THE DEMANDS PLACED ON FUEL SUPPLIES BY EXERCISE PARAMETERS
Rushall notes (1990).
Duration of Exercise
As an exercise duration extends at any intensity, fat becomes an increasingly important energy source. This means that in the early stages of a very intense competition the body may use as much carbohydrate as fat to fuel the work of exercise. However, as the competition progresses the demands on carbohydrate lessen and fat utilization takes its place. The change is largely due to a slowing of the rate of muscle glycogen breakdown and the body's shifting to fat oxidation possibly as part of a survival mechanism.
Intensity of Exercise
As exercise intensity increases, the contribution of carbohydrates to energy production increases. There is a gradient of exercise intensity that demands certain levels of carbohydrate and fat utilization at the outset of exercising: (a) mainly fat oxidation sustains exercise for intensities below 50 percent of VO2max; (b) the use of muscle glycogen increases markedly at 75 percent of VO2max; and (c) at intensities beyond 95 percent of VO2max primarily carbohydrates are burned.
However, exercise intensity and duration interact to produce another effect. What proportions of fuels that are used at the start of any exercise intensity are altered as the exercise progresses with fat being increasingly important and for all but the very most intense efforts, the dominant source of fuel.
State of Training
As specific athletic fitness increases the call on carbohydrate oxidation is reduced and thus, glycogen depletion is stalled. In the early stages of training carbohydrate is used as the preferred fuel when each training activity is initiated. However, as a result of training the body adapts and goes more directly to fat oxidation, that is, it learns to use that source of fuel early in an exercise bout as part of the training response.
Pre-exercise diets affect ensuing performance fuel use. Additional glycogen that is stored as a result of a high carbohydrate diet is used rapidly to fuel higher intensities of performance. Even though a performance is started with an elevated glycogen level, at the end of a contest the depleted carbohydrate level is similar to that which occurs without carbohydrate loading.
The rate of energy production from free fatty acids is determined by the supply of blood to the muscles. Since circulation is improved as a result of training, better fat utilization is possible because of that increased supply. Thus, the use of fats as fuel is improved by training because: a) the body "learns" to use them earlier and in greater amounts (mainly because of an increase in oxidative enzymes and the number of mitochondria), and b) their transport to the muscles is improved because of better circulation.
There are a number of artificial means by which the fuel for exercise can be altered.
[Foster, C., & Costill, D. L. (1978). Effects of preexercise feedings on endurance performance. Medicine and Science in Sports and Exercise, 10, 65. (abstract)]
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