How Soon Should I Carb Load Before an Endurance Race?

December 30, 2025 •

4 min read

According to research published in the journal Sports Medicine, diets high in carbohydrates can improve performance in endurance events lasting over two hours by up to 3%. For athletes aiming for peak performance, knowing how soon should I carb load is a critical strategy to maximize muscle glycogen stores and avoid hitting the infamous 'wall'.

Quick Summary

Carb loading for endurance events typically starts 1-3 days before a race to maximize muscle glycogen stores. Optimal timing, daily intake, and food choices vary by event length and the athlete's individual needs. Plan ahead and practice your strategy during training to avoid performance-impairing mistakes.

Key Points

Start 1-3 Days Before: For endurance events over 90 minutes, begin your carb-loading phase between 1 to 3 days before the race, not the night before.
Consume 8-12g/kg of Carbs: Target a daily intake of 8-12 grams of carbohydrates per kilogram of body weight, adjusting based on your event and personal tolerance.
Choose Low-Fiber, Easily Digested Foods: Focus on simple carbs like white rice, pasta, and potatoes while reducing high-fiber foods to prevent stomach issues.
Practice Makes Perfect: Always test your carb-loading strategy during your longest training runs to see how your body reacts before race day.
Temporary Weight Gain is Normal: Expect a temporary weight gain of 1-2kg due to water retention, which is a sign of successful glycogen storage.
Avoid the Depletion Phase: Modern research shows that the traditional glycogen depletion phase is unnecessary and can be counterproductive.

The Science Behind Carb Loading

Carbohydrate loading, or 'carb loading', is a nutritional strategy used by endurance athletes to maximize the energy stored in their muscles and liver. When we eat carbohydrates, our bodies convert them into glucose. The glucose not immediately used for energy is stored as glycogen. This glycogen serves as a crucial fuel reserve during sustained, high-intensity exercise.

During an endurance event like a marathon, the body's primary fuel source shifts to these glycogen stores after about 90 minutes. Once these reserves are depleted, athletes experience a sudden drop in performance known as 'hitting the wall'. The goal of carb loading is to fill this 'fuel tank' to its maximum capacity, providing a longer and more accessible energy supply for race day.

Optimal Timing: How Soon to Carb Load

For many years, a complex six-day protocol involving a depletion phase was standard practice, but modern sports science has shown this is unnecessary and can cause distress. The current consensus points to a shorter, more effective loading window, timed correctly with a reduced training load, or 'tapering'.

The Modern 1-3 Day Protocol

Most sports dietitians now recommend a 1-to-3-day carbohydrate loading phase leading up to the event, with the duration depending on the athlete's experience and event type. This involves increasing your carbohydrate intake to between 8 and 12 grams per kilogram of body weight per day while tapering your training intensity.

For an event like a half-marathon or shorter triathlon, a 24-hour protocol might be sufficient, while a full marathon or ultra-endurance event may benefit from the full 72-hour loading period. This shorter, more concentrated loading period allows athletes to maximize glycogen storage without the risks associated with the old, restrictive depletion phase. The effects of increased glycogen can last for several days, so you won't lose your advantage if you finish the load a day or two before the race.

Race Week Timing: A Sample Plan

For a Sunday race, a common schedule would involve starting your load on Thursday or Friday morning. The heaviest carbohydrate days would be Friday and Saturday, with a slightly reduced but still high-carb breakfast on race morning. This approach gives your digestive system time to process the food without feeling overly full on the starting line.

What to Eat: The Best Foods for Carb Loading

During the carb-loading phase, the focus shifts from fiber-rich complex carbohydrates to easily digestible, low-fiber options. This helps prevent stomach discomfort and bloating on race day. Your plate should be overwhelmingly carbohydrate-heavy, with reduced fat and protein intake to make room for the extra carbs.

Best Carb-Loading Foods:

White Pasta and Rice: Excellent, easily digestible sources of carbohydrates.
White Bread and Bagels: Familiar and simple, these are great for meals and snacks.
Oats and Low-Fiber Cereals: Options like Rice Krispies or cornflakes can provide carbs without excess fiber.
Potatoes (peeled) and Sweet Potatoes (peeled): Removing the skin reduces fiber content, making them easier to digest.
Bananas: A low-fiber fruit option that is gentle on the stomach.
Fruit Juice: A simple way to boost your carb intake through liquids, which can be helpful if you have a low appetite.
Sports Drinks: Provide both carbohydrates and electrolytes, especially useful for topping off glycogen stores.
Honey, Jam, and Syrups: These simple sugars can be added to meals to increase carbohydrate density.

What to Avoid During Your Carb Load

Just as important as knowing what to eat is knowing what to avoid to prevent potential issues on race day. Avoiding these foods helps keep your gut happy and ready for the demands of the event.

High-Fiber Foods: Raw vegetables, whole grains, and beans can lead to bloating and gut issues during your race.
High-Fat Foods: Fatty meats, fried foods, and rich sauces slow digestion and can make you feel sluggish.
Spicy or New Foods: Stick to familiar, tested foods during your load to avoid any unexpected digestive surprises.
Excessive Protein: While important for muscle repair, too much protein can make you feel full, preventing you from consuming enough carbohydrates.
Large, Infrequent Meals: Spreading your carbohydrate intake across several smaller meals and snacks is often easier on the digestive system than trying to cram in a massive amount at once.

Carb Loading: A Comparison Based on Event Duration


Event Length	Typical Carb Loading Protocol	Daily Carb Target (per kg)	Key Food Focus	Key Considerations
Under 90 mins (e.g., 5K/10K)	Not typically necessary.	Standard athlete intake (5-7g)	Balanced diet, normal foods	Focus on a solid, familiar race-day breakfast.
90 mins to 3 hours (e.g., Half-Marathon)	1 to 2 days prior.	8-10g/kg	Easily digestible, low-fiber carbs.	Practice race-day breakfast timing during training.
Over 3 hours (e.g., Marathon/Ultra)	2 to 3 days prior.	10-12g/kg	High-volume, low-fiber carbs (including liquids).	Practice extensively during training to find what works best.

Conclusion: Fueling Your Performance

Knowing how soon should I carb load and following a disciplined strategy can significantly enhance your endurance performance. Starting your carb load 1 to 3 days before your race, focusing on easily digestible, low-fiber carbs, and continuing to stay hydrated will ensure your glycogen stores are topped off. Just as critical is practicing your carb load during your longest training runs to fine-tune your approach. Proper planning and execution will give you the energy reserves you need to reach the finish line strong and perform your best.

For more information and detailed guidelines on athlete nutrition, consult resources from authoritative sports nutrition organizations, such as Sports Dietitians Australia.

Disclaimer: Always consult a healthcare or sports nutrition professional before making significant changes to your diet, especially if you have an existing health condition like diabetes.

Frequently Asked Questions

For a marathon, it is best to begin carb loading 2 to 3 days before the race. This gradual increase in carbohydrates allows your body to maximize its glycogen stores without causing discomfort from eating too much at once.

No, carb loading is generally not necessary for shorter races like a 5K or 10K. These events don't typically last long enough to deplete your body's normal glycogen stores.

Yes, you should still eat some protein, but you will want to decrease your intake to prioritize carbohydrates. Eating too much protein can make you feel full and prevent you from consuming enough carbs.

Feeling slightly heavier or bloated is common, as glycogen binds with water. To minimize discomfort, focus on easily digestible, lower-fiber carbs like white pasta and bread, and spread your intake over multiple smaller meals.

The key is to spread your carbohydrate intake evenly throughout the day, starting with a carb-heavy breakfast. This helps maintain steady glycogen levels and prevents you from feeling uncomfortably full.

During the carb loading phase, you should be tapering your exercise. Reducing your training volume allows your body to store the extra carbohydrates more effectively.

While storing glycogen too early doesn't cause harm, there's no added benefit to loading more than 3-5 days out. Modern, shorter protocols are more efficient and less burdensome.