Carbohydrate loading, commonly known as "carb loading" or "carbo-loading," is a nutrition strategy that’s often used by endurance athletes to improve their performance by increasing the amount of fuel stored in the muscles.

Carbohydrates serve as the body's main source of energy. The carbs we consume are broken down into glucose, a type of sugar, which is used to fuel cell function. When that energy isn't immediately used, the fuel is converted to glycogen and stored in the liver and muscle tissue. During endurance or high-intensity exercise the body uses more of this store than during lower intensity activities.

Carb loading was introduced in the 1960s to help increase athletes' base levels of muscle glycogen prior to endurance events, said David Rogerson, a principal lecturer in sport and exercise science at Sheffield Hallam University in England. "This is so that when we're doing these prolonged activities like a marathon, we've got more fuel,” he told Live Science.

If your body doesn’t have enough glycogen stored up, you can’t sustain performance at a high level because the body has effectively run out of fuel. So while having all the greatest gear — including one of the best running watches on the market — can be a great asset come race day, carb loading is a vital part of preparation.

What is carb loading?

"Once we start to run out of fuel, people start to feel fatigued and their performance drops because they are not able to exercise at that higher intensity," Rogerson said. "Carb loading is a strategy to reduce the effect of that and to be able to go into an exercise or a competition with higher base levels of stored muscle glycogen." In practice, this means consuming a high quantity of carbohydrates prior to a race or a long, hard training session.