CARBOHYDRATE INTAKE DURING EXERCISE: DOES THE SAME AMOUNT SUIT EVERYONE?

Carbohydrates are our primary energy source during sport.

This is especially true during high-intensity exercise. Depletion of glycogen stores—the form in which carbohydrates are stored in the body—limits performance. Athletes of every level therefore consume large amounts of carbohydrates not only before an event, but also during exercise to supply muscles with this valuable energy substrate.

Until a few years ago

traditional guidelines recommended approximately 60 grams of carbohydrate per hour of exercise because the transporters used to move carbohydrates into muscle cells were thought to have that limit. Later studies showed that combining carbohydrates that use different cellular transporters, such as glucose and fructose, raised the limit. Intakes above 60 grams per hour, and even above 100 grams per hour, produced higher oxidation rates (1,2). In other words, much of the additional carbohydrate was oxidized to produce energy, improving performance. Therefore, more recent guidelines recommended up to 90 grams per hour for long-duration events lasting more than 2 hours, balancing performance benefits against possible gastrointestinal problems (3).

LATEST STUDIES

In recent years, however, evidence has continued to challenge the known limits of carbohydrate oxidation. We now know that high-performance athletes such as cyclists consume more than 120 grams of carbohydrate per hour. Some studies suggest that these very high intakes—120 grams per hour—may attenuate fatigue after highly demanding events such as a mountain trail race compared with 60 or 90 grams per hour (4,5). Along with widespread discussion on social media about carbohydrate intake among professional cyclists, these findings have encouraged athletes at every level, including recreational competitors, to imitate elite performers by maximizing carbohydrate intake during exercise. Before prescribing 120 grams per hour indiscriminately, however, we should ask whether one approach really suits everyone.

Factors affecting carbohydrate oxidation

One of the main factors when deciding how much carbohydrate to consume is the nature of the exercise. Carbohydrate use as an energy source rises with exercise intensity. At 50% of maximum capacity, measured for example by maximal oxygen uptake or VO2max, carbohydrates provide less than half the required energy and fat is the main substrate. As intensity rises—especially above the anaerobic threshold—the importance of carbohydrates increases dramatically. Consequently, carbohydrate intake is less important during very-low-intensity events than during higher-intensity exercise.

Exercise duration must also be considered. In short events lasting <60 minutes, a strategy as simple as rinsing the mouth with carbohydrates may improve performance because glucose in the bloodstream and stored glycogen provide sufficient energy. As duration increases, muscle and liver glycogen are more likely to become depleted, increasing carbohydrate requirements. Therefore, the greater the training load, the greater the carbohydrate requirement. A recent study in Sports Medicineconfirmed a strong correlation between carbohydrate oxidation during an exercise session and its training load, measured using indicators such as Training Stress Score, mechanical work in kilojoules, TRIMPs, or the simple product of perceived exertion and exercise duration (6). Figure 1 shows how carbohydrate-intake recommendations vary with exercise duration and intensity. High doses, such as 90 grams per hour, would in principle be useful only for long-duration exercise lasting >90 minutes when performed at high intensity.

Figure 1. Carbohydrate requirements according to exercise duration—less or more than 90 minutes—and intensity: moderate, below the first or aerobic threshold; high, between the two thresholds; or very high, above the second or anaerobic threshold, also known as critical power. Adapted from Podlogar et al. (7).

Individual characteristics may also affect whether carbohydrate intake should be increased. Recommendations have traditionally used absolute values without considering body weight, unlike protein recommendations. A recent study challenged this practice. Researchers compared heavier cyclists (>70 kg) and lighter cyclists (<70 kg) with exactly the same fitness level—a lactate threshold of 2.3 W/kg—during two hours at the same relative intensity, 95% of lactate threshold. This represented approximately 150 W for lighter cyclists and 180 W for heavier cyclists, although both were working at 2.2 W/kg (8). Every cyclist consumed 90 grams of carbohydrate per hour regardless of weight, as current recommendations advise. At the same relative intensity but a lower absolute power, lighter cyclists oxidized less carbohydrate—33 grams per hour of the 90 consumed—than heavier cyclists, who oxidized 45 grams per hour of the 90 consumed. The amount oxidized correlated strongly with body size (Figure 2). When heavier cyclists exercised at the same wattage as lighter cyclists, however, they oxidized the same amount of carbohydrate. This was mainly due to lower glycogen use rather than a reduction in exogenous carbohydrate use (8). The findings suggest that heavier athletes may have a greater capacity to oxidize carbohydrates even at the same relative intensity, perhaps partly because they produce more absolute power. This could require carbohydrate recommendations to account for anthropometric characteristics.

Figure 2. Correlation between body surface area and the capacity to oxidize exogenous carbohydrates during exercise at a given relative intensity. Adapted from Ijaz et al. (8).

Conclusions

A universal recommendation of 90 or even 120 grams of carbohydrate regardless of the exercise may not be optimal. Relative intensity and duration have a major effect on carbohydrate requirements. Even within the same event, heavier athletes or those producing more absolute power—or running at a higher speed—may require more. Gastrointestinal tolerance is another key factor before intake is increased. Athletes must complete appropriate gut training and practice their planned race-day doses during training before they can tolerate very high intakes.

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Author

Pedro Valenzuela
Researcher in the Physiology Unit at the University of Alcalá and the Performance Monitoring Unit at the Sports Medicine Center (AEPSAD, Madrid High Performance Center).
Website: www.fissac.com

References

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3. Burke LM, Hawley JA, Wong SHS, Jeukendrup AE. Carbohydrates for training and competition. J Sports Sci. 2011;29(SUPPL. 1):37–41.
4. Urdampilleta A, Arribalzaga S, Viribay A, Castañeda-Babarro A, Seco-Calvo J, Mielgo-Ayuso J. Effects of 120 vs. 60 and 90 g/h carbohydrate intake during a trail marathon on neuromuscular function and high intensity run capacity recovery. Nutrients. 2020;12(7):1–17.
5. Viribay A, Arribalzaga S, Mielgo-ayuso J, Castañeda-babarro A. Effects of 120 g/h of Carbohydrates Intake during a Mountain Marathon on Exercise-Induced Muscle. Nutrients. 2020;12.
6. Rothschild JA, Hofmeyr S, McLaren SJ, Maunder E. A Novel Method to Predict Carbohydrate and Energy Expenditure During Endurance Exercise Using Measures of Training Load. Sport Med [Internet]. 2024;(0123456789). Available from: https://doi.org/10.1007/s40279-024-02131-z
7. Podlogar T, Wallis GA. New Horizons in Carbohydrate Research and Application for Endurance Athletes. Sport Med [Internet]. 2022;52(s1):5–23. Available from: https://doi.org/10.1007/s40279-022-01757-1
8. Ijaz A, Collins AJ, Moreno-Cabañas A, Bradshaw L, Hutchins K, Betts JA, et al. Exogenous glucose oxidation during exercise is positively related to body size. Int J Sport Nutr Exerc Metab [Internet]. 2024;In press. Available from: https://www.researchsquare.com/article/rs-4530175/v1