Factors Influencing the Choice of Fuel Used During Exercise.
A number of factors determine whether the body will choose to bum fats, carbohydrates, or protein during exercise.
Intensity of Exercise.
As the intensity of exercise increases, the contribution of carbohydrates to energy production increases. The increased rate of carbohydrate oxidation is due to an increased rate of both muscle glycogen and blood glucose utilization. The increased rate of muscle glycogen utilization becomes progressively more marked at exercise intensities above 75% V02max. At exercise intensities greater than 95% V02max, only carbohydrate is burned (Saltin, 1973).
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The reason for this is not established, but Newsholme and Leech (1983) suggested that the slow rate of transport of free fatty acids into the cell limits the rate at which fats can be converted into usable energy (see Exercises 3.2). Researchers believe that the oxidation of fat alone can sustain exercise only up to levels of about 50% V02max.
Duration of Exercise.
As the duration of exercise increases at any exercise intensity, fat becomes an increasingly important energy source (G. Ahlborg & Felig, 1982; G. Ahlborg et al, 1974).
For example, in the study of G. Ahlborg et al. (1974), in which athletes exercised at 30% V02max for 4 hours, fat oxidation accounted for only 37% of the energy production after 40 minutes of exercise but increased to 62% after 4 hours of exercise (see Exercises 3.9). Costill (1970) found similar changes during 120 minutes of treadmill exercise at 65% V02max. At the start of exercise, 39% of the energy came from fat oxidation, whereas at the finish, 67% of the energy came from that source.
The change from predominantly carbohydrate oxidation to predominantly fat oxidation as the exercise duration increases is due more to a slowing of the rate of muscle glycogen breakdown than to a reduced rate of blood glucose uptake by the muscles. In fact, the rate of glucose uptake by muscle actually increases with increasing duration of exercise (compare 60-minute and 90-minute values with those at 30 minutes in Exercises 3.9). Ultimately, however, liver glycogen depletion occurs, causing the relative contribution of blood glucose to total muscle carbohydrate utilization to fall (see values at 180 minutes and 210 minutes in Exercises 3.9).
State of Athletic Training (Fitness)
The physiological and biochemical adaptations that occur with dynamic exercise training are reviewed in some detail later in this post. The major effect of these adaptations is that during exercise of any intensity or duration, more energy comes from fat oxidation rather than from carbohydrate oxidation (Henriksson, 1977; Hurley et al, 1986). The result is that increased fitness reduces the amount of carbohydrate burned during exercise. Researchers believe that this adaptation.
Exercises 3.9 Changes in the relative contributions of fat, blood glucose, and muscle glycogen to total energy expenditure during prolonged exercise.
Note. From Substrate Turnover During Prolonged Exercise in Man. Splanchnic and Leg Metabolism of Glucose, Free Fatty Acids, and Amino Acids by G. Ahlborg, P. Felig, L. Hagenfeldt, R. Hendler, and J. Wahren. Reproduced from the Journal of Clinical Investigation, 1974, 53, p. 1086, by copyright permission of the American Society for Clinical Investigation.
Allows the trained person to run farther before becoming exhausted due to carbohydrate depletion.