The Science of Carbohydrate Loading: How to Properly Carbo-Load Before a Race

Every endurance athlete has heard the phrase “eat more carbs before race day,” but few truly understand the science behind carbohydrate loading. This article will break down the mechanisms and best practices of carbohydrate loading from a physiological perspective.

What is Carbohydrate Loading?

Carbohydrate loading is a method of strategically adjusting diet and training to increase muscle glycogen stores beyond normal levels. Under normal conditions, the human body stores approximately 300-400 grams of glycogen in muscles; through carbohydrate loading, this can be elevated to 500-700 grams, an increase of 50-100%.

Glycogen is the primary fuel source during moderate-to-high-intensity exercise. When riding intensity exceeds 65% of maximal oxygen uptake (VO2max), the body’s reliance on glycogen increases sharply. Once glycogen is depleted, athletes experience the so-called “bonking,” with performance dropping dramatically.

Traditional Six-Day Method vs. Modified Three-Day Method

Traditional Six-Day Method (Bergström Protocol)

The classic method proposed by Swedish scientist Bergström in 1967:

• Days 1-3: Perform high-intensity depletion exercise while following a low-carbohydrate diet (carbs accounting for 10-15% of total calories)
• Days 4-6: Reduce exercise volume and switch to a high-carbohydrate diet (carbs accounting for 70-80% of total calories)

This method does significantly increase glycogen storage, but the “depletion phase” of the first three days often leaves athletes feeling extremely fatigued, low in mood, and may even affect immune function.

Modified Three-Day Method (Sherman Protocol)

The modified version proposed by Sherman et al. in 1981, and currently the most recommended approach:

• Days 7-4 before race: Maintain normal training and diet
• 3 days before race: Gradually taper training while increasing carbohydrate intake to 8-12 grams per kilogram of body weight
• Race day: Consume 1-4 grams of carbohydrates per kilogram of body weight 3-4 hours before the start

Research shows that the modified three-day method achieves supercompensation effects similar to the traditional six-day method, without the early-phase fatigue and discomfort.

Choosing Carbohydrates

Not all carbohydrates are suitable for carbohydrate loading. Here are stage-specific recommendations:

Priority Choices During the Loading Phase (3 Days Before Race)

Foods to Avoid

• High-fiber foods: Whole grains, legumes (may cause gastrointestinal discomfort)
• High-fat foods: Fried foods, butter (delays gastric emptying)
• New foods: Do not try foods you have never eaten before race day

Practical Example

Using a 70 kg cyclist as an example, with a target daily carbohydrate intake of 8-10 g/kg (560-700 grams of carbohydrates):

Daily Dietary Recommendations for the Three Days Before the Race

Breakfast (approx. 120 g carbohydrates)

• One and a half bowls of white rice with a braised egg
• A glass of juice
• A banana

Lunch (approx. 150 g carbohydrates)

• A large serving of pasta
• Two slices of white bread
• A glass of apple juice

Afternoon snack (approx. 80 g carbohydrates)

• Two red bean mochi
• 500ml sports drink

Dinner (approx. 150 g carbohydrates)

• Two bowls of white rice with simple stir-fried vegetables and fish
• Jelly or pudding

Late-night snack (approx. 60 g carbohydrates)

• Two slices of toast with jam
• A glass of honey water

Common Mistakes and Cautions

Mistake 1: Only Binge-Eating the Day Before the Race

Glycogen synthesis takes time. A single day of massive carbohydrate intake will not achieve supercompensation and may cause gastrointestinal distress. You need at least 2-3 days of sustained high-carbohydrate eating.

Mistake 2: Neglecting Fluid Intake

For every 1 gram of glycogen stored, the body simultaneously retains about 3 grams of water. Therefore, body weight may increase by 1-2 kg during the loading period, which is normal. Also make sure to maintain adequate fluid intake to avoid dehydration.

Mistake 3: Insufficient Carbohydrate Intake

Many athletes think eating one extra bowl of rice counts as carbohydrate loading. In reality, reaching 8-12 grams of carbohydrates per kilogram of body weight requires a very deliberate increase in the carbohydrate ratio, while simultaneously reducing the proportion of protein and fat.

Mistake 4: Assuming Carbohydrate Loading Applies to All Races

Carbohydrate loading is mainly suitable for endurance events lasting 90 minutes or more. For shorter events (such as track races or short time trials), a normal diet provides sufficient energy.

Race-Day Carbohydrate Strategy

Carbohydrate loading is only the starting point; in-race carbohydrate intake is equally important:

• 3-4 hours before the start: Eat a meal with 1-4 grams of carbohydrates per kilogram of body weight
• 30 minutes before the start: You may consume a small amount of easily digestible carbohydrates (such as energy gels)
• During the race: Replenish 60-90 grams of carbohydrates per hour (requires prior gut training)

What Does the Science Say?

A 2019 meta-analysis published in Sports Medicine indicated that carbohydrate loading can improve endurance performance lasting over 90 minutes by approximately 2-3%. For elite athletes, this margin can be enough to decide race results.

Another study on cyclists found that athletes who completed carbohydrate loading produced an average power output 5-8 watts higher than the control group in a 100 km time trial, with significantly less drop-off over the final 20 km.

Summary

Carbohydrate loading is a nutrition strategy validated by decades of scientific research. By using the modified three-day method—increasing carbohydrate intake to 8-12 grams per kilogram of body weight in the three days before the race, combined with a progressive training taper—you can store ample energy for race day. Remember, carbohydrate loading is not binge eating; it is a strategic, planned nutritional approach. Practice it a few times in advance to find the food combinations and portions that work best for you, and make carbohydrate loading a key tool in your race-day arsenal.

Related Reading

• Carb-Loading in the Three Days Before a Race: Complete Plan and Practical Examples
• Optimizing High-Carbohydrate Loading: A Comparative Study of 2-Day vs. 7-Day Protocols
• Pre-Race Glycogen Loading: The Modern Simplified Version Replaces the Grueling Depletion Method
• The Science of High-Carbohydrate Loading: Optimizing for Marathons and Long-Distance Rides