How long to carb load before an event?

December 26, 2025 •

3 min read

According to research published in the journal Sports Medicine, effective carb loading can improve endurance performance by 2-3% in events lasting over 90 minutes. The key to reaping these benefits lies in knowing not just what to eat, but critically, how long to carb load before an event to properly fill your muscles' glycogen stores for race day.

Quick Summary

The ideal carb loading period for endurance events lasting over 90 minutes is typically 1 to 3 days, combined with a tapering of training. Athletes should aim for a higher intake of easily digestible, low-fiber carbohydrates, with recommended amounts varying based on the loading duration. Longer protocols use less intense carb intake over more days, while shorter ones concentrate a higher dose into a tighter window.

Key Points

Endurance Events Need Longer Loading: For events lasting over 90 minutes (e.g., marathons), a 2- to 3-day carb-loading protocol is most effective.
Shorter Events Need Less: A single-day, 24-hour carb load can provide a sufficient top-off for events lasting 60-90 minutes.
Carb Intake Varies with Duration: A longer loading phase (2-3 days) involves a moderate daily intake (7-10g/kg), while a shorter one-day protocol requires a more intense intake (10-12g/kg).
Taper Your Training: Reduce training volume and intensity during your carb-loading period to allow your muscles to maximize glycogen storage.
Focus on Low-Fiber Carbs: Opt for easily digestible carbohydrate sources like white pasta, rice, and sports drinks, especially in the 24-48 hours before the race, to prevent digestive upset.
Hydrate, Hydrate, Hydrate: Increase your fluid intake during carb loading, as each gram of stored glycogen also binds with water.
Practice Makes Perfect: Test your carb-loading strategy during a long training run to see how your body responds before your main event.

Understanding the Carb Loading Timeline

Carb loading, or supercompensation, is a nutritional strategy used by endurance athletes to maximize the amount of glycogen stored in their muscles and liver. This process provides a readily available fuel source for prolonged, high-intensity activity, helping to delay fatigue and improve performance. While the traditional method involved a grueling depletion phase, modern science confirms this is unnecessary and a targeted loading phase is just as effective. The duration of your carb-loading protocol depends primarily on the length of your event and your body's individual response, but research points to optimal windows.

The 1-Day Carb Loading Protocol

For events lasting between 60 and 90 minutes, such as a 10K or short-distance triathlon, a one-day carb-loading protocol is often sufficient. Your body's normal glycogen stores can typically handle activities of this length, but a final 24-hour top-off can provide a performance edge.

Target Intake: Approximately 10-12 grams of carbohydrates per kilogram of body weight.
Strategy: Focus on consuming easily digestible, lower-fiber carbohydrate sources. This includes options like white rice, white bread, low-fiber cereals, and sports drinks. Spreading the intake throughout the day with small, frequent meals and snacks is often easier than forcing large portions.
Example for a 70kg athlete: 700-840g of carbs over 24 hours. A good plan might involve high-carb breakfasts, lunches, and dinners, supplemented with snacks and sports drinks.

The 2-to-3-Day Carb Loading Protocol

This is the most common and widely recommended approach for true endurance events, like marathons, ultramarathons, and Ironman races, which last longer than 90 minutes. This longer window allows athletes to gradually increase glycogen stores without the digestive distress that can accompany a single-day, high-volume intake.

Target Intake: Aim for 7-10 grams of carbohydrates per kilogram of body weight over a 2-3 day period.
Strategy: During this period, reduce your training volume significantly to allow your muscles to store the extra carbohydrates. Reduce your fat and fiber intake, focusing on carb-dense, familiar foods to avoid gastrointestinal issues on race day.
Example for a 70kg athlete: 490-700g of carbs per day for 2-3 days. This can be achieved through regular carb-heavy meals like pasta dinners, rice bowls, or pancakes, along with carbohydrate-rich snacks.

Comparison of Carb Loading Protocols


Feature	1-Day Protocol (Shorter Events)	2-3 Day Protocol (Endurance Events)
Event Duration	Under 90 minutes	Over 90 minutes
Timing	24-36 hours before event	36-72 hours before event
Carbohydrate Target	10-12g per kg of body weight daily	7-10g per kg of body weight daily
Training Taper	Already tapered for shorter event	Coincides with final training taper
Digestibility	Easier to manage volume	Spreads volume out, gentler on stomach
Example Foods	White rice, sports drinks, bananas	Pasta, potatoes, bagels, sports drinks

Practical Tips for a Successful Carb Load

To ensure your carb load is effective and doesn't cause stomach issues, consider these practical tips:

Hydrate Adequately: Glycogen is stored with water, so increased fluid intake is crucial for proper storage and hydration.
Practice Your Plan: Never try a new carb-loading strategy on race week. Practice your fueling plan during a long training run to see how your body reacts.
Avoid High-Fiber Foods: While a great part of a daily diet, high-fiber foods can cause bloating and digestive distress during your event. Opt for lower-fiber, refined carbohydrate sources instead.
Listen to Your Body: What works perfectly for one athlete may not for another. Pay attention to how you feel and adjust your intake accordingly.

Conclusion

Knowing how long to carb load before an event is a critical component of any successful race-day strategy. For endurance events lasting over 90 minutes, a 2-to-3-day approach is recommended to maximize muscle glycogen stores and sustain energy. For shorter, less intense efforts, a one-day protocol can be beneficial. By understanding the principles and tailoring the approach to your specific event and body, you can ensure you cross the finish line feeling strong and energized.

For more in-depth nutritional strategies, consulting a Registered Dietitian with experience in sports nutrition is highly recommended to create a personalized plan.

Frequently Asked Questions

You should start carb loading for a marathon, or any event lasting over 90 minutes, approximately 2 to 3 days before race day. This allows your body to gradually increase muscle and liver glycogen stores for maximum fuel availability.

No, a single high-carb meal the night before is insufficient for fully maximizing glycogen stores, though it can help top off liver glycogen. Proper carb loading requires a consistent increase in carbohydrate intake over 1 to 3 days, combined with reduced training volume.

Focus on easily digestible, low-fiber carbohydrate sources such as white pasta, rice, potatoes, bagels, pancakes, and fruit juices. These are less likely to cause digestive upset on race day compared to high-fiber options.

Carb loading is generally not necessary for events lasting less than 90 minutes, like a 5K. Your body's normal glycogen reserves are typically adequate, so focusing on a balanced diet and proper hydration is key.

Some temporary water weight gain and mild bloating are normal during carb loading, as each gram of stored glycogen also holds water. To minimize discomfort, focus on low-fiber carbohydrates and spread your intake out throughout the day.

For endurance events, aim for 7-10 grams of carbohydrates per kilogram of body weight per day over a 2-3 day period. For example, a 70kg athlete would aim for 490-700g of carbs daily.

While the focus is on carbohydrates, you should not eliminate protein and fat entirely. Instead, reduce your intake of these macronutrients to make room for the higher volume of carbs without excessively increasing your total calorie count.