Why Marathon Runners Eat Carbs Before a Race

January 10, 2026 •

3 min read

According to research published in the journal Sports Medicine, a high-carbohydrate diet can improve endurance performance by up to 3%. This critical practice, known as carb-loading, is why marathon runners eat carbs before a race to maximize their muscle glycogen stores and fuel their long-distance efforts.

Quick Summary

Carb-loading fills muscle glycogen stores, the body's primary energy source for long-duration exercise. It prevents premature fatigue and helps runners maintain their pace, allowing them to perform at their best during a race.

Key Points

Maximizes Energy: Carb-loading saturates the body's glycogen stores, the most efficient fuel source for marathon running.
Prevents Fatigue: Full glycogen tanks help delay or prevent "hitting the wall," the sudden drop in performance caused by fuel depletion.
Timing is Crucial: Successful carb-loading involves a strategic increase of carbohydrates over 2-3 days before the race, not just one large meal.
Choose Easy-to-Digest Carbs: Opt for low-fiber options like white pasta, rice, and potatoes to avoid gastrointestinal issues during the race.
Improves Hydration: Storing glycogen also means storing water, which helps a runner stay hydrated throughout the race.
Test During Training: Always practice your carb-loading and race-day nutrition strategy during long training runs to ensure your body tolerates it well.
Balance Intake: The key is to increase the percentage of calories from carbohydrates, not simply to overeat.

The Science of Carbohydrate Loading

Marathon running demands significant energy, and while the body utilizes both fat and carbohydrates, carbs are the preferred fuel source, especially during intense periods. When a runner consumes carbohydrates, they are converted to glucose and stored as glycogen in the liver and muscles.

Glycogen storage is limited, typically providing enough energy for about 90 minutes of vigorous activity. Since a marathon lasts longer, carb-loading becomes essential. This involves increasing carbohydrate intake in the days leading up to the race to maximize glycogen stores.

How Carb-Loading Prevents "Hitting the Wall"

"Hitting the wall" is a term used to describe the severe fatigue experienced when muscle glycogen is depleted, forcing the body to rely less efficiently on fat for fuel. This metabolic shift results in a significant decline in performance. Carb-loading helps prevent this by increasing glycogen stores, potentially by up to 20%, providing a crucial energy reserve for the later stages of the race.

Practical Steps for Successful Carb-Loading

Effective carb-loading involves a strategic approach over several days, often combined with reduced training volume (tapering). This allows for increased carbohydrate intake to be more effectively stored. In the 2-3 days before a marathon, aim for 70-85% of calories from carbohydrates, which translates to 8-12 grams per kilogram of body weight. Focus on easily digestible, low-fiber carbohydrates like white pasta, rice, white bread, and potatoes to avoid stomach issues on race day.

Carb-Loading vs. Race-Day Fueling


Aspect	Pre-Race Carb-Loading	Race-Day Fueling
Timing	2-3 days leading up to the race.	Immediately before and throughout the race.
Primary Goal	Maximize and top-up glycogen stores in muscles and liver.	Provide a quick, steady supply of glucose to working muscles.
Food Types	Complex carbs (white pasta, potatoes, rice), reduced fiber.	Simple sugars (energy gels, chews, sports drinks).
Volume	High volume of carbs, comprising the majority of caloric intake.	Smaller, frequent doses of quick-absorbing sugars.
Digestion	Low-fiber foods to minimize GI distress.	Easily digestible, non-solid forms to prevent stomach upset during exertion.

Potential Pitfalls to Avoid

Common carb-loading mistakes include consuming too much fiber, which can cause digestive problems, and simply overeating without adjusting macronutrient ratios. Practicing your carb-loading strategy during training is vital to identify what works best for your body.

Hydration and the Glycogen Link

Carb-loading also impacts hydration, as about three grams of water are stored with each gram of glycogen. This natural increase in hydration is beneficial for endurance events and explains the slight weight gain often experienced during the taper.

Conclusion: Fueling for the Finish Line

Understanding why marathon runners eat carbs before a race reveals a scientifically supported strategy to optimize muscle glycogen stores. Effective carb-loading, involving proper timing, appropriate food choices, and avoiding common errors, is crucial for enhancing endurance and performance. This preparation helps runners avoid hitting the wall and finish strong. Mastering pre-race nutrition is as important as training itself. For additional resources on athlete nutrition, the National Institutes of Health offers valuable information. Key takeaways include starting 2-3 days before the race, prioritizing easily digestible low-fiber carbs, and testing strategies during training.

Frequently Asked Questions

Carb-loading is a dietary strategy used by endurance athletes to increase the storage of glycogen in their muscles and liver in the days leading up to a race. This process provides a greater reserve of energy for prolonged exercise.

For optimal results, you should start increasing your carbohydrate intake approximately 2 to 3 days before your marathon. This allows sufficient time for your body to fully top off its glycogen stores.

The best foods for carb-loading are easily digestible, low-fiber carbohydrates. Good options include white pasta, rice, white bread, potatoes, bagels, and low-fiber cereals. Avoiding high-fiber foods helps prevent gastrointestinal distress on race day.

Yes, it is common to gain a small amount of weight (around 1-2 kg) during the carb-loading period. This is a normal and desirable effect, as every gram of stored carbohydrate is accompanied by about three grams of water, which aids hydration during the race.

While a pasta dinner is a classic pre-race meal, it is more effective to spread the increased carbohydrate intake over the 2-3 days leading up to the race. Overeating a single meal can cause bloating and digestive issues.

It is generally recommended to limit whole grains during the final 2-3 days before a race. Their high fiber content can slow digestion and potentially cause stomach discomfort during the marathon.

Carb-loading is most beneficial for endurance events lasting longer than 90 minutes, such as half-marathons and full marathons. For shorter distances like a 5K or 10K, regular glycogen stores from a balanced diet are usually sufficient.

On race day morning, consume a high-carbohydrate breakfast 3-4 hours before the race. Focus on simple, familiar, and low-fiber foods that are easy to digest, such as toast with jam, a bagel, or oatmeal.