In sports, carbohydrate loading, also known as carbo-loading, is a strategy employed by endurance athletes such as marathon runners to maximize the storage of glycogen in the muscles.

The protocol of carbohydrate loading was originally developed in 1967 in Sweden. The original theory of carbohydrate loading was that, if the body's glycogen stores were depleted, it would store more glycogen than normal when carbohydrate intake returned to normal. Consequently, the original carbo-loading regimen began one week before the event, and called for three days of minimal carbohydrate intake and exercise to deplete the body's carbohydrate stores. Then for the next three days, the athlete would consume primarily carbohydrates, and reduce the intensity of exercise to allow for maximum storage.

In the 1980s, further research led to a modified carbo-loading regimen that eliminates the depletion phase, instead calling for increased carbohydrate intake and decreased training for three days prior to the event. Most athletes now follow this modified regimen and it is recommended by many coaches, although there are some athletes who still follow the original carbo-loading regimen.

Carbohydrate loading is generally recommended for endurance events lasting longer than 90 minutes.