Understanding What Happens During Carb Loading: A Guide for Endurance Athletes

October 20, 2025 •

5 min read

Effective carb loading has been shown to improve endurance performance by up to 3% in events lasting over 90 minutes. So, what happens during carb loading in your body to deliver this vital fuel for endurance athletes tackling marathons, triathlons, and other long-distance challenges?

Quick Summary

Carb loading is a nutritional strategy that saturates muscle and liver glycogen stores by combining increased carbohydrate intake with a reduction in training volume, providing a crucial energy reserve to delay fatigue during long-duration athletic events.

Key Points

Enhanced Glycogen Stores: Carb loading maximizes the amount of glycogen stored in your muscles and liver, providing a larger reservoir of energy.
Strategic Macronutrient Shift: The process involves increasing carbohydrate intake while decreasing fat and fiber, optimizing energy storage and reducing digestive discomfort.
Temporary Water Retention: For every gram of glycogen stored, your body also stores water, leading to a temporary weight gain that aids hydration during the event.
Performance Boost: By delaying the depletion of glycogen, carb loading can improve endurance and reduce the onset of fatigue during long-distance efforts.
Tapered Training is Key: Decreasing training volume during the loading phase allows your body to efficiently store carbohydrates without immediately burning them off.
Focus on Low-Fiber Carbs: Choosing refined grains like white pasta and rice helps prevent bloating and stomach issues during your competition.

The Science Behind Glycogen Storage

Carbohydrates are the primary fuel source for high-intensity, prolonged exercise. When you consume carbohydrates, your body breaks them down into glucose. This glucose is either used immediately for energy or converted into glycogen, a long chain of glucose molecules stored mainly in the muscles and liver. The body has a limited capacity to store this glycogen, typically enough to fuel approximately 90 minutes of intense activity. For endurance events that last longer, this limited supply can become a bottleneck, leading to the dreaded 'wall' or sudden fatigue when glycogen stores are depleted.

Carb loading is a systematic approach to super-compensate these glycogen stores. By significantly increasing carbohydrate consumption while simultaneously reducing training volume (tapering), athletes create a surplus of available carbohydrates. The body, no longer burning through fuel at a high rate, is primed to store this excess carbohydrate as glycogen, filling the muscular and hepatic (liver) 'fuel tanks' to maximum capacity. This process can increase an athlete's glycogen stores by 30% or more, providing a substantial reserve of readily available energy for race day.

Physiological Changes During Carb Loading

During the carb-loading phase, several physiological changes occur in the body:

Increased Glycogen Storage and Water Retention

For every gram of glycogen stored in the body, approximately three grams of water are also stored. As athletes consume large quantities of carbohydrates, they will experience a temporary weight gain of 1-2 kg (2-4 lbs) due to this increased water storage. This is not a cause for concern, as the extra water aids hydration during the event, a critical factor for performance. The feeling of being 'heavier' or slightly bloated is a normal indicator that the loading process is working effectively.

Shift in Macronutrient Focus

To accommodate the high carbohydrate intake without a massive increase in total calories, athletes must temporarily shift their macronutrient balance. This means reducing the amount of fat and fiber in their diet. Choosing lower-fiber, easily digestible carbohydrate sources is crucial to prevent gastrointestinal distress during the race. Foods like white rice, white pasta, and refined cereals replace high-fiber whole grains, legumes, and cruciferous vegetables.

Psychological Readiness

Beyond the physical aspects, carb loading plays a crucial psychological role. Knowing that your body is fully fueled and prepared for the intense effort ahead can boost an athlete's confidence. This sense of readiness can be a significant advantage during the mental challenges of a long endurance race. However, this is also where common mistakes occur, such as overeating and causing unnecessary stomach discomfort. Practicing the fueling strategy during long training runs is vital to ensure it works for you.

The Step-by-Step Process of Carb Loading

Modern carb-loading protocols have evolved from the older, more extreme methods. Here is a typical, research-backed approach:

Reduce Training Volume: 3 to 4 days before the event, athletes begin a taper, significantly reducing the intensity and duration of their workouts. This reduces the energy demand on the body, allowing glycogen stores to accumulate.
Increase Carbohydrate Intake: For 2 to 3 days before the event, carbohydrate intake is dramatically increased, typically to 7-12 grams per kilogram of body weight per day. For a 70kg athlete, this could mean consuming between 490g and 840g of carbohydrates daily.
Choose the Right Foods: Focus on easily digestible, lower-fiber carbohydrates to maximize absorption and prevent digestive issues. Examples include white rice, potatoes, white bread, pasta, fruit juices, and sports drinks.
Stay Hydrated: Continue to drink plenty of fluids, especially water. The increased water retention is beneficial and requires adequate fluid intake.
Race-Day Meal: Eat a final high-carb, low-fat, low-fiber meal 2-4 hours before the event to top off liver glycogen stores, which can be depleted overnight.

Comparing Carb Loading Strategies


Feature	Classic (Old) Method	Modern (Current) Method
Duration	6 to 7 days	2 to 3 days
Depletion Phase	Required. 3-4 days of intense training and low-carb diet to deplete stores.	Not required. Research shows it provides no added benefit and risks fatigue.
Loading Phase	3-4 days of high-carb diet and rest.	2-3 days of high-carb diet and tapering.
Carb Intake	Increased to over 70% of total calories during the loading phase.	7-12 g/kg of body weight per day.
Overall Approach	More extreme and unpleasant for the athlete.	More practical, sustainable, and less stressful.
Effectiveness	Can lead to high glycogen stores but risks psychological and physical stress.	Equally effective at maximizing glycogen and less burdensome.

Practical Tips and Potential Pitfalls

Practice your fueling strategy: Never try a new food or carb-loading method on race day. Use your long training runs to experiment and find what works for your body.
Avoid unfamiliar foods: Stick to foods you know your body tolerates well to prevent unexpected digestive issues.
Limit high-fiber foods: While normally a healthy choice, too much fiber in the final 48 hours can cause bloating and gut discomfort.
Don't overeat: The goal is to increase the percentage of calories from carbs, not simply to gorge on food. Excess calories from fat will not aid glycogen storage and can lead to lethargy.
Consider liquid calories: If you struggle to consume the required volume of food, sports drinks and smoothies can be an easy way to increase carbohydrate intake without feeling overly full.
Be smart about snacks: Adding energy gels, honey, or pretzels throughout the day can help meet your carbohydrate goals.

Conclusion: Is Carb Loading Right for You?

What happens during carb loading is a calculated physiological process designed to saturate your body's energy reserves for optimal endurance performance. For athletes competing in events lasting over 90 minutes, it's a proven and effective strategy to delay fatigue and maintain pace. By following a modern, two- to three-day protocol of increased carbohydrate intake and tapered training, you can significantly enhance your body's fuel reserves. While temporary weight gain and careful food choices are necessary, the payoff can be a stronger, more sustained performance. However, for shorter events, the benefits are negligible, and a balanced diet is sufficient. A final key is to practice your nutrition plan well in advance to avoid any unwelcome surprises on event day. For more detailed nutritional guidance, consider consulting an expert sport dietitian or resources like the International Society of Sports Nutrition stand on Nutrient Timing.

Frequently Asked Questions

Carbohydrates are converted into glucose and stored as glycogen in your muscles and liver. Carb loading is a strategy to intentionally increase these glycogen stores by consuming higher-than-normal amounts of carbohydrates over a few days while reducing training intensity.

Carb loading is primarily beneficial for endurance athletes in events lasting 90 minutes or longer, such as marathons or triathlons. It is not necessary for shorter events like 5K or for casual exercise and may lead to unnecessary weight gain.

A modern carb-loading protocol typically lasts 2 to 3 days before the event. Older methods suggested a longer period with a depletion phase, but recent research shows this is not necessary.

Yes, temporary weight gain of 1-2 kg (2-4 lbs) is normal during carb loading. This weight is primarily water, as your body stores water along with glycogen. The benefits of increased energy stores outweigh the minor weight increase.

Focus on easily digestible, low-fiber carbohydrates to prevent gastrointestinal upset. Good options include white rice, pasta, white bread, potatoes, bananas, and fruit juices. Limit high-fiber foods like whole grains and legumes.

No, a single high-carb meal is not enough to significantly increase glycogen stores. Effective carb loading requires a strategic, multi-day increase in carbohydrate intake along with tapered training.

To prevent stomach upset, choose low-fiber and low-fat carbohydrate sources. Avoid overly spicy, greasy, or unfamiliar foods in the 48 hours before your race. Practice your eating plan during training to identify any sensitivities.