During endurance exercise energy derives mainly from oxidation of carbohydrates and fat. Carbohydrates are stored in muscle and liver glycogen, fat is stored in adipose subcutaneous tissue and in intramuscular triglycerides. The intramuscular stores are the preliminary sources of fuel during muscular exercise. The principles of fatloading and carboloading are useful in increasing the intramuscular stores before exercise. The increased muscular reserves enable the athlete to maintain a higher intensity for longer time. During exercise depletion of glycogen may be delayed by ingestion of carbohydrates. Due to the limited resorption and oxidation of carbohydrates, efficacy of ingested carbohydrates is clearly limited. Due to the limitation in energy stores in the muscles and the limitation of energy ingestion during exercise, a considerable energy deficiency in long lasting events over days has to be considered.