Is Carb Loading Good for Endurance Athletes? The Definitive Guide

December 21, 2025 •

4 min read

Studies have shown that maximizing muscle glycogen stores can boost performance by up to 2-3% and endurance by as much as 15-25% in events over 90 minutes. So, is carb loading good for endurance athletes, and how can they do it effectively to gain a competitive edge? The science and experience of top athletes confirm its benefits, provided the strategy is executed correctly.

Quick Summary

This article explores the scientific principles behind carbohydrate loading, its performance benefits for endurance athletes in long-duration events, and how to implement a proper, low-fiber protocol while tapering. It also addresses common mistakes, potential side effects like bloating, and clarifies loading recommendations for different event durations.

Key Points

Event Duration Matters: Carb loading is most beneficial for endurance events over 90 minutes, not shorter races.
Prioritize Low-Fiber Carbs: Choose easily digestible, low-fiber carbohydrates like white rice and pasta to avoid gastrointestinal issues during your event.
Plan Ahead: A successful carb load involves increasing carbohydrate intake over 1-3 days, not just with one large meal the night before.
Embrace Water Weight: Temporary weight gain from water retention is a normal and expected sign of a successful glycogen supercompensation.
Practice During Training: Experiment with your carb loading strategy during long training runs to find what works best for your body before race day.
Consult a Professional: Individuals with underlying health conditions like diabetes should consult a physician before attempting to carb load.

Understanding the Science of Carb Loading

Carbohydrate loading, or 'carbo-loading,' is a strategic dietary manipulation used by endurance athletes to maximize the storage of glycogen in their muscles and liver. Glycogen is the body's primary fuel source for high-intensity, prolonged exercise. An athlete's body can only store a finite amount of glycogen, typically enough to sustain about 90 minutes of intense activity before depletion and fatigue occur, a phenomenon many runners dread as 'hitting the wall'. By increasing carbohydrate intake while tapering exercise in the days leading up to an event, athletes can significantly boost these glycogen reserves, providing a larger fuel tank for race day.

The Glycogen Supercompensation Effect

Once the body's glycogen stores are intentionally depleted during training, a period of high carbohydrate intake can trigger a 'supercompensation' effect. This means the muscles and liver store more glycogen than their usual capacity. This process is most effective when combined with a tapered training schedule, allowing muscles to rest and focus on replenishment. The result is a more resilient fuel system that can power the athlete for longer before fatigue sets in, which can be the difference between a personal best and a disappointing finish.

Who Needs to Carb Load?

Carb loading is not necessary for everyone. It is most beneficial for endurance events that last 90 minutes or longer, such as marathons, long-distance cycling, or triathlons. For shorter races like a 5K or 10K, normal daily carbohydrate intake is typically sufficient, and aggressive loading may only lead to unnecessary weight gain and discomfort. For events over 90 minutes, proper fueling is critical to stave off fatigue. Even with a full fuel tank, athletes must still consume carbohydrates during the race to maintain energy levels and avoid hitting the wall.

How to Execute a Proper Carb Load

Effective carb loading is a science, not an excuse for a junk food binge. A proper regimen involves careful planning and smart food choices, typically starting 1-3 days before the event. The goal is to increase the percentage of total daily calories from carbohydrates, not necessarily to consume more calories overall, especially if training volume has decreased.

Key Strategies for Successful Carb Loading:

Start Tapering: Begin reducing your exercise intensity and duration several days out from your event. This allows your muscles to recover and become more receptive to glycogen storage.
Prioritize Low-Fiber Carbs: While complex carbohydrates are generally healthy, the high fiber content can cause gastrointestinal distress, bloating, and gas when consumed in large quantities. In the final 24-48 hours, switch to easily digestible, low-fiber options.
Reduce Fat and Protein: Minimize your intake of fats and proteins during the loading phase. These nutrients are filling and slow down digestion, making it harder to consume the required volume of carbohydrates.
Stay Hydrated: Glycogen is stored with water, so increased fluid intake is necessary. This will also help with digestion and hydration on race day.
Practice in Training: Never try a new fueling strategy on race day. Practice your carb loading during a longer training run to see how your body responds.

Optimal Carb Choices for Loading

To achieve the high carbohydrate intake (7-12g per kg of body weight), focus on these easily digestible sources:

White bread, bagels, crackers, and English muffins
White pasta, rice, and rice noodles
Peeled potatoes and sweet potatoes
Bananas and other low-fiber fruits
Fruit juice and sports drinks
Dried fruits like raisins and medjool dates
Low-fat dairy products like yogurt and milk

Comparison Table: Smart vs. Poor Carb Loading


Aspect	Smart Carb Loading	Poor Carb Loading
Timing	Starts 1-3 days before event with taper.	One large meal the night before.
Carb Type	Prioritizes easily digested, low-fiber sources like white rice and pasta.	Relies on high-fiber whole grains and beans, causing GI issues.
Nutrient Balance	Consciously reduces fat and protein to make room for carbs.	Unintentionally high in fat from creamy sauces or greasy sides.
Hydration	Increases fluid intake alongside carbs.	Neglects hydration, which is essential for glycogen storage.
Practice	Tested during training cycles for long runs.	Never practiced, leading to race-day surprises.
Result	Optimized performance and delayed fatigue.	Potential GI distress and lethargy from overeating.

Potential Downsides and Considerations

While highly effective, carb loading isn't without potential drawbacks. The most common side effect is a temporary weight gain of 1-2kg, caused by the extra water stored alongside glycogen. This is a normal and beneficial part of the process and not a cause for concern. Other issues can include bloating, digestive discomfort, and feeling sluggish, especially if low-fiber options aren't prioritized. For individuals with diabetes, any significant dietary change, including carb loading, must be discussed with a doctor to manage blood sugar levels. There is also some conflicting research regarding gender differences in how women and men store glycogen, suggesting some women may need to increase their total calorie and carb intake more to maximize stores.

Conclusion: Making Carb Loading Work for You

For endurance athletes competing in events longer than 90 minutes, carb loading is good for endurance athletes and a proven strategy to boost performance and delay fatigue. It works by maximizing muscle glycogen stores, which serve as the body's primary fuel during sustained, high-intensity efforts. The key to a successful carb load lies in proper execution, involving a gradual increase in low-fiber carbohydrates paired with reduced training volume in the final days before the event. Experimenting with different foods and timings during your training is crucial to personalize your approach and avoid race-day surprises. While there may be some minor side effects like temporary weight gain, the performance benefits for long-distance athletes far outweigh these small inconveniences when done correctly. Consult with a sports dietitian for a personalized fueling plan to ensure you cross the finish line feeling strong.

BarrUltra provides a helpful guide on carb loading strategies for endurance athletes.

Frequently Asked Questions

For optimal results in events over 90 minutes, you should start increasing your carbohydrate intake 1 to 3 days before your race, combined with a reduction in training volume.

General recommendations for carb loading are 7-12 grams of carbohydrates per kilogram of body weight per day during the loading phase, depending on the event and individual needs.

Yes, it can cause a temporary weight gain of 1-2kg. This is due to the extra water stored with glycogen and is not a concern, as the performance benefits outweigh the minor weight increase.

It is better to switch to low-fiber options like white pasta and white rice in the final 24-48 hours before a race. High-fiber foods can cause digestive discomfort and bloating when consumed in high volumes.

Feeling a little heavy or sluggish is normal when your glycogen stores are filling up. To minimize bloating, choose easily digestible, lower-fiber carbohydrate sources and drink plenty of fluids.

Research suggests there may be differences in how men and women store glycogen. Some women may need a slightly higher total calorie and carbohydrate intake to maximize their stores, so it's important to listen to your body and potentially consult a dietitian.

Yes, carb loading fills your initial fuel tank, but you still need to consume carbohydrates during the race to prevent your glycogen stores from depleting and to delay fatigue.