the effects of carbohydrate ingestion before and at regular intervals during a 60-min cycle in which subjects had to perform as much work as possible. In this study, performance was improved by almost 11% in the carbohydrate trial compared with placebo. Further studies were performed in hot conditions. Below et al. (4) exercised trained cyclists in 31°C (and 54% humidity) for 50 min at 80% V˙O2max followed by a time trial that lasted approximately 10 min. They observed a 6% improvement in time trial performance when carbohydrate was ingested throughout exercise. In a study by Carter et al. (9), subjects exercise to exhaustion at 73% V˙O2max in 35°C (and 30% humidity). Time to exhaustion increased by 14% in the carbohydrate trial compared with placebo. Thus there are several studies that confirm the work from our laboratory (23).
There are, however, also some studies that did not observe performance effects with carbohydrate feeding in these conditions (17,32,35). There are several possible explanations for the differences between the studies that did and did not find a positive effect on performance. The majority of the studies that did not find an effect actually observed a positive effect on performance, but this did not reach statistical significance. It could therefore be that the negative findings are a result of the lack of statistical power. However there may be another reason why the conclusions of these studies are different. The studies that showed a difference generally had a longer duration of preceding starvation prior to the performance trial. This possibility will be discussed in more detail later.
Carbohydrate ingestion seems to work when the exercise is longer than 30 min. Exercise shorter than that did not benefit from carbohydrate intake (24,38). It is likely that with this relatively short-duration and high-intensity exercise, time to absorb carbohydrate in these events is too short, and other causes of fatigue override any effect the carbohydrate ingestion might have. In summary, it seems that the majority of studies observed a performance improvement when carbohydrate is ingested during high-intensity exercise lasting approximately 1 h, and it is unlikely that the cause of this improvement is related to energy delivery to the working muscle.
Carbohydrate Mouth Rinse and Performance
In order to study the potential role of carbohydrate as a fuel, cyclists were asked to perform a 40-km time trial (11). On one occasion, they were infused with a glucose solution, and on another occasion, saline was infused (11). It was observed that when glucose was infused, blood glucose concentrations were twice as high and glucose disappearance (Rd glucose) also was doubled (11). However although glucose was taken up (presumably into the muscle) and oxidized (26), there was no effect on performance (11). This provides evidence for the thought that the effects of carbohydrate during this type of exercise are not metabolic, and thus, there must be an alternative explanation for the ergogenic effect.
In a follow-up study, cyclists were asked to repeat the 40-km time trial but only rinse their mouth with a carbohydrate solution without swallowing it (10). The carbohydrate used in this study was a nonsweet, tasteless maltodextrin solution. The rinsing protocol was standardized; subjects rinsed their mouth for 5 s with the drink and then spat the drink out into a bowl. The results were remarkable: performance was improved with the carbohydrate mouth rinse compared with placebo, and the magnitude of the effect was the same as we had seen in the early study with carbohydrate ingestion (23). It was unlikely that much, if any, carbohydrate had been absorbed from the mouth rinse, yet performance was improved by about 3% (10), very similar to the 2.3% improvement observed with carbohydrate feeding (23) (Table 1).
After this initial study by Carter et al. (10), several other studies reproduced these findings. Rollo et al. (43) reported that mouth rinsing with a carbohydrate solution increased total distance covered during a self-selected 30-min run in comparison with a color- and taste-matched placebo. This was the first running study that showed an effect and the first study where exercise was as short as 30 min. However it is important to note that this study was not a performance study. Participants were asked to run at speeds that equated to a rating of perceived exertion of 15, mouth rinsing with either a 6% carbohydrate or taste-matched placebo solution. In addition to recording self-selected speeds and total distance covered, the authors assessed the runners’ subjective feelings. The total distance covered was greater during the carbohydrate than during the placebo trial. The authors also observed that faster speeds selected during the first 5 min of exercise corresponded with enhanced feelings of pleasure when mouth rinsing with the carbohydrate solution.
In a follow-up study, Rollo et al. (41) studied the effect of a carbohydrate-electrolyte mouth rinse during a 60-min self-paced run. The treadmill was modified so that the runners could change velocity without the need for manual input or visual feedback from the runner (i.e., the treadmill velocity increases or decreases as the runner moves to the front or the back of the treadmill belt, respectively). Runners covered 211 m more distance during the carbohydrate trial (14298 ± 685 m) compared to the placebo trial (14086 ± 732 m), which was an improvement of 1.5%.
In another study, the influence of ingestion and mouth rinse with a carbohydrate solution on performance during a high-intensity time trial (approximately 1 h) was investigated in trained subjects (39). Subjects rinsed around the mouth or ingested a 6% carbohydrate solution or placebo before and throughout a time trial. In the mouth rinse conditions, time to complete the test was shorter with the carbohydrate mouth rinse (61.7 ± 5.1 min) than with placebo (64.1 ± 6.5 min) (39). Interestingly the investigators did not see a difference between placebo (62.5 ± 6.9 min) and carbohydrate (63.2 ± 6.9 min) conditions when drinks were consumed (39), which is in contrast to a number of other studies that observed performance improvements with carbohydrate ingestion during exercise of similar duration (1,4,9,23,34).
Further evidence for a performance-enhancing effect of an oral carbohydrate mouth rinse came from another study at the University of Birmingham in the United Kingdom. Chambers et al. (12) showed a 1.9% improvement in cycling time trial performance with a carbohydrate mouth rinse compared with a nonnutritive sweetened placebo.
Although all these studies confirmed the initial findings by Carter et al. (10), there are also a couple of studies that did not find this effect (3,50). There may be several reasons for these discordant findings. It is possible there was a lack of statistical power. The study by Whitham and McKinney (50), for example, had only seven subjects and used a performance measurement that may have been less reliable and/or sensitive. Runners had to adjust treadmill speed manually as opposed to the modified treadmill used by Rollo et al. (41) wherein runners could change velocity without need for manual input or visual feedback. Another explanation was offered by Beelen et al. (3) who gave their subjects a meal 2 h before the performance trial more in line with current recommendations (7,40). It was suggested that when fed, the effects of a mouth rinse are diminished. This will be discussed in more detail in one of the following sections.
Other Types of Exercise
Most studies have investigated the effects of a carbohydrate mouth rinse on endurance exercise performance in events between 30 and 60 min. Potential effects during supramaximal exercise, intermittent exercise, resistance exercise, or very prolonged exercise have not been studied extensively (Table 2). Chong et al. (15) studied impact of carbohydrate mouth rinse during a 30-s sprint on a cycle ergometer and concluded that the use of a 5-s mouth rinse with an isoenergetic amount of either maltodextrin or glucose is not beneficial for maximal sprint performance (Table 2). Painelli et al. (37) came to a similar conclusion for maximum strength or strength endurance performance (Table 2). No studies to date have investigated carbohydrate mouth rinse during very prolonged endurance performance, during intermittent exercise, or in the later stages of prolonged endurance exercise. Additionally no studies have investigated potential effects on cognitive functioning, decision making, reaction time, timing, and so on, all of which can impact performance in a variety of sports.
The Role of the CNS
It has been suggested that the alterations in power output commonly observed during a self-paced exercise task is under the influence of a “central governor” that controls the recruitment of motor units during exercise to ensure that homeostasis is maintained (27,36). The “central governor” is postulated to alter power output using afferent signals from peripheral physiological systems and receptors that detect changes in the external and internal environment (29). It is therefore plausible that during exercise, the positive central responses to an oral carbohydrate stimulus could counteract the negative physical, metabolic, and thermal afferent signals arising from muscles, joints, and core temperature receptors that are sent to the brain and consciously or unconsciously contribute to central fatigue and an inhibition of motor drive to the exercising muscles (49). For example, the dopaminergic system of the ventral striatum has been implicated in arousal, motivation, and the control of motor behavior (6), and increased activity of this pathway during exercise has been postulated to attenuate the development of central fatigue (13,16,21). This would suggest that the beneficial effects of carbohydrate feeding during exercise are not confined to its conventional metabolic advantage and may serve not as an energy substrate but as a positive afferent signal capable of modifying motor output.
Chambers et al. (12) used functional magnetic resonance imaging (fMRI) to investigate the responses of the human brain to a carbohydrate and placebo mouth rinse (12). The study revealed that tasting both sweet (glucose) and nonsweet (maltodextrin) carbohydrate solutions activated areas of the brain, such as the anterior cingulate cortex and ventral striatum, that were unresponsive to artificial sweetener (saccharin). Other neuroimaging investigations also have reported that an oral carbohydrate solution activates additional brain regions compared with an artificial sweetener (20,22), suggesting there may be taste transduction pathways that respond to carbohydrate independently of those for sweetness. This is in line with the observation that performance was enhanced compared with the control condition with both a sweet and a nonsweet carbohydrate (8).
Mechanisms and Brain Regions Involved
The receptors involved in signal transduction after a mouth rinse have not yet been identified. It is known that whenever food or drink is placed in the mouth, taste receptor cells (TRCs) are stimulated, providing the first analysis of potentially ingestible food (5,14,47). TRCs exist in groups of 50 to 100 in taste buds, which are distributed across different papillae of the tongue, soft palate, and epiglottis (45). Electrical activity initiated by a taste cue is transmitted to gustatory neurons (cranial nerves VII, IX, and X) that innervate the taste buds (46,48). This information converges on the nucleus of the solitary tract in the medulla and is subsequently relayed via 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 in the orbitofrontal cortex (47). 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 (28,44). The fact that many of these higher brain regions have been reported to be activated by oral carbohydrates and not nonnutritive sweeteners (12,20,22) may provide a mechanistic explanation for the positive effects of a carbohydrate mouth rinse on exercise performance. It is not known, however, what exactly is detected because most taste receptors respond to sweetness, not carbohydrate content.
Experimental data from rodent studies supports the existence of mammalian taste transduction pathways that respond to carbohydrate independently of those for sweetness. The mammalian sweet taste receptor combines two G-protein-coupled receptors, T1R2 and T1R3, which form a heterodimer that responds to both natural sugars and artificial sweeteners (33). It has, however, been suggested that homodimers of T1R2 and T1R3 might also exist and function as sugar detectors (25). Further research is warranted to fully understand the separate taste transduction pathways for various carbohydrates and sweeteners and how these differ between mammalian species, particularly in humans.
The Effect of the Preexercise Fasting Period
The studies that report a performance enhancement appear to involve subjects commencing exercise following an overnight fast (12,41) or in a postabsorptive state (>4 h) (10). Conversely it appears that investigations that fail to report an ergogenic action from a carbohydrate mouth rinse tend to be the studies where subjects received a carbohydrate-rich meal 2 to 3 h prior to exercise (3). There are also a few studies that do not support this view. Whitham and McKinney (50) showed no effects after a 4-h fast, and Pottier et al. (39) did see effects even when performance trials were completed 2 h after a meal.
Nevertheless it is likely that the difference in the preexercise fasting period influences the central neural response to an oral carbohydrate stimulus. An fMRI study compared the cortical responses to oral sucrose following an overnight fast (12 h) and after ingestion of a 700-kcal liquid meal (22). There was significantly greater activity within a number of brain regions, including the ventral striatum, amygdala, and hypothalamus, following a prolonged fast compared with in a postprandial state. In contrast, the cortical responses to tasting an artificial sweetener were very similar in both physiological conditions. Variations in the homeostatic signals that follow food intake, such as plasma ghrelin, PYY, and leptin, previously have been shown to modulate the activity of brain areas, such as the anterior cingulate cortex, ventral striatum, and hypothalamus (2,19,31). The central responses to oral carbohydrate, which are capable of modifying motor output, may therefore be dependent on the preexercise nutritional state of the body.
However, two studies now have compared directly the oral carbohydrate mouth rinse effects on performance after an overnight fast versus 2 h postprandially. In both these studies, performance effects were observed after an overnight fast as well as 2 h postprandially. However the magnitude of the performance benefit was greater after an overnight fast, which seems in agreement with the observations from fMRI studies.
So overall, although not all studies confirm the findings, the effect of a carbohydrate mouth rinse is rather convincing and seems to be present at exercise lasting 30 to 60 min. It is not clear whether shorter exercise can benefit, and it is unlikely that the mouth rinse effect can overrule some of the other factors that cause fatigue during more prolonged exercise.
Athletes may want to consider rinsing with CHO instead, which may provide a similar ergogenic benefit with a lower likelihood of possible gastrointestinal discomfort. It may not be necessary to ingest large quantities of CHO during exercise lasting 30 min to 1 h. In most conditions, the performance effects with the mouth rinse were similar to ingesting the drink, so there does not seem to be a disadvantage of taking the drink, although occasionally, athletes may complain of gastrointestinal distress when ingesting larger quantities. In these situations, small sips or a mouth rinse could be a solution. When the exercise is prolonged (2 h or more), CHO becomes an important fuel and it is essential to ingest CHO. It is highly unlikely that a carbohydrate mouth rinse can prevent fatigue when carbohydrate stores are being depleted, although it is possible that for a short period, ratings of perceived exertion may be reduced after a mouth rinse. A potential application would be during very prolonged events when athletes struggle with gastrointestinal discomfort and absorption may be impaired. In that situation, a carbohydrate mouth rinse may still provide some advantage. There are currently no studies that have looked into this, but anecdotally, athletes have started to use this approach. Future research will lead to a better understanding of the role of carbohydrate receptors and the underlying mechanisms of observed performance improvements after oral carbohydrate sensing.
Carbohydrate during exercise has been demonstrated to improve exercise performance even when the exercise is of high intensity (>75% VO2max) and relatively short duration (approximately 1 h). It has become clear that the underlying mechanisms for the ergogenic effect during this type of activity are not metabolic but may reside in the CNS. Carbohydrate mouth rinses have been shown to result in similar performance improvements, which suggest that the beneficial effects of carbohydrate feeding during exercise are not confined to its conventional metabolic advantage. Carbohydrate also may serve as a positive afferent signal capable of modifying motor output. These effects appear to be specific to carbohydrate and are independent of taste. Further research is needed to fully understand the separate taste transduction pathways for various carbohydrates and sweeteners as well as the practical implications in different sports.
The author declares no conflicts of interest and does not have any financial disclosures.
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