What exactly does carb loading do? A Comprehensive Guide for Endurance Athletes

December 12, 2025 •

3 min read

According to research, proper carb loading can improve performance in endurance events over 90 minutes by 2-3% and delay fatigue. This strategic dietary practice, designed to maximize the body's glycogen stores, answers the question, "what exactly does carb loading do?" for serious athletes seeking a competitive edge.

Quick Summary

Carb loading is an endurance sports strategy involving a high carbohydrate intake in the days before a long event. It maximizes glycogen reserves in the muscles and liver, providing a greater energy supply to delay fatigue and improve performance. This technique is most beneficial for activities lasting over 90 minutes.

Key Points

Maximizes Glycogen: Carb loading is a strategy to increase the amount of energy (glycogen) stored in your muscles and liver before an endurance event.
Boosts Performance: For events lasting over 90 minutes, it can improve performance and delay fatigue by 2-3% by providing a sustained energy source.
Requires Specific Timing: Start increasing carbohydrate intake 1-3 days before your competition while reducing your training volume (tapering).
Causes Temporary Weight Gain: For every gram of glycogen stored, three grams of water are also retained, leading to a temporary increase in body weight that also aids hydration.
Emphasize Low-Fiber Carbs: Focus on easily digestible carbohydrates like white rice and pasta, and reduce high-fiber foods to prevent gastrointestinal issues during the event.
Depletion Phase Not Required: Modern science shows that effective carb loading can be achieved without the old, stressful depletion phase of low-carb dieting.

Understanding the Core Function of Carb Loading

Carb loading is a nutritional strategy for endurance athletes participating in events lasting over 90 minutes. Its main goal is to maximize the body's stored form of glucose, known as glycogen, which is kept in muscles and the liver. These glycogen stores serve as a primary energy source, and their depletion can lead to fatigue or "hitting the wall" during prolonged exercise. By adjusting diet and training in the days before an event, athletes can increase these glycogen reserves, effectively creating a larger energy supply for race day.

The Physiological Mechanism of Glycogen Supercompensation

When carbohydrates are consumed, they are converted into glucose for immediate energy or stored as glycogen. During carb loading, specific physiological changes enhance glycogen storage:

Increased Insulin Sensitivity

A reduction in training volume during the taper period before an event increases muscle sensitivity to insulin, which helps muscles absorb and store glucose as glycogen more effectively.

Maximized Glycogen Synthesis

High carbohydrate intake combined with reduced exercise stimulates glycogen synthase enzymes, leading to increased conversion of dietary carbs into stored glycogen. This process, called supercompensation, can boost muscle glycogen levels significantly, sometimes by 30% or more.

Water Retention for Enhanced Hydration

Storing glycogen also involves storing water; approximately three grams of water are stored with each gram of glycogen. This can cause a temporary weight gain of 1-2kg during carb loading, which is beneficial for hydration during the event.

Benefits and Applications for Athletes

Effective carb loading provides significant advantages for endurance athletes in events over 90 minutes, such as marathons, long-distance cycling, or triathlons. However, it is generally unnecessary for shorter events like 5K or 10K races.

Performance Benefits

Delayed Fatigue: A larger glycogen supply means a longer reliance on the body's most efficient fuel source.
Improved Endurance: Increased energy reserves allow for sustained higher intensity.
Enhanced Glycogen Sparing: Full glycogen stores help maintain blood glucose levels.

How to Carb Load Effectively

The modern approach to carb loading focuses on increasing carbohydrate intake and reducing training in the final days before an event, unlike older depletion methods.

The Modern Protocol

Timing: Begin 36-72 hours before the event.
Carb Intake: Aim for 8-12 grams of carbohydrates per kilogram of body weight daily.
Reduce Fiber and Fat: Prioritize carbs and minimize fiber and fat to prevent digestive issues and optimize digestion.
Stay Hydrated: Drink plenty of fluids to support glycogen storage and hydration.
Choose the Right Foods: Select easily digestible, low-fiber carbohydrate sources that are familiar to you.

Optimal Food Choices for Carb Loading

Low-Fiber Grains: White rice, white pasta, white bread, low-fiber cereals.
Starchy Vegetables: Potatoes and sweet potatoes (without skin).
Fruits: Bananas, applesauce, fruit juice.
Sugars: Honey, maple syrup, sports drinks.

Avoiding Common Carb Loading Mistakes

Avoid high-fat or high-fiber foods that can cause digestive problems on race day. Don't wait until the night before to start; follow the recommended 2-3 day timeline. Focus on replacing fat and protein calories with carb calories rather than simply overeating. Always practice your carb loading strategy during long training runs to ensure it works for you.

Comparison of Carb Loading Strategies


Feature	Modern 1-3 Day Method	Outdated 6-Day Method (Depletion-Loading)
Duration	1 to 3 days	6 days (3 days low-carb, 3 days high-carb)
Exercise	Reduced training load (taper)	High-intensity exercise in depletion phase, reduced in loading phase
Daily Carb Intake	8-12 g/kg body weight	Low (5-15%) initially, high (>70%) later
Complexity	Simple, focuses on diet adjustment and tapering	More complex, involves a potentially stressful depletion phase
Effectiveness	Highly effective for maximizing glycogen stores	Effective, but newer research shows similar results without depletion
Common Side Effects	Potential bloating, water weight gain	Greater risk of fatigue and mood swings from depletion

Conclusion: Is Carb Loading Right for You?

Carb loading is a beneficial strategy for endurance athletes in events over 90 minutes. It maximizes glycogen stores, providing extra fuel to delay fatigue and improve performance. The modern method is simpler and more effective, focusing on increased carb intake and reduced training for 1-3 days. It's crucial to practice this strategy during training to find what works best and avoid digestive issues. Understanding glycogen supercompensation and following practical guidelines helps athletes optimize this technique for peak performance. Resources from organizations like the National Strength and Conditioning Association (NSCA) can offer further guidance.

Frequently Asked Questions

Carb loading is primarily recommended for endurance athletes preparing for events that last longer than 90 minutes, such as marathons, long-distance cycling, or triathlons.

For optimal results, a modern carb loading strategy should be implemented over a 1 to 3 day period leading up to the event, during which high carbohydrate intake is combined with a taper in training.

No, a single large meal the night before is not enough. Proper carb loading involves a strategic increase in carbohydrate intake over several days to fully maximize glycogen stores and should not be left to the last minute.

Easily digestible, low-fiber carbohydrates are best for carb loading to avoid stomach upset. Examples include white rice, white pasta, potatoes without skin, and bagels.

Yes, carb loading can cause a temporary weight gain of 1-2kg due to the water that is stored alongside glycogen. This is a normal and beneficial part of the process.

No, the depletion phase is now considered outdated. Modern approaches have shown that similar glycogen supercompensation can be achieved by simply tapering training and increasing carb intake for 1-3 days, without the stress of depletion.

Common mistakes include eating too much fat or fiber, starting the loading too late, and not practicing the strategy during training. It's also a mistake to carb load for shorter events (under 90 minutes) where it's not necessary.