What is Carb Loading Before a Marathon?

October 2, 2025 •

2 min read

Carbohydrate loading, or 'carb loading', is a well-known nutrition strategy, with studies showing it can improve endurance performance by 2-3% in events lasting over 90 minutes. This specialized eating plan is designed to maximize the glycogen stores in your muscles and liver, providing a critical energy reserve for the grueling 26.2 miles of a marathon.

Quick Summary

This guide explains the science behind carbohydrate loading, a strategic approach for endurance athletes to maximize muscle glycogen stores before a marathon. It details timing, what and how much to eat, common mistakes to avoid, and the benefits of practicing this nutritional strategy during training.

Key Points

Strategic Fueling: Carb loading maximizes glycogen stores in muscles, the body's primary fuel for endurance events over 90 minutes, delaying fatigue and improving performance.
Timing is Crucial: Start the carb loading phase 36 to 48 hours before the marathon, not just with one large meal the night before.
Balanced Intake: Aim for 8-12 grams of carbohydrates per kilogram of body weight daily during the loading period, while reducing fat and fiber intake.
Low-Fiber Focus: Choose low-fiber, easy-to-digest carbs like white rice, pasta, and bagels to avoid gastrointestinal issues on race day.
Practice Makes Perfect: Test your carb-loading strategy during your long training runs to find out what works best for your body before the actual race.
Don't Overeat: Carb loading involves shifting macronutrient ratios, not just eating more calories, to avoid bloating and sluggishness.
Stay Hydrated: Increased water intake is essential, as glycogen stores with water. Dehydration can hinder the effectiveness of carb loading.

The Science Behind Carb Loading

Carbohydrates are the primary energy source for endurance activities. When consumed, they are converted into glucose and stored as glycogen in muscles and the liver. These glycogen reserves are crucial for events lasting over 90 minutes, like a marathon. Depleting these stores can lead to severe fatigue, commonly known as "hitting the wall". Carb loading aims to fill these glycogen reserves to their maximum capacity, providing a larger fuel source for the race. This is achieved by combining reduced training (tapering) with increased carbohydrate intake in the days leading up to the event. The reduced energy expenditure, paired with higher energy intake, allows the muscles to store more glycogen than usual.

How to Execute a Successful Carb Load

Effective carb loading is a planned strategy over several days, not just a large meal the night before.

Timing: Start increasing your carbohydrate intake 36 to 48 hours before the marathon. If your race is on Sunday, begin on Friday.
Dosage: Aim for 8-12 grams of carbohydrates per kilogram of body weight daily during the loading period. A 70kg runner would target 560-840 grams of carbs per day. Beginners may start at the lower end.
Tapering and Nutrition: Alongside increased carbs, decrease fat and fiber intake. High fiber can cause stomach issues during a race. Tapering training is also vital to conserve energy and promote glycogen storage.

Comparing High-Carb Foods for Carb Loading

Choose easy-to-digest, high-glycemic, low-fiber options to prevent stomach upset. More details on food choices can be found on {Link: STYRKR website https://styrkr.com/blogs/training-and-nutrition-hub/carb-loading-before-marathon}.

Common Mistakes to Avoid

Avoid these common errors to ensure a successful carb load. These include overeating, consuming the wrong types of foods, trying to load only the night before, not practicing during training, and neglecting hydration. More information on common mistakes can be found on {Link: STYRKR website https://styrkr.com/blogs/training-and-nutrition-hub/carb-loading-before-marathon}.

Practicing Your Carb Load Strategy

Just like training your body, train your gut for race-day nutrition. Test carb loading meals during long runs to identify what works best for your digestive system. Liquid carbs like energy drinks or gels can help meet targets without excessive bulk. Personalization is key; what suits one runner may not suit another.

Conclusion: Fueling for the Finish Line

Proper carb loading is a science-backed method to boost marathon performance by maximizing glycogen stores, helping runners avoid "hitting the wall." Key steps include understanding glycogen's role, timing carb intake over 2-3 days, and choosing low-fiber, high-glycemic foods. Avoiding common mistakes like last-minute loading or poor hydration is crucial. Successful carb loading, combined with training and race-day nutrition, ensures you are optimally fueled for a strong finish.

Frequently Asked Questions

You should start increasing your carbohydrate intake approximately 36 to 48 hours before your marathon. For a Sunday race, this means beginning the process on Friday morning or evening.

Without proper carb loading, your glycogen stores may be depleted before the race ends, leading to severe fatigue often called 'hitting the wall.' Your body will be forced to rely on less efficient energy sources, significantly slowing you down.

Focus on high-glycemic, low-fiber carbohydrates that are easy to digest. Good options include white rice, white pasta, bagels, potatoes (without the skin), oats, and simple fruits.

Yes. While fiber is generally healthy, an excess amount in the days leading up to a race can cause gastrointestinal discomfort, bloating, and diarrhea. Stick to low-fiber options to avoid issues.

Yes, a temporary weight gain of 2-4 lbs is common during effective carb loading. This is due to the extra water that is stored with glycogen in your muscles. The added weight is beneficial for hydration and performance, not a negative side effect.

No, carb loading is most beneficial for endurance events lasting over 90 minutes. For shorter races like a 5K or 10K, maintaining a normal, balanced diet with sufficient carbohydrates is generally enough.

The night before the race, eat a familiar, high-carb, low-fat, and low-fiber meal. Think simple pasta with a light tomato sauce or rice with grilled chicken. Avoid spicy or heavy, rich foods.