Is Carb-Loading Effective for Endurance Athletes?

"First we eat, then we do everything else." – M.F.K. Fisher

 

I remember pre-pandemic wandering through Boston the night before the marathon and spotting the long line for the BAA pre-race pasta dinner winding out of the building and spilling onto the streets. Marathoners clearly knew the value of carbohydrate loading. Yet, research shows many endurance athletes still do not carb-load correctly- or enough. While the term is familiar, there is widespread confusion about when, how, and why to carb-load, as well as whether it actually boosts performance. This post breaks down the science, strategy, and practical guidelines for effective carb-loading.

 

What is Carb-Loading?

If you have raced or trained in long-distance events, you have likely heard of carb-loading (also known as carbo-loading or glycogen loading). In essence, it is a strategy to maximize glycogen stores in muscles, and sometimes the liver, before competition. Higher glycogen stores delay fatigue, improve endurance, and enhance performance.

The recommended approach is to increase carbohydrate intake in the 24 to 48 hours preceding a prolonged event. Most experts suggest a high-carbohydrate diet of about 8–12 g/kg body weight per day. For example, a 60 kg athlete would aim for roughly 480–720 g of carbohydrates daily. The goal is not just eating more food, but specifically topping up glycogen reserves (Hawley, 1997) (Thomas, 2016) (Nadieri, 2023).

Does Carb Loading Help Endurance Performance?

Yes. Muscle and liver glycogen availability strongly influences endurance performance. Low glycogen at the start of exercise leads to early fatigue, reduced power output, and impaired performance. In contrast, elevated glycogen can delay fatigue by ~20% in events longer than 90 minutes and improve performance by 2–3% (Hawley, 1997).

A field study of middle-distance runners found that consuming a very low-carb diet (<1.5 g/kg for 2 days after glycogen depletion) slowed a 1500 m time trial by ~2% compared to a high-carb diet (>5 g/kg). That’s a measurable advantage to maintaining adequate carbohydrate intake and proper glycogen loading (Venckunas 2024).

The benefits are most relevant for events lasting longer than 90 minutes. For shorter events, if you eat a carbohydrate-rich diet daily, the performance gains are minimal.

 

Are Athletes Carb-Loading Effectively?

Despite strong evidence supporting carb-loading, many athletes do not practice it correctly. A 2023 study from Liverpool John Moores University revealed many endurance athletes have limited knowledge of carbohydrate guidelines. Only 28% of athletes know when carb-loading is necessary and how much carbohydrate to consume (Sampson, 2023).

Pre-event carbohydrate intakes often fall below recommended levels. Athletes typically eat between 2.5–7.3 g/kg/day rather than the recommended 8–12 g/kg/day. This gap exists among both elite and amateur athletes, including runners, cyclists, and triathletes. Experienced athletes who have access to expert advice from sports dietitians tend to fare better, whereas amateurs often rely on social media or experience (Sampson, 2023).

This mismatch explains why carb-loading mistakes are common and why race-day performance can suffer as a result (Heikura, 2017). Nutrition education, planning, and practice are crucial.

 

Effective Carb-Loading Strategies

The following factors influence how carb-loading affects performance:

Event duration: 

Glycogen loading benefits moderate to high intensity events lasting more than 90–120 minutes. Shorter events may yield limited benefits and experience minor downsides, such as temporary weight gain due to water bound to glycogen. 

Training status and glycogen capacity: 

Highly trained athletes have a greater capacity to store and utilize glycogen, which may influence how they respond to carbohydrate loading.

Pre-event and in-event carbohydrate intake: 

Even if you load well, if you fail to ingest enough carbohydrates during the event, or start with depleted liver glycogen (e.g., due to an overnight fast), performance gains will be compromised. It is important to ensure that liver glycogen is topped up in the morning before a race.

Execution & tolerance: 

Carb-loading is not just “eat lots of pasta.” It requires reducing training load, emphasizing carbohydrates while avoiding excessive calorie intake, and selecting digestible foods to minimize gastrointestinal issues. (Podlogar, 2022) (Nadieri, 2023)

 

Practical Strategies for Carb-Loading

Here is how to carb-load effectively:

1. Taper Training

Reduce training volume and intensity 3–7 days before your event. This reduction allows your body to store more fuel than it uses by preventing substantial glycogen depletion. Many athletes undermine carb-loading by maintaining heavy training loads.

2. Increase Carbohydrate Intake

• Aim for 8–12 g/kg/day of carbohydrates 24–48 hours pre-event. This equates to 540-800 g/day for a 68 kg (150 lb) person.
• Prioritize low-fat, low-fibre, high-carb foods for easy digestion, such as white rice, pasta, potatoes, low-fibre bread, fruit juices, sweets and moderate amounts of fruit.
• 1–4 hours pre-race: eat ~1–4 g/kg in a meal to top off liver and muscle glycogen, especially if racing in the morning, after an overnight fast. This equates to ~70-270 g for a 150 lb (68 kg) person.

3. Practice in Training

Each athlete’s gastrointestinal tolerance and preferences differ. Trial carb-loading during training to test:

• Food tolerance
• Body weight changes
• Energy levels

This ensures your carb-loading strategy works before race day.

4. Consider Event Length & In-Race Fueling

Extra stored glycogen helps delay fatigue and improve performance in events lasting longer than 90–120 minutes. However, you still need to take in carbohydrates during the race. Many athletes under-consume carbohydrates during long events: only ~50% of triathletes, 30% of cyclists, and 15% of marathoners meet the recommended intake of 60–90 g/h during events lasting more than 2.5 hours.

5. Monitor Body Mass & Hydration

Glycogen binds water in the amount of ~3 g water per 1 g glycogen. As such, expect a temporary weight gain. This is functional, not fat. In weight-sensitive sports, the effect is minor and transient.

 

Key Takeaways

Carb-loading works when executed correctly, particularly for events lasting longer than 90 minutes:

• Taper training and reduce glycogen use in the days before racing.
• Increase your carbohydrate intake without exceeding your total calorie limit. Load carbs not calories!
• Choose digestible, whole-food carbohydrate sources.
• Practice your carb-loading strategy in training.
• Fuel during the race to maintain glycogen availability.
• Tailor intake to body mass and event demands.

Carb-loading is not a universal solution; it offers little benefit for short events and requires careful planning. However, for long, intense endurance events, it can be a small change with measurable impact, giving athletes the energy reserves to push through fatigue and maximize performance.

 

References

Hawley, J. A., Schabort, E. J., Noakes, T. D., & Dennis, S. C. (1997). Carbohydrate-loading and exercise performance: An update. Sports Medicine, 24(2), 73–81. https://doi.org/10.2165/00007256-199724020-00001.

Heikura, I. A., Stellingwerff, T., Mero, A. A., Uusitalo, A. L. T., & Burke, L. M. (2017). A mismatch between athlete practice and current sports nutrition guidelines among elite female and male middle- and long-distance athletes. International Journal of Sport Nutrition and Exercise Metabolism, 27(4), 351–360. https://doi.org/10.1123/ijsnem.2016-0316.

Janse van Rensburg, D. C., Sparks, I. M., Fletcher, L., & Jansen van Rensburg, A. (2018). A cross-sectional study of 2,550 amateur cyclists shows lack of knowledge regarding relevant sports nutrition guidelines. South African Journal of Sports Medicine, 30(1), 1–6.

McLeman, L. A., Ratcliffe, K., & Clifford, T. (2019). Pre- and post-exercise nutritional practices of amateur runners in the UK: Are they meeting the guidelines for optimal carbohydrate and protein intakes? Sport Sciences for Health, 15, 511–517. https://doi.org/10.1007/s11332-019-00564-4.

Naderi, A., Gobbi, N., Ali, A., Berjisian, E., Hamidvand, A., Forbes, S. C., Koozehchian, M. S., Karayigit, R., & Saunders, B. (2023). Carbohydrates and endurance exercise: A narrative review of a food-first approach. Nutrients, 15(6), 1367. https://doi.org/10.3390/nu15061367.

Sampson, G., Morton, J. P., & Areta, J. L. (2023). Mind the gap: Limited knowledge of carbohydrate guidelines for competition in an international cohort of endurance athletes. Journal of Nutritional Science, 12, e68. https://doi.org/10.1017/jns.2023.68.

Sports Dietitians Australia. (2024). Fuelling for endurance events. https://www.sportsdietitians.com.au

Thomas DT, Erdman KA, Burke LM. Position of the Academy of Nutrition and Dietetics, Dietitians of Canada, and the American College of Sports Medicine: Nutrition and Athletic Performance. J Acad Nutr Diet. 2016 Mar;116(3):501-528. 

Venckunas T, Minderis P, Silinskas V, Buliuolis A, Maughan RJ, Kamandulis S. Effect of Low vs. High Carbohydrate Intake after Glycogen-Depleting Workout on Subsequent 1500 m Run Performance in High-Level Runners. Nutrients. 2024 Aug 19;16(16):2763. doi: 10.3390/nu16162763. PMID: 39203899; PMCID: PMC11357641.

 

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Written by: Ashley Leone, RD, MSc, IOC. Dipl. Sport Nutr with editing from ChatGpt

Ashley is a sports and performance dietitian and the Founder of Fuel Kit and the Owner of Gazelle Nutrition Lab. Ashley has been a dietitian for over 25 years and holds a Master of Science degree from the University of Toronto, as well as a Diploma in Sports Nutrition from the International Olympic Committee.