The type of substrate (fuel) and the rate at which it is utilized during exercise is largely dependent on the intensity and duration of the exercise. During strenuous exercise there is an obligatory demand for carbohydrate (CHO) oxidation that must be met; fat oxidation cannot substitute. In contrast, there is an increase in fat oxidation during prolonged moderate intensity exercise as carbohydrate fuels are depleted. Fats are also more dominant in long duration exercise since fat oxidation requires more oxygen than does carbohydrate oxidation.
Fuel Selection During Exercise
Again, fats are the primary source of fuel for muscle during low-intensity exercise (70% of VO2max). Proteins contribute less than 2% of the substrate used during exercise of less than one hour0s duration but may increase up to 5-15% during the final minutes of exercise lasting 3-5 hours.
At about 40% of VO2max there is a 0crossover point0 where carbohydrate becomes more dominant than fat in supplying fuel to the muscle. This shift to CHO metabolism is caused by two factors: the recruitment of fast twitch fibers (which are better equipped to metabolize CHOs) and increasing levels of epinephrine in the blood (which contribute to glycogen breakdown). This shift can also be seen when examining […]

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