Carbohydrates are the main energy source in the body. Almost half of the dietary caloric intake is provided by carbohydrates. Athletes should increase carbohydrates intake to ±60% of total calories and during periods of intense training to 70% (McArdle et al., 2016). Carbohydrates are mainly found in plant foods and grains. As previously explained, all macronutrients can serve as energy fuel in the tissues. However, the brain is an exception since it can only use glucose or ketones as fuel. To ensure a constant glucose supply to the brain, blood glucose is closely regulated in the body. Glucose is constantly broken down and stored to guarantee that blood glucose is maintained.
Carbohydrates are categorised based on the complexity of their structure: monosaccharides, disaccharides, and complex carbohydrates. Monosaccharides and disaccharides are called simple sugars and provide a rapid source of energy. Monosaccharides have one sugar molecule and can be absorbed in the small intestine without further digestion. There are three types of monosaccharides: glucose, fructose, and galactose. Disaccharides consist of a combination of two monosaccharides and the bond must be broken down to make absorption possible. Three disaccharides are found in the diet: maltose (glucose + glucose), sucrose (glucose + fructose), and lactose (glucose + galactose). Each disaccharide has a specific enzyme that can break the bond between the monosaccharides. Complex carbohydrates have more health benefits than simple sugars and include fibres and other polysaccharides, like starches and glycogen. Simple sugars, like mono- and disaccharides, are quickly absorbed and result in insulin-peaks and high blood glucose levels. Starch and glycogen are built from multiple simple sugars and can be digested by enzymes and absorbed in the small intestine. Digestion and abortion take more time to be absorbed compared to simple sugars, resulting in a steadier release of glucose into circulation. Fibres cannot be broken down by enzymes but are fermented by microbiota in the large intestine instead (Goodman, 2010). Fermentation products like short-chain fatty acids (SCFAs), can be used for energy production as well.
The absorption of monosaccharides in the small intestine is facilitated by transporters. Sodium-dependent glucose transporter 1 (SGLT1) moves glucose and galactose across the membrane into the intestinal absorptive cell called an enterocyte, and glucose transporter 5 (GLUT5) facilitates fructose uptake from the lumen (see Figure 3) (Harada & Inagaki, 2012). Fructose is mostly converted into glucose inside enterocytes and the remaining fructose diffuses to the bloodstream via glucose transporter 2 (GLUT2). GLUT2 also transports glucose and galactose to the blood. Glucose can enter the enterocyte without sodium, but the binding affinity of glucose with SGLT1 is much higher in the presence of sodium (Goodman, 2010).
After absorption, galactose and fructose first need to be converted into glucose by the liver before they can serve as fuel or be stored as glycogen or triglycerides. Glycogen is a branched polysaccharide from glucose that is mainly stored in the liver and muscle tissue. Glycogen has a lower energy density compared to triglycerides, because it retains more water. Liver glycogen is used to maintain blood glucose level and muscle glycogen to provide energy for muscle contraction. Glycogen synthesis is influenced by blood glucose levels, degree of glycogen depletion, and hormones influencing glucose transport into muscle cells, like insulin and glucocorticoid (Murray & Rosenbloom, 2018). Regular exercise increases glycogen storage capacity, positively affecting exercise performance by delaying fatigue (Balsom et al., 1999). Post-exercise carbohydrate ingestion results in an additional increase in glycogen storage capacity, called supercompensation (Hingst et al., 2018).
The storage of glycogen in the liver and muscle is limited. During prolonged exercise, muscle glycogen depletion and reduced blood glucose concentrations (hypoglycaemia) are the main causes of fatigue. Ingestion of carbohydrates during prolonged endurance exercise (>2h) is known to improve endurance capacity (time to exhaustion) and performance (Koopman et al., 2004). Ingestion of carbohydrates delays muscle glycogen depletion and ensures stable blood glucose concentration. The adviced amount of carbohydrates to ingest depends on the type of sport and exercise duration and intensity. In the following paragraph we will discuss the carbohydrate needs of endurance and team sport athletes.
Carbohydrates and fat are the primary sources for the energy metabolism during endurance exercise. Although the skeletal muscle tissue can both oxidize carbohydrates and fat, the relative contribution to energy metabolism depends on the intensity and the duration of the exercise. When the intensity of exercise increases, the contribution of carbohydrates becomes more important. Therefore, when exercising, it is important to have enough carbohydrates available for oxidation. One way to establish the carbohydrate availability is the ingestion of carbohydrates during prolonged endurance exercise. It has been known that carbohydrate ingestion during prolonged (>2h) moderate-to-high intensity exercise can significantly improve performance. Athletes are advised to ingest carbohydrates when performing exercise longer than 2-3 hours at a rate of 60 g·h-1 (~1.0-1.1 g·min-1) (Van Loon et al., 2001). This rate ensures maximum exogenous carbohydrate oxidation when the carbohydrates come from one source. The limiting factor for this carbohydrate oxidation rate is SGLT1 saturation. SGLT1 is responsible for the transport of glucose inside the intestine (Harada & Inagaki, 2012). The carbohydrate oxidation rate can be increased by the ingestion of multiple transportable carbohydrate sources, like fructose, which uses a different transporter, to ±1.75 grams of carbohydrates per minute. Moreover, mixed carbohydrates ingestion is less likely to cause GI discomfort during exercise and improves fluid delivery (A. Jeukendrup, 2013; A. E. Jeukendrup, 2008, 2010). Well-trained endurance athletes can this way consume up to 90-120 g per hour (e.g., 1.2 g·min-1 glucose plus 0.6 g·min-1 of fructose) (Urdampilleta et al., 2020). Carbohydrate ingestion is also important during intermittent/team sports. Although they have different needs based on exercise duration and intensity, team sporters ingesting 30-60 grams carbohydrates per hour show enhanced performance (Oliveira et al. 2017, Williams & Rollo, 2015). Furthermore, athletes who exercise multiple times per day benefit from carbohydrate ingestion during exercise to replent glycogen stores (Oliveira et al. 2017, Williams & Rollo, 2015).
Carbohydrate ingestion does not only contribute to delaying fatigue by providing fuel, but mouth rinsing with carbohydrate-electrolyte beverages improves self-pace time-trial performance as well. No clear metabolic explanation is found yet, but it is hypothesised that carbohydrate and electrolyte sensation already have a ‘non-metabolic’ or ‘central effect’ on endurance performance (Rollo & Williams, 2011).
Before cells can either store or use glucose, glucose must be transported over the cell membrane with insulin-dependent glucose transporter GLUT4. During and shortly after intense exercise, GLUT4 translocation to the membrane is increased, which enhances the number of transporters present on the membrane and thus enhances the uptake capacity of glucose. This attributes to increased carbohydrate oxidation during exercise. Once inside the cell, glucose oxidation starts with the glycolysis outside the mitochondria. When oxygen is available, the oxidation proceeds aerobically with the TCA cycle and oxidative phosphorylation in the mitochondria. Glucose is the main macronutrient that can be metabolised without oxygen, because it can produce lactate instead of entering the TCA cycle. Furthermore, carbohydrate oxidation yields more ATP per volume of oxygen compared to lipid oxidation (Spriet, 2014a). High-intensity exercise requires energy rapidly and in high amounts which can be provided only by carbohydrate oxidation, making it the preferred fuel during high-intensity exercise (Achten & Jeukendrup, 2004).
• Carbohydrate consumption during prolonged exercise reduces glycogen use, increases blood glucose concentrations and improves performance.
• Intake of 30-60 g of carbohydrates per hour is beneficial for team athletes.
• Endurance athletes are recommended to ingest 60 g of carbohydrates per hour, competing 1-1.5 hours per match.
• Well-trained endurance athletes can ingest 90 g of carbohydrates per hour that are derived from multiple carbohydrate sources in athletes competing > 2 hours.
• Strength athletes do not benefit from carbohydrate consumption during exercise.