Are Simple or Complex Carbs Better for Carb Loading? The Strategic Approach for Endurance Athletes

October 25, 2025 •

4 min read

Endurance performance can improve by 2-3% by maximizing muscle glycogen stores through carb loading. The crucial question for athletes isn't just how much to eat, but rather, "Are simple or complex carbs better for carb loading?" The answer lies in understanding the timing and purpose of each type of carbohydrate to avoid GI issues and properly fuel for race day.

Quick Summary

For effective carb loading, endurance athletes should strategically favor lower-fiber, easy-to-digest carbohydrates—often refined simple or complex options—in the final days before an event to maximize glycogen storage and prevent gastrointestinal distress.

Key Points

Timing Matters: While complex carbs are generally better for daily health, simple or refined complex carbs are preferred in the final 1-3 days before an endurance event.
Prioritize Low-Fiber: For carb loading, focus on easy-to-digest, lower-fiber carbs like white rice, white bread, and sports drinks to prevent gastrointestinal discomfort on race day.
Maximize Glycogen Storage: Carb loading is most effective for events lasting 90 minutes or longer, with the primary goal of fully saturating muscle and liver glycogen stores.
Watch for GI Issues: High-fiber complex carbs can cause bloating and make it difficult to consume the high volume of carbs needed for effective loading.
Practice Makes Perfect: Always test your carb-loading plan during training runs or similar efforts to see what foods and timing work best for your body.
Reduce Fat and Protein: During the final loading days, slightly decrease fat and protein intake to make more room for the necessary increase in carbohydrate consumption.

The Foundation of Performance: Glycogen and Carb Loading

Carbohydrate loading is a nutritional strategy used by endurance athletes to maximize the storage of glycogen in their muscles and liver. Glycogen is the body's primary fuel source for high-intensity, prolonged exercise, and depleted stores can lead to severe fatigue, a phenomenon known as "hitting the wall". An effective carb-loading plan is designed to top off these energy reserves, providing a crucial performance edge for events lasting 90 minutes or longer, such as marathons, triathlons, and long-distance cycling races.

To understand which carbs to prioritize, athletes must first grasp the distinction between simple and complex carbohydrates and how their body processes them.

Simple vs. Complex Carbohydrates: The Key Differences

Simple carbohydrates, or sugars, consist of one or two sugar molecules. They are broken down and absorbed quickly, providing a rapid release of glucose into the bloodstream. Complex carbohydrates, or starches, are long chains of sugar molecules. Their complex structure requires more time to digest, resulting in a slower, more sustained release of energy.

For general health, complex carbohydrates are often favored due to their higher fiber content and more stable energy release. However, when it comes to the specific, high-volume demands of carb loading, the rules change.

The Strategic Use of Carb Types for Loading

During a carb-loading phase, which typically starts 1-3 days before an event, an athlete aims to consume an unusually high volume of carbohydrates—around 8-12 grams per kilogram of body weight. In this specific context, a strategic approach that prioritizes simpler, lower-fiber carbs over whole-grain complex carbs is often recommended.

The Case for Simple and Refined Carbs

White Rice and Pasta: These foods are complex carbohydrates, but the refining process removes most of the fiber. This makes them easier to digest and absorb, preventing gastrointestinal upset during the crucial days before a race.
Sports Drinks and Juices: These liquid carbs are rapidly absorbed and can help an athlete reach their daily carb target without feeling excessively full. They also contribute to hydration, which is vital for optimal glycogen storage.
Bagels and White Bread: These provide a dense source of carbohydrates with low fiber, making them ideal for topping off glycogen stores without weighing you down.

The Downside of High-Fiber Complex Carbs

While nutritious, high-fiber complex carbs like brown rice, oatmeal, and whole-wheat pasta are often not the best choice during the final 48 hours of carb loading for two main reasons:

Gastrointestinal Distress: High fiber intake can lead to bloating, gas, and an upset stomach, which can negatively impact an athlete's comfort and performance on race day.
Satiety and Volume: Fiber is filling. Eating a high-fiber diet makes it difficult for athletes to consume the massive quantity of carbs needed to fully saturate their glycogen stores.

A Comparison of Carb Types for Loading


Feature	Simple & Refined Complex Carbs (e.g., White Rice, Pasta)	High-Fiber Complex Carbs (e.g., Brown Rice, Whole-wheat Pasta)
Digestion Speed	Rapid, quick energy conversion	Slow, sustained energy release
Best for Carb Loading	Yes, excellent choice for the 1-3 days before a race due to lower fiber and easy absorption.	Not ideal for the final days, as high fiber can cause GI issues and premature fullness.
Best for General Health	Should be limited in a regular diet; can cause blood sugar spikes.	Generally superior for overall health due to fiber, vitamins, and minerals.
Risk of GI Distress	Low risk, assuming tried-and-tested foods	Higher risk due to increased fiber content.
Feeling of Fullness	Less satiating, allowing for higher carb intake.	Very satiating, making it difficult to consume high volumes.

How to Create Your Strategic Carb-Loading Plan

Successful carb loading involves more than just eating a large meal the night before. Follow these steps to maximize your glycogen stores without discomfort:

Plan the Timing: Start your high-carb phase 2-3 days before your event, coinciding with your training taper. Tapering allows your muscles to heal and store more glycogen, rather than using it for intense workouts.
Choose Lower-Fiber Options: In the final 48 hours, switch from whole-grain choices to refined, lower-fiber options. Focus on white rice, white pasta, bagels, potatoes without the skin, and fruit juices. These are gentle on the digestive system.
Reduce Fat and Protein: While protein is essential for muscle repair, too much can displace carbohydrates and slow digestion. In the final days, slightly reduce your intake of fat and protein to make room for the higher volume of carbs required.
Embrace Liquid Carbs: If you struggle to eat enough solids, incorporate liquid sources. Sports drinks, fruit juices, and smoothies can be an easy way to increase your carb intake without feeling bloated.
Practice in Training: Never try a new carb-loading strategy on race week. Test your plan during your long training runs to understand what foods and quantities work best for your body.
Stay Hydrated: Drink plenty of fluids alongside your meals. Carbohydrates require water to be stored as glycogen, so proper hydration is crucial for the process to work effectively.

Conclusion: A Strategic Approach to Carb Loading

Ultimately, the question of are simple or complex carbs better for carb loading has a nuanced answer. While complex carbohydrates are generally healthier for daily nutrition, the specific, high-volume demands of pre-race fueling require a strategic shift. For the 1-3 days leading up to an endurance event, prioritizing easy-to-digest, lower-fiber carbohydrates—including refined options like white pasta, rice, and bagels—is the most effective way to maximize glycogen stores and avoid potential gastrointestinal distress. Complex, high-fiber carbs should be reserved for regular, daily fueling. By approaching your carb-loading with a clear strategy, you can arrive at the starting line fully fueled and ready to perform your best. For more in-depth guidance on planning your race nutrition, consider resources like TrainingPeaks, which provides expert insights from sports nutritionists.

Frequently Asked Questions

No, a single large meal is not enough. Effective carb loading requires 1-3 days of consistently higher-than-normal carbohydrate intake, typically 8-12 grams per kg of body weight, to fully saturate glycogen stores.

Consuming too much fiber during carb loading can cause gastrointestinal issues like bloating, gas, and stomach upset on race day. This is why lower-fiber, refined carbs are recommended in the final days before an event.

Yes. During your regular training, and not in the final days of loading, it is beneficial to consume high-fiber complex carbs (like whole grains, beans, and vegetables) for their sustained energy and nutritional benefits.

Yes, liquid carbohydrates from sports drinks and juices are an excellent way to help meet high carb targets during the loading phase, especially if you struggle to eat enough solid food.

Good examples include white rice, white pasta, bagels, potatoes (peeled), crackers, ripe bananas, and fruit juices. These are easy to digest and absorb quickly.

Carb loading is not typically necessary for events shorter than 90 minutes, as your normal glycogen stores are usually sufficient. For shorter races, focus on a balanced diet and a good pre-race meal.

You should reduce your fat intake during carb loading. Fat is more calorie-dense and can make it difficult to eat enough carbohydrates. High-fat meals also slow down digestion.