A Review of Carbohydrate Supplementation Approaches and Strategies for Optimizing Performance in Elite Long-Distance Endurance

The process of digesting and assimilating consumed carbohydrates into muscle and liver tissue as glycogen stores requires a minimum of four hours [13]. Therefore, pre-competition carbohydrate supplementation can be categorized into three phases: (1) carbohydrate loading to maximize glycogen storage, (2) the final pre-competition meal, and (3) supplementation between the final meal and the start of the competition.

It has been established that for every gram of glycogen stored, approximately 2.7 g of water are retained, which results in an increase in body weight during the glycogen loading phase. This increase may, in turn, lead to a slight reduction in exercise efficiency. Therefore, pre-competition carbohydrate supplementation is closely linked to the competition duration [14]. For events with a duration of less than 90 min, the standard recommendation is to consume 6–12 g·kg⁻¹ of carbohydrates during the 24 h prior to the competition, with the aim of restoring glycogen levels to their normal state. For prolonged events exceeding 90 min duration, a glycogen-loading strategy is recommended, initiated 36–48 h before competition with a daily carbohydrate intake of 10–12 g·kg⁻¹ [15]. Additionally, for the athletes competing in afternoon or evening events, attention to their carbohydrate intake at breakfast should be paid. Research suggests that even if lunch provides sufficient carbohydrates, a low-carbohydrate breakfast may reduce liver glycogen storage, negatively affecting endurance performance [16].

Typically, elite long-distance endurance athletes prepare for glycogen loading 10 days before the major event. During the 7–10 days before the competition, they primarily consume natural foods rich in carbohydrates, with three meals per day, avoiding additional carbohydrate powders or gels. During non-specialized training sessions, athletes may also reduce or eliminate carbohydrate drinks. In the final week (1–6 days before the event), the athletes increase their carbohydrate intake, including carbohydrate-rich foods at each meal, and resume carbohydrate drinks during training, along with supplementation of carbohydrate powders or gels. To avoid gastrointestinal discomfort, the total amount of carbohydrate supplementation is usually increased gradually. It is important to note that the carbohydrate intake should not exceed 75 g in the final pre-race meal [14], which should be consumed at least 2 h before the competition [17]. During the period from the final pre-race meal to the start of the race, including the time spent traveling to the competition venue, warm-up, and waiting in the check-in area, athletes often choose to consume small amounts of habitual carbohydrate solutions to maintain stable blood glucose levels and optimal hydration. It is recommended not to consume high doses of carbohydrates 60 to 30 min before the race. During the wait in the check-in area, athletes may rinse their mouths with carbohydrate drinks.

Carbohydrate distribution within skeletal muscle varies, with glycogen stored in the subsarcolemmal, inter- and intra-myofibrillar compartments, which account for 5–15%, 75%, and 5–15% of total muscle glycogen, respectively. During exercise, glycogen utilization in the intra-myofibrillar and subsarcolemmal regions takes precedence over that in the inter-myofibrillar region. Glycogen loading increases subsarcolemmal glycogen above normal levels, thereby sparing intra-myofibrillar glycogen use and improving endurance performance [4]. The final pre-competition meal and small doses of carbohydrate supplementation before the start avoid premature glycogen utilization and large fluctuations in blood glucose before the competition. A sudden rise or excessively high levels of blood glucose can lead to increased insulin release, which may impair fat oxidation, cause hypoglycemia, or trigger rebound hypoglycemia during the competition [15]. Most studies suggest that athletes should begin the competition with a normal blood glucose range [18].

For endurance events lasting longer than 150 min, athletes are generally advised to ingest carbohydrates at a rate of 60–90 g·h⁻¹. For events lasting between 60 and 150 min, athletes may choose to rinse with carbohydrate drinks or consume carbohydrates at a rate of 30–60 g·h⁻¹. In events lasting ≤60 min, rinsing with carbohydrate-containing drinks is typically recommended. The total amount, timing, and frequency of carbohydrate supplementation are primarily determined by factors such as endogenous glucose production, exogenous carbohydrate oxidation rates, exercise intensity and duration, and environmental conditions [19].

Endogenous glucose production mainly occurs through hepatic glycogenolysis and gluconeogenesis, with a basal rate of approximately 0.1 g·kg⁻¹·h⁻¹. However, during periods of exercise at 55% of maximal output power and lactate threshold intensity, this rate can increase to 0.36 g·kg⁻¹·h⁻¹ and 0.48 g·kg⁻¹·h⁻¹, respectively [7]. The rate of exogenous carbohydrate oxidation exhibits a non-linear relationship with carbohydrate intake, suggesting that within a certain intake range, oxidation increases as the intake rises. However, beyond a certain intake threshold, the oxidation rate plateaus [20]. Thus, for endurance events exceeding 150 min, a carbohydrate intake rate greater than 90 g·h⁻¹ may not lead to additional performance improvements [21]. Conversely, in circumstances where glycogen stores are insufficient, an intake rate exceeding 90 g·h⁻¹ has been demonstrated to enhance endurance performance [22]. For events with a duration shorter than 150 min, there is an absence of evidence supporting the notion that a carbohydrate intake rate exceeding 60 g·h⁻¹ results in additional performance enhancements [23]. Regarding the timing and frequency of supplementation, it is primarily determined by individual glucose kinetics and the available aid conditions during the competition. The time required for blood glucose levels to change following the intake of carbohydrate drinks is at least 15 min [24]. In a marathon, athletes can only retrieve their own drinks at fixed points every 5 km. In a 10 km swimming race, aid stations are provided every 1.6 km. In race walking events, aid stations are available every 1 km. Therefore, in major Olympic events, athletes should aim to supplement each time they pass an aid station.

During competition, as muscle glycogen levels decline, the continuous intake of exogenous carbohydrates has been shown to maintain glucose oxidation rates, preserve glycogen storage, and stabilize blood glucose levels. These effects collectively delay the onset of fatigue and reduce its severity [19]. Moreover, even in the glycogen depleted state, the supplementation of carbohydrates has been reported to enhance endurance performance [25]. When endogenous glucose production is sufficient to meet the energy demands of the event, the necessity of exogenous carbohydrate supplementation diminishes. In such cases, adequate carbohydrate intake can still effectively reduce the consumption of endogenous glucose, thus helping to maintain blood glucose balance while minimizing hepatic glycogen breakdown [7]. Without carbohydrate supplementation, plasma glucose contributes 19% to energy expenditure, whereas with high-concentration glucose intake during 2 h of exercise, this contribution increases to 42%. Therefore, blood glucose is considered the most important energy substrate in the later stages of prolonged exercise [26]. During intense exercise, hypoglycemia can occur in less than a minute, as the rate of glucose utilization can easily exceed the rate of hepatic glycogen breakdown and gluconeogenesis [1]. The most effective approach to carbohydrate supplementation during competition is to maximize exogenous carbohydrate intake without causing gastrointestinal discomfort. This approach ensures that carbohydrate oxidation does not become a limiting factor for performance. Maintaining stable blood glucose levels, or allowing them to gradually increase during the competition, is likely an ideal state for meeting the energy demands of long-duration endurance events and optimizing performance.

After glycogen depletion, it requires at least 24 h for muscle glycogen and 11 h for liver glycogen to recover to pre-exercise levels [27]. The first 4 h post-competition, referred to as the “metabolic window”, are considered the most optimal time for carbohydrate supplementation at a rate of 1–1.2 g·kg⁻¹·h⁻¹ to promote glycogen recovery [19]. Within 24 h, a carbohydrate intake of 8–10 g·kg⁻¹ is sufficient to restore glycogen stores [28]. Moreover, a sustained high carbohydrate intake (8 g·kg⁻¹·day⁻¹) over 36–48 h can lead to muscle glycogen supercompensation, effectively doubling muscle glycogen levels [29]. Furthermore, a positive correlation has been observed between an athlete’s skeletal muscle mass and their muscle glycogen storage capacity [29], suggesting that athletes with higher muscle mass should consume more carbohydrates to ensure adequate glycogen recovery.

A dose-dependent relationship has been observed between post-exercise carbohydrate intake and the recovery of endurance performance. Muscle glycogen recovery has been identified as a key determinant of subsequent endurance performance, while liver glycogen recovery has been shown to influence the timing of fatigue onset during later exercise bouts [30]. Consequently, the restoration of glycogen post-exercise is particularly crucial for athletes engaged in multiple events or back-to-back competitions. Glycogen restoration is initiated through gluconeogenesis at a rate of 1–2 mmol·kg⁻¹·h⁻¹. However, a high carbohydrate intake has been shown to accelerate glycogen resynthesis to 10 mmol·kg⁻¹·h⁻¹, persisting for up to 4 h [27]. The increase in plasma insulin concentrations following high carbohydrate intake post-exercise, in combination with enhanced insulin sensitivity, has been demonstrated to promote glucose transport into skeletal muscle [31].

Carbohydrates can be sourced from natural foods or processed supplements, which are available in various forms, such as solid bars, powders, gels, pastes, or liquids. The effects of different forms of the same carbohydrate on glycogen synthesis are generally similar. Therefore, gastrointestinal comfort is often the primary consideration when athletes select their carbohydrate sources [12]. However, it is crucial to note that the insulin dynamics, absorption pathways, transport carrier availability, gastric emptying rates, osmolarity, and gastrointestinal irritation vary across different carbohydrate types, leading to distinct absorption rates, oxidation rates, and supplementation effectiveness.

The classification of carbohydrates is based on their degree of hydrolysis and molecular weight, which categorizes them into four categories: monosaccharides, disaccharides, oligosaccharides, and polysaccharides. The blood glucose dynamics associated with these carbohydrates are influenced by hormones, such as insulin and glucagon. Monosaccharides and disaccharides have been shown to cause a rapid increase in blood glucose, while the blood glucose from oligosaccharides and polysaccharides occurs more gradually. Based on glycemic response velocity, foods are classified into high- or low-glycemic index (GI) categories. According to total carbohydrate content per serving, they are further categorized as high- or low-glycemic load (GL). Additionally, advancements in biotechnology have enabled the development of derivatives, such as isomaltose and trehalose, which, in comparison to glucose, may potentially reduce insulin responses and contribute to the maintenance of blood glucose stability [32].

The primary limitation for the absorption and transport of glucose and galactose is the sodium–glucose cotransporter 1 (SGLT1) on intestinal epithelial cells, while fructose is primarily transported by glucose transporter protein 5 (GLUT5). However, GLUT5 is less abundant than SGLT1, resulting in glucose being absorbed more rapidly and efficiently than fructose [33]. High doses of either glucose or fructose alone can cause gastrointestinal issues, such as diarrhea, abdominal pain, and bloating. However, the combination of glucose and fructose in supplementation has been shown to reduce or prevent gastrointestinal discomfort [19]. The concentration of carbohydrate drinks influences its absorption efficiency by modulating osmolarity and gastrointestinal function [34]. Biotechnological modifications of high-molecular-weight carbohydrates, such as highly branched cyclodextrin solutions, exhibit very low osmolarity, which has been found to accelerate gastric emptying and reduce gastrointestinal discomfort [32]. Similarly, gel forms of carbohydrates have also been demonstrated to speed up gastric emptying and facilitate absorption [35]. Therefore, athletes should strategically select the most suitable types of carbohydrates based on their physiological state and gastrointestinal function during different phases of competition. The precise adjustment of carbohydrate ratios and concentrations has the potential to minimize digestive discomfort, enhance absorption efficiency, and optimize glycogen storage and blood glucose homeostasis.

Prior to competition, carbohydrate loading with carbohydrate-fortified natural foods has been shown to enhance glycogen stores and provide essential nutrients and higher safety levels. To achieve adequate carbohydrate intake, elite athletes typically prioritize natural foods while also incorporating carbohydrate supplements. This approach is often complemented by multiple meals or snacks to further boost glycogen reserves. In the 48 h preceding the event, athletes are advised to consume familiar, easily digestible foods that are low in fat, dietary fiber, and GI, and high in carbohydrate content, to avoid gastrointestinal distress and prevent drastic fluctuations in blood glucose levels. After the final meal and before the race begins, the consumption of a low-GI beverage is particularly beneficial for maintaining energy metabolism during the competition [2].

During competitions, athletes commonly select mixed carbohydrate solutions to enhance the absorption and oxidation rate of exogenous carbohydrates. Additionally, certain long-distance endurance athletes choose whole foods, such as bananas, during the race. The maximum oxidation rate of glucose alone is around 1.0–1.2 g·min⁻¹. However, when fructose is added, the oxidation rate can increase to approximately 1.5 g·min⁻¹. Fructose has been shown to be particularly beneficial for liver glycogen replenishment [19]. The recommended ratio of fructose to glucose is typically 1:2, though some studies suggest that a ratio closer to 0.8:1 optimizes carbohydrate utilization efficiency, gastrointestinal comfort, and endurance performance [22].

It has been customary to recommend that athletes consume low-concentration or isotonic carbohydrate solutions during prolonged exercise, as these solutions are more easily absorbed and help maintain hydration [34]. A 6% glucose solution or an 8–10% glucose-fructose mixture is commonly advised. High-concentration solutions have been shown to slow gastric emptying. However, both the amount of carbohydrate delivered to the intestine and the oxidation rate of exogenous carbohydrates increase linearly with higher concentrations [34]. Due to the high intensity and gastrointestinal factors during long-distance endurance competitions, athletes typically consume smaller quantities of liquid (such as a mouthful) at each carbohydrate intake point. To meet their energy and carbohydrate demands, some athletes opt for higher-concentration solutions, even those exceeding 200 g·L⁻¹, with optimal performance outcomes. Further research is needed to investigate this dose-response relationship and its effects on gastrointestinal function.

Additionally, carbohydrate supplementation during competition must consider the specific conditions of the event. To minimize errors, it is advisable to simplify the options available at aid stations, ideally combining carbohydrate and electrolyte solutions. However, athletes who cannot tolerate mixed carbohydrate–salt solutions should be allowed to maintain their preferred supplementation habits. In the post-competition phase, it is recommended to prioritize high-GI natural foods and various types of carbohydrate supplements. These have been shown to stimulate a stronger insulin response, which, in turn, promotes glycogen synthesis and enhances subsequent endurance performance [19].

The combination of moderate amounts of protein, sodium, and caffeine in carbohydrate supplementation has been shown to have a synergistic effect, promoting carbohydrate metabolism, aiding functional recovery, reducing muscle damage, and improving endurance performance. Pre-competition supplementation with protein has been found to mitigate the insulin response triggered by substantial carbohydrate intake, which can impede fat mobilization and promote carbohydrate utilization. This approach has been observed to prevent the onset of premature fatigue, reactive or rebound hypoglycemia, and promote fat oxidation [18]. One study indicated that athletes engaged in 60 min of exercise at 70% VO_2max, followed by 80% VO_2max, until reaching exhaustion. The athletes were given 1.2 g·kg⁻¹ of carbohydrates, and either 0.4 g·kg⁻¹ of protein 30 min before or at the start of the high-intensity portion. This supplementation reduced levels of alanine aminotransferase, aspartate aminotransferase, and creatine kinase 24 h after exercise [35]. Furthermore, in-competition supplementation with 0.25 g·kg⁻¹·h⁻¹ of protein has been shown to effectively mitigate fatigue and muscle damage [36]. Following exhaustive exercise, which resulted in a decline of muscle glycogen to 125 mmol·kg⁻¹, the effects of two different nutritional interventions were examined. The first intervention involved the administration of 1.2 g·kg⁻¹·h⁻¹ of carbohydrates, while the second intervention consisted of a combination of carbohydrates (0.8 g·kg⁻¹·h⁻¹) and protein (0.4 g·kg⁻¹·h⁻¹). The latter group exhibited an 8.4-min increase in time to exhaustion during a 5-h cycling test compared to the carbohydrate-only group [37]. When carbohydrate intake is below 0.8 g·kg⁻¹·h⁻¹ post-exercise, adding protein (0.2–0.4 g·kg⁻¹·h⁻¹) can enhance the insulin response, promoting faster glycogen recovery [36]. However, when carbohydrate intake is sufficient, the increasing protein intake does not further improve endurance performance [30]. Given that many long-distance endurance athletes experience gastrointestinal discomfort, pre-competition carbohydrate loading often includes easily digestible, high-quality protein. During the 48 h leading up to the event, protein intake should be moderately reduced. During competition, given the increase in solution viscosity associated with the addition of protein powder, adjustments should be made based on race conditions, such as temperature and the athlete’s preferences. Immediately after the competition, protein supplementation (0.2–0.4 g·kg⁻¹ body weight) can be calculated based on the athlete’s weight and either mixed with carbohydrates or consumed separately, according to the athlete’s habits.

Sodium, a fundamental electrolyte in the body, plays an essential role in maintaining extracellular volume, osmotic pressure, and gastrointestinal function [38]. Glucose absorption in the intestines is facilitated by co-transport with sodium, and the ingestion of sodium at concentrations ranging from 30 to 50 mmol·L⁻¹ has been shown to enhance glucose uptake and utilization [39]. However, excessive sweating in high-temperature environments can disrupt this balance, leading to muscle cramps and hyponatremia. Consequently, athletes have been recommended to supplement sodium either prior to or during competition. Electrolyte salts are typically consumed in combination with carbohydrate solutions or separately. Symptoms such as vomiting, weakness, and muscle cramps during competitions may indicate hyponatremia. On the other hand, excessive sodium intake can lead to hypernatremia, which can cause similar issues and impair performance. Therefore, monitoring sodium loss through sweating during competition enables athletes to adjust their sodium intake in real time, thereby minimizing the risk of both hyponatremia and hypernatremia, while achieving the synergistic effects of combined sodium and carbohydrate supplementation. Additionally, some athletes have reported experiencing gastrointestinal discomfort, such as diarrhea, when consuming electrolyte salts, highlighting the importance of individualizing sodium supplementation and “training” in advance.

Furthermore, the integrated combination of nutrient supplementation has been shown to enhance the effects of carbohydrate intake. Caffeine, a widely used ergogenic aid among endurance athletes [40], has been found to promote fat breakdown and oxidation while reducing muscle glycogen depletion when ingesting 6–9 mg·kg⁻¹ of caffeine 60 min before exercise [3]. In the 4-h recovery period following exhaustion, the combination of carbohydrates (1.2 g·kg⁻¹) and caffeine (8 mg·kg⁻¹) has been shown to accelerate muscle glycogen recovery [41]. However, the effects of caffeine are influenced by genetic factors, with athletes metabolizing caffeine at different rates depending on their genotype [40]. Some athletes may experience adverse effects, such as insomnia or dehydration, which can negatively impact performance. Therefore, the decision to use caffeine, as well as the appropriate dosage and timing, should be tested and refined during training before being applied in competition. Additionally, while caffeine may reduce the perception of fatigue, it does not eliminate fatigue itself. In other words, athletes may feel less fatigued after caffeine intake, but the underlying fatigue remains. For athletes who benefit from caffeine, it is essential to implement enhanced recovery protocols following training or competition. Furthermore, the tolerance to caffeine can vary among athletes, particularly during periods of pre-competition anxiety, which can exacerbate adverse effects, such as insomnia. Consequently, the dosage of caffeine should be adjusted based on the athlete’s training cycle, caffeine half-life, and emotional state prior to competition [42].

Gastric emptying and intestinal absorption capacity are key determinants of glucose uptake and utilization. Gastrointestinal discomfort, however, significantly limits the oxidation rates of exogenous glucose [43]. Exercise-induced gastrointestinal syndrome (EIGS) is commonly observed in endurance athletes, with an incidence ranging from 30% to 90% [44,45]. The high prevalence of EIGS among endurance athletes is attributed to multiple factors, such as intense training, high-dose carbohydrate supplementation, elevated temperatures, and dehydration [46]. In major events like the Tokyo 2021 and Paris 2024 Olympics, many long-distance endurance athletes experienced gastrointestinal distress, including symptoms like vomiting and abdominal pain, which either led to race withdrawals or forced them to slow down to finish the event.

The etiology of gastrointestinal distress is multifactorial, encompassing factors such as insufficient athlete conditioning to sustain high race speeds or tactics. Additional contributing elements include insomnia, elevated stress levels, and discomfort from environmental factors (e.g., temperature) upon arrival at the competition venue. A rapid escalation of carbohydrate intake rates during competition, combined with sports beverages exceeding recommended carbohydrate concentrations and maintained at suboptimal temperatures, alongside elevated environmental heat and humidity levels during the event, may collectively contribute to gastrointestinal functional disturbances through synergistic physiological stressors. Collectively, these factors increase the likelihood and severity of gastrointestinal disturbances. It is noteworthy that the health status of athletes in the 10 days leading up to competition, particularly if they are experiencing diarrhea, can serve as a significant risk factor for gastrointestinal issues during the event [47]. Consequently, disparities in gastrointestinal function play a critical role in determining the effectiveness of carbohydrate supplementation.

The repeated intake of high carbohydrates before and during training has been demonstrated to condition the gastrointestinal system, improving its function, increasing tolerance, and reducing the incidence and severity of EIGS. These effects may, in turn, offer potential performance benefits [48]. On the one hand, high carbohydrate intake enhances gastric pressure adaptation, promoting more efficient gastric emptying. On the other hand, it has been observed to boost intestinal motility and transport, further optimizing overall gastrointestinal function [49]. Moreover, gastrointestinal tolerance training, which stimulates glucose transporter activity (such as SGLT1 and GLUT5), has the potential to augment the absorption capacity [49]. A study [50] assessed the impact of gastrointestinal tolerance training on endurance running performance. The study involved ten sessions of training over a two-week period, during which participants ran at 60% VO_2max for 60 min while ingesting a gel containing 30 g of carbohydrates at 0, 20, and 40 min. The results showed a significant reduction in gastrointestinal discomfort and a noticeable improvement in endurance performance. Thus, a scientifically designed “gastrointestinal function training” regimen emerges as an effective strategy for maximizing the efficacy of carbohydrate supplementation (Figure 2).

Athletes exhibit considerable individual variation in their ability to digest and absorb carbohydrates, their gastrointestinal responses, and blood glucose kinetics. Moreover, substantial interindividual differences in glycogen storage have been documented, with muscle glycogen content ranging from 300 to 1000 mmol·kg⁻¹, and liver glycogen content varying from 270 to 400 mmol·kg⁻¹ [6]. These findings emphasize the need for personalized carbohydrate supplementation strategies during both training and competition. For instance, one study developed a model to provide individualized pacing and carbohydrate intake recommendations for ultramarathon runners, incorporating factors such as altitude, terrain, distance, and the athlete’s body weight, cardiovascular health, and psychological state [51]. Another study employed continuous glucose monitoring (CGM) to investigate the relationship between dynamic blood glucose levels and pacing during competition, further highlighting the importance of individualized carbohydrate supplementation [2,7]. Furthermore, endurance athletes tend to reach their peak performance at older ages and sustain a longer athletic career, leading to a broader age range in competitive events [52]. Age influences athletic performance in various ways, with factors such as changes in basal metabolic rate [53], mitochondrial capacity and insulin sensitivity [54], appetite and glucose homeostasis [54], and nutrition habits and beliefs [55], all serving as important determinants of energy expenditure and carbohydrate metabolism. Consequently, different age stages, from adolescence to adulthood, should be considered essential when developing personalized carbohydrate supplementation strategies for athletes. However, research on this area remains limited.

Gender-based differences in carbohydrate metabolism and supplementation efficacy have been well-documented. Female athletes, compared to males, benefit from the effects of estrogen, particularly 17-β estradiol, which promotes fat breakdown and enhances the availability of fatty acids [56]. During exercise, women tend to exhibit greater lipid oxidation and relatively lower glycogen utilization. Additionally, the depletion of intramyocellular glycogen during exercise has been found to be significantly higher in males compared with females [57]. Hormonal fluctuations throughout the menstrual cycle, including changes in 17-β estradiol and progesterone, influence the balance between carbohydrate and fat utilization during both rest and exercise [58], which may contribute to the increased difficulty women experience in glycogen storage during the follicular phase. Additionally, these hormonal fluctuations affect appetite and food preferences [59]. Menstrual irregularities are more common among long-distance endurance athletes [60], whereas athletes with regular menstrual cycles demonstrate a greater ability to maintain blood glucose levels during prolonged exercise [61].

Given the impact of gender differences and hormonal fluctuations on carbohydrate metabolism, athletes require periodized nutritional protocols to address their carbohydrate needs across all phases of their menstrual cycle [58]. Especially during the follicular phase, female athletes typically enhance glycogen stores through multiple smaller meals. Athletes with menstrual irregularities may gain an advantage from increased intake of high-glycemic foods to counteract these effects and improve performance. Further research is required to clarify how hormonal variations during the menstrual cycle impact carbohydrate metabolism in female athletes, which could facilitate the development of more tailored supplementation strategies.

Moreover, approximately 80% of long-distance endurance athletes exhibit at least one symptom of relative energy deficiency in sport (RED-S) [62]. Insufficient carbohydrate intake is a primary contributor to RED-S [63]. The negative energy balance associated with RED-S further reduces the availability of glucose for circulation, thereby decreasing exogenous glucose oxidation rates [19] and endurance performance [64]. However, excessive carbohydrate intake can lead to exercise-induced gastrointestinal syndrome (EIGS), which also impairs athletic performance [48]. Therefore, individualized carbohydrate intake, in line with recommended guidelines, is crucial for preventing both RED-S and EIGS, while promoting optimal exogenous glucose oxidation.

Endurance athletes often compete in extreme environments, such as the high-temperature and high-humidity conditions at the 2019 IAAF World Athletics Championships in Doha, the 2021 Tokyo Olympics, and the 2023 World Athletics Championships in Budapest. Other notable events include the 2015 IAAF World Cross Country Championships in Guiyang, China (elevation 1275 m), the 2017 IAAF World Cross Country Championships in Kampala, Uganda (elevation 1210 m), and the Russian marathon held in −53 °C conditions. Environmental factors, including high altitude, heat, humidity, and extreme cold, have been shown to alter an athlete’s basal metabolic rate, endogenous and exogenous glucose oxidation rates, sweat rate, gastrointestinal function, blood volume, appetite, and food preferences. These changes may lead to fatigue, injury, and disruptions in energy metabolism and physiological function, ultimately influencing athletic performance [46] (Figure 3).

In major competitions, such as the Olympic Games, pre-competition anxiety and nervousness are unavoidable [79], and those emotions tend to intensify as the event date approaches [43]. The level of nervousness experienced prior to competition has been demonstrated to affect an athlete’s appetite and blood glucose levels. Long-distance endurance major events typically involve many highly skilled competitors. For instance, in the 2023 World Athletics Championships, 50 athletes participated in the men’s 20 km race walk. Following the starting gun, the leading group was large, with numerous competitors employing high-paced tactics. The top three finishers recorded times of 1:17:32, 1:17:39, and 1:17:47, respectively, while the 4th to 10th place finishers had times between 1:18:03 and 1:18:59, and the 11th to 18th placed athletes finished between 1:19:01 and 1:19:55. The competition was intense, and, therefore, the pressure and anxiety during the race were also inevitable.

When athletes experience anxiety, their stress-response metabolic pathways are activated. The sympathetic nervous system is the primary pathway involved, leading to the release of catecholamines, such as norepinephrine and epinephrine, from the adrenal medulla. These catecholamines regulate a series of stress responses through the sympathetic–adrenal–medullary axis [80]. In the context of normal or elevated glycogen levels, the injection of epinephrine (20 μg/100 g body weight) over a period of 3 h has been observed to result in a significant decrease in muscle glycogen levels, exceeding 30% [81]. Conversely, catecholamine injections (20 μg/100 g body weight) over a duration of 2 h have resulted in a 41% reduction in muscle glycogen [82].

The second pathway involves the activation of the hypothalamic–pituitary–adrenal (HPA) axis, which triggers the release of the corticotropin-releasing hormone (CRH). This, in turn, induces the secretion of the adrenocorticotropic hormone (ACTH) from the pituitary gland and stimulates the release of glucocorticoids, such as cortisol [83]. Cortisol activates glycogen metabolism in both the liver and muscles by stimulating glycogen phosphorylase, glucagon, and epinephrine [84]. Furthermore, emotional stress has been demonstrated to exacerbate gastrointestinal discomfort [43].

Athletes participating in long-distance endurance events often experience a decrease in appetite, unexplained diarrhea, and insomnia during pre-race training or upon arrival at the competition venue. Those symptoms suggest that heightened anxiety and emotional stress during the competition may increase glycogen consumption while impairing the effective absorption of carbohydrates due to gastrointestinal discomfort, further diminishing glycogen reserves. Consequently, athletes experiencing elevated anxiety levels due to high-intensity training and various pressures may benefit from an advanced carbohydrate loading period and an increased total carbohydrate intake, in combination with other forms of supplementation. This strategy helps optimize endogenous glycogen stores and alleviate anxiety. In cases where gastrointestinal distress limits carbohydrate intake during competition, increasing the concentration of the carbohydrate solution to boost exogenous carbohydrate intake may effectively mitigate early or excessive glycogen depletion caused by anxiety. This approach ensures an adequate energy supply during the competition.