Carbohydrate supercompensation protocols, or carbohydrate loading, involve diet and exercise routines over the course of the week preceding an endurance exercise performance. The function of these dietary manipulations is to maximize muscle glycogen stores and improve performance in both time-to-fatigue and time trial protocols (Hawley et al. 1997). A high-carbohydrate meal consumed before an exercise performance, generally 3–5 h prior, has been shown to contribute to muscle glycogen stores and improve endurance performance (Hargreaves 2004). However, carbohydrate ingestion at this point may be more important for maximizing liver glycogen stores, especially if the enhanced diet is consumed at breakfast after an overnight fast. Ample liver glycogen is crucial to the maintenance of blood glucose levels necessary for prolonged exercise. The ergogenic effect of 3- to 5-day dietary supercompensation can then be enhanced with further carbohydrate ingestion in the hour before and throughout a prolonged bout of exercise (Jeukendrup and Gleeson 2010).

Carbohydrate ingestion during exercise is proposed to improve endurance exercise performance via a number of metabolic mechanisms (Jeukendrup and Gleeson 2010). Compared to a placebo, blood glucose levels and carbohydrate oxidation rates are maintained with carbohydrate ingestion, prolonging endurance performance (Coyle et al. 1986). Supplementing blood glucose with ingested carbohydrate decreases the rate of liver glycogen breakdown during exercise, sparing glycogen for later use as an energy source (Jeunkendrup et al. 1999). This metabolic pathway secondary to carbohydrate ingestion occurs for muscle glycogen use during running (Tsintzas et al. 1995), but possibly not cycling (Jeunkendrup et al. 1999).

Part of the ergogenic effect of carbohydrate ingestion in prolonging endurance exercise may be related to the role of carbohydrate substrate availability as a physiological exertional mediator. An increased carbohydrate availability, indicated by increased blood glucose levels and subsequent carbohydrate oxidation rates, may be an important peripheral mediator of perceived exertion (Pandolf 1982). The maintenance of neurological function and skeletal muscle contraction through enhanced glucose availability could help sustain exercise performance (Utter et al. 1997). The attenuation of RPE with carbohydrate ingestion as compared to a placebo has been shown to coincide with improved endurance performance in studies involving time trial (Burgess et al. 1991; Kang et al. 1996; Utter et al. 1997, 1999) and time-to-exhaustion protocols (Wilber and Moffatt 1992). The ergogenic effect has been most notable near the end of prolonged exercise involving 2–2.5 h of moderately high intensity (70–80 % VO₂max/peak) running or cycling (Burgess et al. 1991; Kang et al. 1996; Utter et al. 1997, 1999).

Historically, carbohydrate was not thought to induce an ergogenic effect during shorter-term exercise lasting 1 h or less. This occurred because little of the specific carbohydrate ingested during exercise was able to enter the bloodstream in time to prevent time-dependent decreases in blood glucose. This may explain why at least one recent study has confirmed the lack of ergogenic properties of acute carbohydrate ingestion for comparatively short-term, high intensity exercise (Timmons and Bar-Or 2003). However, carbohydrate ingestion during exercise has resulted in significant improvements in 1-h endurance and intermittent high-intensity exercise performance (Jeukendrup et al. 1997; Winnick et al. 2005). Ingested carbohydrate may interact with receptors in the mouth or stomach, inducing an effect on the central nervous system long before the carbohydrate reaches the blood. This effect, which appears to reflect on attenuation of fatigue perception, has been shown in studies of hypoglycemia in which significant relief is experienced almost immediately after carbohydrate ingestion (Jeukendrup and Gleeson 2010). During exercise, the stimulation of glucose receptors in the mouth may cause central nervous system effects such as the activation of CNS reward centers and an increase in central drive/motivation. This neurosensory response results in attenuated exertional perceptions and a less negative mood shifts that may have salutary effects on exercise performance independent of carbohydrate substrate availability (Carter et al. 2004; Jeukendrup and Gleeson 2010). Experimental evidence supporting such a central nervous system mechanism is provided via studies that employed a carbohydrate mouth rinse during exercise rather than actual ingestion (i.e., the carbohydrate drink is spat out rather than swallowed) (e Silva et al. 2014). The mouth rinse has resulted in similar improvements in 1-h time-trial performance as observed for traditional dietary carbohydrate ingestion (Carter et al. 2004; Pottier et al. 2010). However, not all studies support this neurological mechanism for the observed ergogenic effect (Beleen et al. 2009; Whitham and McKinney 2007). Acute carbohydrate ingestion or a carbohydrate mouth rinse may be more likely to affect the central nervous system when subjects are performing high-intensity exercise in a fasted state (Carter et al. 2004; O’Neal et al. 2013), a less practical situation with lower ecological validity than a fed, or postprandial, state (Beleen et al. 2009).

A number of recent investigations involving various types and intensities of exercise lasting 1 h or less have compared the perceptual and psychosocial effects of carbohydrate ingestion or mouth rinses to placebo conditions. For example, Backhouse and colleagues (2005) compared the effects of in-task consumption of a 6.4 % carbohydrate–electrolyte beverage to placebo during 2 h of cycling at 70 % VO₂max in endurance trained males after an overnight fast. Throughout exercise, RPE and FS ratings of AR were regularly measured. RPE was significantly lower in the carbohydrate than the placebo condition, but not until the 75th minute of exercise. There was an overall main effect reported for affect such that the carbohydrate condition yielded a more positive response compared to the placebo condition throughout exercise. The more positive mood with carbohydrate ingestion was evident beginning at the 30-min time point (Backhouse et al. 2005).

In contrast, O’Neal and colleagues (2013) compared the ergogenic effect of in-task consumption of a 6 % carbohydrate–electrolyte beverage to a non-caloric electrolyte beverage during 50 min of cycling at 60–65 % of heart rate reserve followed by three Wingate anaerobic tests. A Wingate test involves 30 s of cycling at maximum speed against a set resistance. The subjects were active young adults who reported to the laboratory at least 2 h after a meal (i.e., in a postprandial state). The study found no differences between beverage conditions for any performance outcome, momentary RPE during submaximal exercise or session RPE following the final Wingate test (O’Neal et al. 2013).

Winnick and colleagues (2005) compared the ergogenic effects of in-task consumption of a 6 % carbohydrate beverage to a placebo condition during 1 h of intermittent high-intensity exercise intended to mimic a basketball game. Subjects began the exercise protocol after a 12-h fast. Healthy college-aged men and women performed the exercise in 15-min quarters with 5-min breaks separating first-second and third-fourth quarters and a 20-min halftime period. Improved performances were noted in carbohydrate versus placebo condition during the last 15 min of exercise, including faster sprint times and higher jump heights. The Profile of Mood States questionnaire was used to assess overall mood changes as well as alterations in specific feelings. The questionnaire was administered before the simulated basketball game, at halftime, and after exercise. The questionnaire results indicated that overall mood declined significantly throughout exercise in the placebo condition in comparison to the carbohydrate condition. The between group differences were particularly notable for feelings of fatigue and vigor. Whole response levels for these mood constructs were maintained throughout exercise with carbohydrate supplementation (Winnick et al. 2005).