Carb loading, formally known as glycogen supercompensation, is a short term nutrition strategy designed to increase muscle glycogen stores by 20 to 40 percent above normal levels before long endurance events. When applied correctly, it can improve performance by roughly 2 to 3 percent in races longer than 90 minutes by delaying fatigue and providing extra fuel for the final miles of a marathon, the last hours of an Ironman, or any other long event. This guide covers how it works, the modern protocols, exact dosing by body weight, the right foods, who benefits, and common mistakes to avoid.
What Is Carb Loading and Glycogen Supercompensation
Carb loading is the process of systematically increasing carbohydrate intake in the three to four days before a long endurance event to maximize glycogen stores in the muscles and liver. Glycogen is the stored form of glucose, the primary fuel source during moderate to high intensity exercise.
Normally, an active person stores roughly 400 to 500 grams of glycogen (about 300 to 400 grams in muscle and 80 to 100 grams in the liver). A proper carb loading protocol can increase this by 20 to 40 percent, reaching 600 to 700 grams, providing 2400 to 2800 extra calories of available fuel.
Supercompensation is the scientific term for this above normal increase. It occurs when high carbohydrate intake is strategically timed after a period of glycogen depleting exercise, creating a short window where muscles store glycogen more efficiently than usual.
Why Carb Loading Works and Who Needs It
The main reason carb loading works is that normal glycogen stores are limited. Even a well fueled person has enough glycogen fuel for roughly 90 to 120 minutes of moderate to high intensity exercise. After that, the body must rely more heavily on fat burning, which is slower and less efficient, forcing a slowdown in pace.
By increasing glycogen stores by 20 to 40 percent, you can push that point from 90 to 120 minutes to 110 to 140 minutes or more, delaying fatigue and improving performance in the final hours of the race.
Who Benefits from Carb Loading
Carb loading is only useful for athletes participating in continuous endurance events lasting more than 90 minutes at moderate to high intensity. This includes marathons, longer half marathons (for slower runners taking over 90 minutes), triathlons, Ironman races, long distance cycling races, long distance swimming, and competitive rowing.
You do not need to carb load for strength training, high intensity interval training, five or ten kilometer races, or any activity lasting less than 90 minutes. Normal glycogen stores are sufficient for these activities.
The Modern Carb Loading Protocol
The old method of carb loading required a depletion phase (one week of low carbohydrates and hard exercise to deplete glycogen) followed by a loading phase (three to four days of high carbohydrates). This approach was uncomfortable, hard on the digestive system, and increased the risk of injury or illness before the race.
The modern protocol is much simpler and more effective. It relies on the fact that normal pre race training (tapering) already provides sufficient glycogen depletion, so all you need is the loading phase.
The Step by Step Protocol
Three to four days before the race, begin gradually increasing carbohydrate intake while reducing training volume (tapering). The goal is to consume 8 to 12 grams of carbohydrates per kilogram of body weight daily during this period.
For example, a 70 kilogram runner should consume 560 to 840 grams of carbohydrates daily. This equals 2240 to 3360 calories from carbohydrates alone, so fats and protein should be reduced to keep total calories reasonable.
The carbohydrates should come from easily digested low fiber sources to avoid digestive discomfort. Good foods include white rice, pasta, white bread, potatoes, bananas, fruit juices, and sports drinks. Avoid high fiber, fatty, or spicy foods that may cause digestive issues.
Reduce training volume to short easy sessions (20 to 30 minutes of easy running or complete rest). This allows muscles to store glycogen efficiently without depleting it through hard training.
On the day before the race, maintain high carbohydrate intake (8 to 10 grams per kilogram) but avoid overeating or trying new foods. Stick with what you know and what you have tested in training.
On race morning, eat a high carbohydrate breakfast two to three hours before the start (100 to 150 grams of easily digested carbohydrates). This tops up liver glycogen stores which drop overnight.
Exact Dosing by Body Weight
The correct dose depends on your body weight. Here are the daily targets during the three to four day loading period:
- 60 kilograms: 480 to 720 grams of carbohydrates daily
- 70 kilograms: 560 to 840 grams of carbohydrates daily
- 80 kilograms: 640 to 960 grams of carbohydrates daily
- 90 kilograms: 720 to 1080 grams of carbohydrates daily
Most athletes should start at the lower end of the range (8 grams per kilogram) and only increase if they tolerate it well and need more fuel. Larger, faster athletes and those competing in longer events (Ironman, ultra marathons) may benefit from the higher end.
The Best Foods for Carb Loading
Choose easily digested low fiber carbohydrates to minimize the risk of digestive discomfort. Good foods include:
- White rice: one cup cooked contains 45 grams of carbohydrates
- Pasta: one cup cooked contains 40 grams of carbohydrates
- White bread: two slices contain 30 grams of carbohydrates
- Potatoes: one medium contains 35 grams of carbohydrates
- Bananas: one large contains 30 grams of carbohydrates
- Fruit juice: one cup contains 25 to 30 grams of carbohydrates
- Sports drinks: one 500 milliliter bottle contains 30 grams of carbohydrates
- Pancakes, bagels, sweetened cereals: easy dense sources
Avoid high fiber foods (whole grains, legumes, lots of vegetables), fatty foods (fried, creamy), or spicy foods that may cause stomach problems on race day.
Common Mistakes to Avoid
The first mistake is overeating instead of carb loading. Carb loading does not mean eating everything in sight, it means strategically increasing carbohydrates while reducing fats and protein to keep total calories reasonable. Eating too many total calories leads to bloating, weight gain, and discomfort on race day.
The second mistake is trying new or unfamiliar foods during the loading period. Stick with foods you know and tolerate well. The week before the race is not the time to try a new restaurant or exotic dish.
The third mistake is carb loading for short events. If your race is less than 90 minutes, you do not need a full loading protocol. Normal nutrition is sufficient.
The fourth mistake is not reducing training enough. Carb loading requires a significant reduction in training volume to allow muscles to store glycogen. Continuing hard training during the loading period defeats the purpose.
The fifth mistake is neglecting hydration. Every gram of stored glycogen binds with roughly 3 grams of water, so you will gain water weight (1 to 2 kilograms) during the loading period. This is normal and expected. Drink adequate water to support glycogen storage.
The Research on Carb Loading and Performance
The research on carb loading is extensive and consistent. A comprehensive study published in Frontiers in Physiology in 2025 confirmed that the magnitude of glycogen supercompensation is influenced by basal glycogen levels, glycogen content after exercise, and the relative carbohydrate content of the diet. The study showed that increasing carbohydrate intake to 8 to 12 grams per kilogram daily after glycogen depletion (through training or tapering) leads to a 20 to 40 percent increase in glycogen storage above normal levels.
Multiple other studies have shown that this increase in glycogen stores translates to performance improvements ranging from 2 to 3 percent in endurance events longer than 90 minutes. This improvement is large enough to make a noticeable difference in finishing time, especially in the final hours of the race when glycogen stores are depleted.
The research is also clear that carb loading offers no benefit for shorter events, intermittent exercise, or strength training. The benefit is specific to continuous events lasting more than 90 minutes.
The Bottom Line
Carb loading, or glycogen supercompensation, is a scientifically proven nutrition strategy that can improve performance by 2 to 3 percent in endurance events longer than 90 minutes. The modern protocol is simple: increase carbohydrate intake to 8 to 12 grams per kilogram daily for three to four days before the race, reduce training volume, and stick with easily digested low fiber foods.
When applied correctly, carb loading can increase glycogen stores by 20 to 40 percent, providing extra fuel for the final miles or hours of the race and delaying fatigue. Avoid overeating, new foods, and other common mistakes, and you will be ready to perform at your best on race day.