Carbohydrate and fat are the main fuel sources for muscle during prolonged exercise. The amount of these fuel sources depends on factors such as the exercise intensity, duration, and the training status of the athlete.
Carbohydrate is stored in the body mainly as muscle glycogen. However, the amount of muscle glycogen stored is relatively small and may become depleted during exercise.
The amount of carbohydrates and fat used during exercise depends on the exercise intensity. In rest, glucose and fat from the blood (plasma) were the main fuel sources.
When subjects started exercising at a relatively low intensity (40% Wmax), they started using muscle glycogen and other fat sources (e.g. fat stored in the muscle). With a further increase in exercise intensity (55% Wmax), there was an increase in both carbohydrate and fat oxidation (burning as fuel).
However, when the exercise intensity was increased further (75% Wmax), fat oxidation actually went down again. However, there was a very large increase in the amount of muscle glycogen used.
This means that during relatively high-intensity exercise, muscle glycogen is the main fuel source that is being used. As muscle glycogen stores are relatively small, you run out of muscle glycogen relatively quickly during high-intensity exercise (60-90 min).
This provides the rationale for ingesting carbohydrates during prolonged exercise to improve performance (e.g. sports drinks).
Does this mean that exercising at around 55% of your Wmax is best for losing fat as fat oxidation is the highest at this intensity?
While a more moderate exercise intensity burns more fat during exercise, this neglects what happens afterwards. A higher intensity burns more muscle glycogen. During your next meal, the carbohydrates would be used to refill muscle glycogen instead of being stored as fat. This is somewhat of a simplification, but the main message is that caloric balance (calories in versus out) determines fat loss.
Fuel selection is important for sports nutrition, caloric balance is important for fat loss.
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- Study: https://www.ncbi.nlm.nih.gov/pubmed/11579177
- US National Library of Medicine National Institutes of Health – 7776237
- US National Library of Medicine National Institutes of Health – 2ISPxT4
- US National Library of Medicine National Institutes of Health – 21448723
- US National Library of Medicine National Institutes of Health – 2394662