Metabolism of substrates: energy substrate metabolism during exercise and as modified by training.

The question of what is the source of fuel for oxidation by muscle during exercise has been addressed. A review of experiments spanning more than 60 years supports the concept that the major energy source for the metabolism of exercise is the oxidation of fats and carbohydrates. The relative contribution of these major substrates to the total body metabolism depends on factors such as the intensity and duration of the exercise, the diet consumed on the days before the exercise, and the state of physical training. With light prolonged exercise there is a progressively greater use of fat until it can contribute up to 80% of the total caloric expenditure. However, the relative contribution of fat to the metabolism is less and that of carbohydrate greater as exercise intensity increases. Consumption of a diet rich in fat and protein produces a shift toward a greater use of fat with a concomitant reduction of both the intensity and duration of effort that can be sustained. Conversely, ingestion of a carbohydrate-rich diet increases the percentage of carbohydrate used and increases endurance. The concentration of glycogen in muscle is reduced by fat-protein diets and elevated by carbohydrate-rich diets. Endurance training results in a shift of the metabolism toward a greater use of fat during the same absolute and relative exercise loads. This produces a glycogen sparing that is associated with improving endurance capacity.