Free fatty acid and glucose metabolism during increased energy expenditure and after training

The amount of fat available as substrate to provide the energy needed for submaximal exercise is almost unlimited; therefore, it stands to reason that the organism will adapt so that it uses fat as the major energy substrate during very prolonged exercise. Nevertheless, the quantitatively smaller body stores of carbohydrate, which contain only one to two percent as many calories as the fat stores, play a very important role during exercise, since depletion of either muscle or liver glycogen will force an individual to terminate strenuous muscular work. In normal dogs during long-lasting exercise, at energy expenditures ranging from the resting state of 0.73 kcal/m2 min to a work load of 4.66 kcal/m2 min, the FFA mobilization, and participation of FFA oxidation in total energy expenditure increases. During prolonged exercise in trained dogs, 50-90% of the energy may derive from plasma FFA, while plasma glucose contributes not more than 10% to the energy expenditure. However, there is an inverse relationship between the amount of glycogen stored inside the muscle, its rate of depletion, and muscular endurance during prolonged strenuous work. Oxidation of FFA spares muscle glycogen and in this way increases work endurance.