How Many Days Before a Marathon Should You Carb Load for Success?

October 23, 2025 •

3 min read

Athletes who properly execute a carb-loading strategy can increase their muscle glycogen stores by as much as 30% to 50%, providing a critical energy reserve for race day. Knowing how many days before a marathon should you carb load is the first step toward maximizing this performance-boosting technique.

Quick Summary

Carb-loading for a marathon is a strategic, multi-day process to maximize muscle glycogen stores, not a single large meal. The optimal timing is typically 2 to 3 days before the race, focusing on increased intake of easily digestible carbohydrates to prevent premature fatigue.

Key Points

Start 2-3 Days Before: Begin carb-loading 36-72 hours before the marathon, aligning with your taper.
Spread Out Your Intake: Distribute carbohydrate consumption across multiple meals and snacks.
Prioritize Low-Fiber Carbs: Focus on easily digestible carbs to avoid digestive issues.
Stay Hydrated: Drink plenty of fluids during carb-loading to aid glycogen storage and hydration.
Practice in Training: Test your carb-loading plan during long training runs.
Reduce Fiber and Fat: Lower intake of fat and fiber in the days before the race.

The Science of Carb-Loading

Carbohydrate loading is a nutritional strategy used by endurance athletes to maximize the glycogen stored in their muscles and liver. Glycogen is a primary energy source for prolonged, high-intensity exercise like marathons. As a race progresses, the body increasingly relies on these finite glycogen stores. Depletion of these stores can lead to a significant drop in performance, often referred to as "hitting the wall". Properly carb-loading ensures these energy stores are full before the race, helping to maintain pace and avoid energy crashes. While fueling during the race is also important, carb-loading provides the essential energy base.

So, How Many Days Before a Marathon Should You Carb Load?

Sports nutritionists generally recommend starting to increase carbohydrate intake 2 to 3 days before a marathon. This timeframe, about 36-72 hours prior to the event, allows the body to maximize glycogen storage while training volume is reduced. It's more effective to distribute carbohydrate intake over several meals and snacks during this period rather than consuming a large amount the night before, which can cause discomfort.

The Timing and Tapering Connection

The 2-3 day carb-loading phase aligns with the final taper in marathon training, where mileage is reduced to allow for muscle recovery. This combination of decreased energy expenditure and increased carbohydrate intake creates an ideal environment for glycogen storage. For a Sunday marathon, for instance, carb-loading would typically begin on Thursday evening or Friday morning.

What to Eat During Your Carb-Load

During the 2-3 day carb-loading period, the focus is on consuming easily digestible carbohydrates while reducing fiber and fat intake to minimize the risk of digestive issues on race day.

Recommended Low-Fiber Carb Sources:

White pasta, white rice
White bread, bagels, English muffins
Potatoes (without the skin)
Pancakes or waffles with syrup
Low-fiber cereals
Pretzels and rice cakes
Bananas and canned fruit
Fruit juice and carbohydrate-rich sports drinks
Energy gels and chews

Foods to Reduce or Avoid:

High-Fiber Foods: Brown rice, whole wheat bread, oats, beans, legumes, and most raw vegetables. These can cause stomach issues.
High-Fat Foods: Creamy sauces, fried foods, and fatty meats, which can slow digestion.
New or Exotic Foods: Stick to familiar foods to avoid unexpected digestive problems.

The Role of Hydration in Carb-Loading

Hydration is important during carb-loading because for every gram of glycogen stored, about 3 grams of water are also stored. This stored water aids overall hydration during the race. Consistent fluid intake, including water and potentially electrolyte drinks, is recommended.

Carb-Loading Strategies: Comparison Table


Feature	Recommended 2-3 Day Strategy	Old-School 1-Night Binge
Timing	Starts 2-3 days before the race.	Single, large meal the night before.
Carb Distribution	Spreads high-carb meals and snacks throughout the day.	Concentrates all extra carbs into one meal.
Digestion	Easier on the stomach, reduces risk of GI issues.	Can cause bloating, discomfort, and poor sleep.
Food Choices	Prioritizes lower-fiber, easily digestible carbs.	Often includes high-fiber pasta and rich sauces.
Performance Impact	Maximizes glycogen stores for sustained endurance.	Can lead to incomplete glycogen storage and sluggishness.

Conclusion: Practice Makes Perfect

Understanding when to carb-load is key, but practicing your nutrition strategy during training is equally important. Experiment with foods and quantities before long runs to determine what works best for you. Proper pre-race fueling is about strategically adjusting your diet to ensure you are well-fueled for a successful marathon performance. Consulting a sports dietitian can provide further personalized guidance. Learn more about marathon nutrition strategies on Runner's World.

Frequently Asked Questions

The optimal time to start carb-loading for a marathon is 2 to 3 days before the race.

Starting too early, such as a week before, is generally unnecessary and won't effectively increase glycogen stores before the training taper.

Ideal foods are high in carbohydrates and low in fiber and fat, such as white pasta, white rice, bagels, and peeled potatoes.

Yes, include moderate amounts of lean protein and healthy fats, but carbohydrates should be the primary focus.

Some bloating is normal as water is stored with glycogen, contributing to hydration during the race.

Carb-loading is typically not needed for races under 90 minutes.

For the 2-3 day loading phase, lower-fiber simple carbs are often preferred for easier digestion in large quantities, while complex carbs are better for regular training.