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.

1. The ingestion of caffeine in sufficient quantities (about 5 mg/kg of body weight) can cause free fatty acid levels to peak after about 60 minutes and remain elevated for about three hours at about three to four times that of normal levels. The effect is delayed by about two hours if sugar is also taken at the same time.

2. The drug Heparin has similar properties to that of caffeine. Although it has been used in an attempt to extend endurance performances, research has not been consistent in replicating the effects and benefits that it is suggested to produce.

3. A high carbohydrate meal causes blood insulin to rise and stay elevated for 60 to 90 minutes. Since insulin inhibits performance because it slows free fatty acid mobilization and the breakdown of glycogen in the liver, the body has to rely primarily on muscle glycogen and a small amount of glucose in the blood for energy. Those sources are used rapidly, hypoglycemia could result (evidenced by dizziness, a feeling of weakness, or nausea), and endurance is reduced. Foster and Costill (1978) found reductions of 19 percent in endurance capacity in subjects who ingested 75 grams of glucose prior to performing a maximum exercise at 80 percent of VO2max. This would suggest it is not wise to ingest any form of carbohydrate within two hours before a performance. This no longer is generally recommended although it is necessary for individuals who are susceptible to reactive hypoglycemia. Thus, testing for reactivity in athletes is important so that the best precompetition regimen can be established.

   [Foster, C., & Costill, D. L. (1978). Effects of preexercise feedings on endurance performance. Medicine and Science in Sports and Exercise, 10, 65. (abstract)]

4. The ingestion of glucose or carbohydrates during exercise can marginally prolong performance. It has no effect on muscle glycogen but it does spare the use of liver glycogen if it can be assimilated into the circulatory system in time. The rate of emptying from the stomach and absorption into the blood stream determine the value of this supplement. Emptying is facilitated by the glucose being diluted as a cool drink taken in resting or calm circumstances.

5. The rate of muscle glycogen use appears to be increased in hot conditions.

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