Carbohydrate is one of the most important nutrients to athletic performance. Carbohydrate plays the major role in supplying your brain and body with power. The body cannot supply enough carbohydrate on its own and therefore it needs to come from foods. Exercising with low levels of carbohydrate leads to fatigue. Carbohydrates are the ideal fuel for muscular work.

Carbohydrate is stored as glycogen, which is stored in the muscles and liver. Liver glycogen is used to maintain blood sugar, which, in turn, fuels the brain, nervous system and other cells. Optimal blood sugar levels are important for clear, brain function and therefore critical to sharp, high speed mental performance. Low blood sugar results in weakness and fatigue. Muscle glycogen fuels muscle cells during exercise. Muscle glycogen and fat supply energy during endurance activities. Maximizing glycogen stores is one of the primary goals of sports nutrition.

When exercising hard there is a continual loss of glycogen from the active skeletal muscle during the prolonged exercise. When the glycogen stores become depleted the athlete will not be able to exercise intensely and will experience fatigue. A gradual decline in muscle glycogen is related to the chronic fatigue often experienced by athletes during repetitive strenuous training conditions. Chronic fatigue often limits an athlete’s ability to comply with a progressive training program and subsequently to compete at maximal potential. If you do not eat enough carbohydrates to refill the stores that are depleted in each workout, you may not have enough carbohydrates available during ensuing workouts. Therefore, consuming carbohydrates during endurance exercise can postpone fatigue and prolong peak performance.

Diet and endurance training influence the amount of glycogen stored in muscle and the time it takes to exhaustion. A high carbohydrate diet can raise the initial muscle glycogen concentration and thus there will be a greater time to exhaustion. Diet provides the body with the needed fuels, while training promotes muscles to store more carbohydrate and help improve the body's utilization of fuel. More muscle glycogen will help increase endurance. An individual that is more fit uses less glycogen, is better able to conserve the limited glycogen stores in the body, and utilizes more fat as a fuel source during endurance events.

Athletes who follow a high-carbohydrate diet can maintain high-intensity exercise for a longer period than those on a lower-carbohydrate diet. There is substantial evidence for a benefit of carbohydrate intake for the performance of brief, high power events if the competitor has been consuming a reduced energy diet.

Total carbohydrates are made up of simple sugars, complex carbohydrates, and fiber.

Simple carbohydrates are commonly known as sugars. Sources of simple carbohydrates include table sugar, candies and other sweets, sodas and bakery goods. These foods provide empty calories, i.e., calories that supply no vitamins and minerals and should therefore be minimized.

Complex carbohydrates include all the complex starches and fiber, such as those found in grains, cereals, breads and starchy vegetables like potatoes, corn, peas and beans. Milk, fruit and vegetables also contain carbohydrate.

Complex carbohydrates contain many essential nutrients and are the body's most effective source of energy to the athlete. Complex carbohydrates increase glycogen stores more efficiently than sugars, or simple carbohydrates.

Complex carbohydrates are ideal because they are quickly digested and absorbed into the bloodstream, leaving the stomach quickly so there is less chance of indigestion and nausea during the event.

Carbohydrates should make up the largest portion of the athlete's diet. Research suggests that to maintain adequate carbohydrate stores during heavy training, carbohydrate intake should range from 7-10 grams/kg of body weight/day or 55-70% carbohydrate.

Athletes who train exhaustively on successive days, or who compete in more prolonged endurance events, would benefit from a diet that contains 65% to 70% of total calories from carbohydrates.