What is the Best Source for Carb Loading? A Guide for Endurance Athletes

October 5, 2025 •

4 min read

For endurance events lasting over 90 minutes, a proper carbohydrate loading strategy can increase muscle glycogen stores by up to 30%. This technique helps delay fatigue, but for many athletes, the question remains: what is the best source for carb loading to achieve optimal results without digestive issues?

Quick Summary

Effective carb loading involves strategically increasing intake of easily digestible carbohydrates like white rice, pasta, and potatoes in the final days before an endurance event. Selecting low-fiber sources is key for maximizing glycogen and avoiding gastrointestinal distress.

Key Points

Prioritize Easily Digested Carbs: In the 2-3 days before an event, focus on low-fiber, refined carbohydrate sources like white rice, white pasta, and potatoes to prevent digestive issues.
Start Loading 2-3 Days Out: Begin your increased carbohydrate intake 48-72 hours before your race to give your body enough time to maximize glycogen stores.
Aim for 8-12g/kg Body Weight: Consume 8-12 grams of carbohydrates per kilogram of body weight per day during the loading phase to effectively top off your energy reserves.
Avoid High-Fiber and High-Fat Foods: Steer clear of whole grains, fibrous vegetables, and fatty sauces in the final days to aid digestion and ensure maximum carb intake.
Stay Well-Hydrated: Drink plenty of fluids, as each gram of stored glycogen binds with water, which is essential for performance and hydration.
Practice Your Plan: Use your long training runs to experiment with your carb-loading meals and timing to identify what works best for your body.

The Science Behind Carb Loading

Carbohydrate loading is a nutritional strategy employed by endurance athletes, such as marathon runners and triathletes, to maximize glycogen stores in their muscles and liver. Glycogen is the body's primary and most readily available fuel source for high-intensity exercise. While the body typically has enough glycogen for about 90 minutes of continuous activity, events exceeding this duration can deplete these reserves, leading to what is known as 'hitting the wall'. By increasing carbohydrate intake and simultaneously tapering training in the days leading up to an event, athletes can supercompensate their glycogen stores. This results in a larger 'fuel tank' for race day, which can improve endurance performance by 2-3%.

The Importance of Easily Digestible Carbs

Not all carbohydrates are created equal, especially in the context of pre-race preparation. While nutrient-dense whole grains are beneficial for everyday health, their high fiber content can cause digestive distress, bloating, and gas in the days leading up to a race. This is why sports nutritionists recommend focusing on easily digestible, low-fiber, refined carbohydrates for the 2-3 day carb-loading phase. These options allow for rapid absorption of glucose to top off glycogen stores efficiently without taxing the digestive system.

Complex vs. Simple Carbs for Loading

It's important to understand the role of different types of carbohydrates. Complex carbohydrates, found in foods like brown rice and whole-wheat pasta, provide a slower, sustained release of energy and are excellent for daily fueling. However, for the final 48-72 hours of carb loading, many athletes switch to more refined, low-fiber options to prevent stomach issues and facilitate maximum glycogen storage. Simple carbohydrates, such as those in energy gels and drinks, are crucial for quick, intra-race energy, but can also be used strategically during the loading period to meet high carbohydrate targets.

Here are some of the best sources to consider:

White Rice: Easy to digest and a versatile staple for carb loading meals. It's low in fiber and can be consumed in large quantities without causing digestive discomfort.
White Pasta: Another classic choice, pasta with a simple, low-fat sauce (like tomato-based) is a great option. Choose a white variety over whole-wheat during the final days.
Potatoes: White and sweet potatoes are both excellent sources of easily digestible carbs, especially when prepared without the skin.
Oats: A good option for breakfast, especially if it's a familiar food that your body tolerates well.
Bread and Bagels: White bread, plain bagels, and English muffins provide a high dose of carbs with minimal fiber.
Low-Fiber Fruits: Ripe bananas, peeled apples, fruit juices, and applesauce are easy on the stomach and provide quick energy.
Energy Gels and Sports Drinks: For athletes struggling to meet high carb intake goals through food alone, supplements like sports drinks and energy gels can be an effective way to pack in extra carbs.

Best Carb Loading Sources Compared


Carb Source	Type	Fiber Content	Ease of Digestion	Recommended Use	Notes
White Rice	Refined Complex	Low	Very High	2-3 days before event	Versatile for meals.
White Pasta	Refined Complex	Low	High	2-3 days before event	Avoid creamy, fatty sauces.
Sweet Potato	Complex	Low-Medium (without skin)	High	2-3 days before event	Nutrient-dense.
Bananas	Simple	Low	Very High	2-3 days before event, pre-race snack	Excellent source of potassium.
Whole-Wheat Pasta	Complex	High	Low-Medium	During training, not for pre-race	High fiber can cause gut issues.
Brown Rice	Complex	Medium	Medium	During training, not for pre-race	Healthier for daily diet.
Sports Drinks	Simple	Very Low	Very High	During loading and pre-race	Good for boosting intake when solid food is too filling.

Timing and Amount for Optimal Fueling

The carb-loading process is more than just a single large meal; it's a strategic period of increased intake paired with reduced activity. Research suggests starting 2-3 days before your event. The recommended amount is typically 8-12 grams of carbohydrates per kilogram of body weight each day. For a 70kg athlete, this means consuming 560-840 grams of carbs daily. Spreading this intake across smaller, more frequent meals can prevent a feeling of being overly full or bloated.

Hydration and What to Avoid

Alongside a high-carb diet, staying well-hydrated is crucial, as every gram of stored glycogen is bound to 3 grams of water. This extra water weight is a positive sign of a successful load, not fat gain. During the loading period, it's equally important to know what to avoid:

High-fiber foods: Limit intake of high-fiber vegetables (like broccoli and beans) and whole grains to avoid gut residue and potential GI issues during the race.
High-fat foods: Heavy, creamy sauces, fried foods, and fatty cuts of meat should be reduced, as they slow digestion and can displace valuable carbohydrate calories.
New foods: Never experiment with a new food or meal plan right before an event. Stick to what you've practiced during training.

Conclusion

There is no single 'best' source for carb loading, but rather a spectrum of high-carbohydrate foods that should be chosen based on timing and digestive tolerance. For the final 48-72 hours before a major endurance event, focusing on easily digestible, low-fiber sources like white rice, pasta, and potatoes is the most effective strategy to maximize glycogen stores without risking digestive discomfort. By combining this dietary approach with proper tapering and hydration, athletes can ensure they arrive at the starting line fully fueled for peak performance. It is always recommended to practice your fueling strategy during long training sessions to find what works best for your body. You can read more about glycemic index and diet planning from sources like the Better Health Channel.

Frequently Asked Questions

No, a single high-carb meal is not sufficient to fully maximize glycogen stores. Effective carb loading requires a consistent, increased carbohydrate intake over a 2- to 3-day period leading up to the event.

Carb loading is generally not necessary for events lasting less than 90 minutes. Your body's normal glycogen stores are usually sufficient for shorter, less demanding efforts.

During the final days of loading, you should avoid high-fiber foods (like whole grains and cruciferous vegetables) and high-fat foods, which can cause digestive issues and slow down carb absorption.

Short-term weight gain (1-2 kg) from carb loading is not fat. It is primarily water weight, as glycogen molecules bind with water in your muscles. This is a sign of successful fueling for your event.

During the final 2-3 days before an event, focus on easily digestible, low-fiber carbs like white rice and pasta. While complex carbs are great for daily health, their high fiber can cause stomach issues right before a race.

Proper hydration is crucial because the process of storing glycogen requires water. For every gram of glycogen your body stores, it also stores an extra 3 grams of water, helping you stay well-hydrated for the race.

Women can benefit from carb loading, though some research suggests they may require slightly different carbohydrate intakes. Practicing and fine-tuning an individual protocol during training is key.