[Notion of metabolic adaptation to exercise]

During muscular exercise, adenosine triphosphate (ATP) must be resynthesized in order to allow subsequent muscle contraction. ATP can be immediately resynthesized from adenosine diphosphate (ADP) and from creatine phosphate (CP). However, such resynthesis from phosphorylated compounds is not possible in very short-duration exercise. For further exercise an energy source, mainly carbohydrates and lipids, is needed. While anaerobic (glycolysis and glycogenolysis) and aerobic (mitochondrial oxidation) pathways are used for carbohydrates, lipids can only undergo oxidation. Carbohydrates are used preferentially with rising exercise intensity because of increased muscle uptake, progressive recruitment of type II fibers and catecholamine release. Lipid oxidation rises with the duration of exercise and falls with increasing intensity. The observation that training plays an important role in the use of these energy substrates has led to the development of the crossover theory by which the use of carbohydrates and lipids in all individuals depends at all times on the combined effect of training and exercise intensity. Training facilitates lipid oxidation but, by allowing higher intensity, also implies inevitable use of carbohydrates. Thus there is an exercise level, or a crossover point, at which carbohydrate energy predominates over lipid energy. Beyond this point, increasing energy requirements are satisfied by preferential use of carbohydrates. Finally, although it was incorrectly thought that proteins play a very minor role, actually amino acids coming from the voluminous tissue mass provide an important source of energy during prolonged exercise.