Why Endurance Athletes Eat Carbohydrates Before a Long Race

January 13, 2026 •

3 min read

According to sports nutrition research, carbohydrate loading can improve an athlete's endurance performance in events lasting over two hours by up to 3%. This critical strategy of eating carbohydrates before a long race is designed to super-compensate muscle glycogen stores, which are the body's primary fuel source for high-intensity, prolonged exercise.

Quick Summary

This article explains the scientific reasons behind pre-race carbohydrate consumption, focusing on glycogen storage, its role as the most efficient fuel source for intense exercise, and how it prevents premature fatigue in endurance athletes.

Key Points

Glycogen Storage: Carbohydrates are stored as glycogen in the muscles and liver, serving as the body's primary, readily available fuel for endurance efforts.
Efficient Fuel: Glycogen provides a more rapid and efficient source of energy for moderate-to-high intensity exercise compared to fat.
Fatigue Prevention: Maximizing glycogen stores through carb loading prevents the energy crash known as "hitting the wall," which occurs when stores are depleted.
Carb Loading Strategy: Proper carb loading involves increasing carbohydrate intake over 2-3 days prior to a race, combined with reduced training volume (tapering).
Pre-Race Topping Off: A final high-carb, low-fiber meal 2-4 hours before the event helps replenish liver glycogen stores that are used overnight.
Practice in Training: Athletes should practice their carb loading and race-day meal strategy during training to test what works best for their body and avoid gastrointestinal issues on race day.
Water Retention: Carb loading naturally leads to some temporary water weight gain, which is a sign of proper glycogen saturation and is beneficial for performance.

The Science of Carbohydrates and Endurance

For endurance athletes, understanding macronutrients is crucial, and carbohydrates are arguably the most important. The body stores ingested carbohydrates as glycogen in the muscles and liver. During intense, prolonged exercise, the body primarily relies on this stored glycogen for energy. The limited nature of these stores means athletes must strategically increase them to prevent early fatigue, a phenomenon colloquially known as "hitting the wall".

Glycogen: The High-Octane Fuel

While the body can also use fat for energy, glycogen is its preferred and most efficient fuel source for moderate to high-intensity efforts. The metabolic pathway for breaking down glycogen is faster and more direct than that for fat. This is why when an athlete's glycogen stores are depleted, they must slow their pace significantly as their body transitions to a less efficient fat-burning metabolism. By maximizing glycogen reserves, athletes can sustain higher speeds and power outputs for longer periods.

The Art of Carb Loading

Carbohydrate loading, or "carb loading," is the practice of increasing carbohydrate intake in the days leading up to an endurance event. This is not simply about eating a single large pasta meal the night before, which is a common misconception. A proper carb-loading strategy involves a period of tapering training intensity combined with a high-carbohydrate diet for several days (typically 2-3 days) before the race. This process ensures the muscle and liver glycogen stores are topped off, providing a larger fuel tank for the race.

Strategic Timing of Pre-Race Meals

Just as important as the days leading up to the race is the final meal. An athlete's liver glycogen can be partially depleted overnight, so a pre-race meal serves to top off these stores. This meal, typically consumed 2-4 hours before the event, should consist of easily digestible, low-fiber carbohydrates to prevent gastrointestinal distress. Foods like white rice, oatmeal, or a bagel are popular choices. This final nutritional step ensures a steady supply of blood glucose at the start of the race, delaying the use of precious muscle glycogen.

The Glycogen and Performance Difference


Feature	Carbohydrate/Glycogen	Fat
Energy Source	Primary fuel for moderate to high-intensity endurance exercise	Primary fuel for low-intensity exercise and a reserve tank for all activity
Storage Capacity	Limited; stored in muscles and liver (around 2,000 kcal)	Almost unlimited for most people (around 100,000 kcal)
Metabolic Efficiency	High; rapidly converted to energy during intense activity	Lower; slower to process and requires more oxygen for conversion
Water Retention	Stored with water, which can cause temporary weight gain	Stores with little to no water, allowing for more compact energy storage
Performance Effect	Essential for sustained high performance, delaying fatigue and preventing 'hitting the wall'	Cannot sustain high intensity; depletion of glycogen forces a slower pace

A Complete Pre-Race Nutrition Strategy

It's not only about the pre-race meal but a holistic strategy that extends to training and recovery. Adequate carbohydrate intake during training is essential for replenishing glycogen stores and ensuring the athlete can perform the necessary workouts. After a hard session, consuming a combination of carbohydrates and protein helps speed up glycogen resynthesis and muscle repair. An athlete who consistently fuels their body correctly is better prepared for the demands of a long race. Furthermore, some athletes may consider "fueling for the work required," periodizing their carbohydrate intake to match the intensity of their training sessions, though this is a more advanced strategy. The right nutritional plan is a marathon, not a sprint, and should be practiced and perfected during training to avoid race-day surprises. You can read more about periodized carbohydrate intake from research published in Nutrition & Metabolism.

Conclusion: A Full Tank for Peak Performance

Ultimately, endurance athletes eat carbohydrates before a long race to ensure their body's primary high-intensity fuel source is fully stocked. This strategic approach, involving careful carb loading and a well-timed pre-race meal, maximizes muscle and liver glycogen stores. The resulting energy reserve is critical for sustaining peak performance, delaying the onset of fatigue, and preventing the drastic performance drop associated with glycogen depletion. By embracing this fundamental sports nutrition principle, athletes give themselves the best possible chance to succeed, perform at their highest level, and cross the finish line feeling strong.

Frequently Asked Questions

Carb loading is a nutrition strategy used by endurance athletes to maximize the storage of glycogen in their muscles and liver in the days leading up to a long event, like a marathon.

While the body can use fat for energy, it is a slower, less efficient process. Glycogen is the body's preferred fuel for high-intensity exercise, and relying on fat alone would significantly reduce an athlete's speed and power.

Most experts recommend starting the carb-loading process 2 to 3 days before an endurance event. This involves increasing your carbohydrate consumption while tapering your training volume.

If an athlete's glycogen stores are not properly topped off, they risk experiencing premature fatigue, a performance decline, and the sensation of "hitting the wall" during the race.

On the morning of the race, consume a high-carbohydrate, low-fiber meal 2 to 4 hours beforehand. Good options include oatmeal, a bagel, or pancakes, as these are easily digestible and replenish liver glycogen.

Yes, temporary weight gain is normal during carb loading because every gram of glycogen is stored with several grams of water. This is a positive sign that your body is adequately fueling its energy reserves.

No, it is crucial to stick to familiar foods and drinks that you know your body tolerates well during carb loading and on race day to avoid any unexpected gastrointestinal issues.