When to Carb Load Before an Event for Peak Performance

October 23, 2025 •

2 min read

For every one gram of stored glycogen, the body stores three to four grams of water, according to sports dietitians, which is vital for hydration. Mastering when to carb load before an event is key for any endurance athlete aiming to optimize performance.

Quick Summary

This article discusses the best timing and methods for carbohydrate loading before endurance events. It explains glycogen storage, outlines various strategies for different race lengths, and provides tips for maximizing fuel reserves.

Key Points

Timing is Critical: Start your high-carbohydrate diet 2-3 days before an endurance event.
Taper and Fuel: Combine carb-loading with a reduction in training volume to maximize glycogen storage.
Choose Low-Fiber Carbs: Opt for easily digestible, low-fiber carbohydrates to avoid gastrointestinal issues.
Prioritize Hydration: Increase fluid intake alongside carbs, as water is essential for storing glycogen.
Practice in Training: Practice any carb-loading strategy during training; never try it for the first time on race day.
Adjust for Event Length: Carb loading is most beneficial for events lasting longer than 90 minutes; it's not typically necessary for shorter races.
Shift Macronutrients: Focus on increasing the percentage of calories from carbs while reducing fat and protein.

The Science Behind Carb Loading

Carbohydrate loading, or 'carb loading', is a nutritional strategy used by endurance athletes to increase glycogen stores in the liver and muscles. Glycogen is the body's main energy source for moderate to high-intensity exercise. For events lasting over 90 minutes, such as a marathon, adequate glycogen levels are essential to prevent fatigue. By increasing carbohydrate intake and reducing training intensity, athletes can maximize their glycogen reserves.

Modern Carb-Loading Protocols

Unlike traditional methods, modern carb loading doesn't require a depletion phase. The focus is on increasing carbohydrate consumption while tapering exercise.

Events 90 minutes or longer: Increase carbohydrate intake 2-3 days before the event.
Shorter events (under 90 minutes): Carb loading is usually not necessary.
The Taper: Reducing training volume and intensity allows muscles to store more glycogen.

The Optimal Timing for Carb Loading

The most effective window for carb loading starts 36-48 hours before your event. This involves a gradual increase in carbohydrate intake over two days, making carbs the primary macronutrient while reducing fat and protein.

Two Days Before: Increase carb intake to 7-10 grams per kilogram of body weight daily. Choose easily digestible, low-fiber options like white rice or pasta to prevent stomach issues. Stay well-hydrated.
One Day Before: Continue a high-carb, low-fiber diet with familiar foods. Have a moderate, carb-rich dinner earlier in the evening.
Morning of Event: Eat a light, high-carb breakfast 2-4 hours before the start to replenish liver glycogen.

The Importance of Practice

Always practice your carb-loading strategy during training before a major event to see how your body responds.

Comparison Table: Carb-Loading for Different Event Durations


Event Duration	Carb-Loading Strategy	Rationale	Examples
Under 90 minutes	Not necessary	Normal glycogen stores are sufficient.	5K, 10K race, most sprint triathlons.
90 minutes to 3 hours	Moderate increase over 1-2 days	Helps top off glycogen stores to delay fatigue.	Half-marathon, Olympic distance triathlon.
Over 3 hours	Significant increase over 2-3 days	Maximizes glycogen for sustained high-intensity output.	Marathon, Ironman, Ultra-marathons.

Avoiding Common Carb-Loading Mistakes

Overeating: Focus on shifting macronutrients, not just eating more.
Too much fiber: High fiber can cause digestive problems. Choose low-fiber options.
New foods: Stick to tested foods to avoid stomach upset.
Leaving it too late: Begin 2-3 days prior for full glycogen saturation.

What Foods to Choose When Carb Loading

Prioritize easily digestible, low-fat, low-fiber carbohydrates:

Grains: White pasta, white rice, bagels, low-fiber cereals.
Fruits: Bananas, fruit juice, applesauce.
Starches: Peeled potatoes, sweet potatoes, low-fat pancakes.
Beverages: Sports drinks and fruit juices can supplement solid food intake.

Conclusion: Fuel Your Success Strategically

Knowing when to carb load before an event is vital for endurance performance. Starting 2-3 days out with a high-carbohydrate, low-fiber diet and tapering training ensures maximum glycogen stores. Practicing your strategy allows you to find what works best, helping you finish strong. For more detailed nutrition guidance, consult resources like the American College of Sports Medicine.

Frequently Asked Questions

The optimal time to start carb loading for a marathon is 2 to 3 days before the race. A significant increase in carbohydrate intake during this period, combined with a reduction in training, effectively maximizes your muscle and liver glycogen stores.

No, carb loading is generally not necessary for a 5K race or other events lasting less than 90 minutes. Your body's regular glycogen stores are sufficient for these shorter durations, provided you have a balanced diet.

For carb loading, focus on easily digestible, low-fiber carbohydrates to avoid stomach upset. Good options include white rice, white pasta, bagels, pancakes, peeled potatoes, fruit juice, and sports drinks.

During the 2-3 day carb-loading window, you should aim for a daily carbohydrate intake of approximately 7 to 10 grams per kilogram of your body weight. This will be a significant increase from your normal diet.

The night before a race, have a carbohydrate-heavy but moderate-sized meal composed of familiar, low-fiber foods. Eating earlier in the evening allows for proper digestion and better sleep. Avoid heavy, fatty foods or anything you haven't tested in training.

Yes, temporary weight gain is a normal side effect of carb loading. This is because for every gram of glycogen stored, your body also stores several grams of water, which adds to your body weight. This extra fluid is beneficial for hydration during the race.

Both solid and liquid carbohydrates can be effective. If you find it difficult to consume the large quantity of solid food needed, incorporating liquid sources like sports drinks and fruit juice can be very helpful to meet your carbohydrate goals.