February – 2018
– 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 training status
– Carb 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
– At rest, glucose and fat from the blood (plasma) are the main fuel sources
– At low intensity 40% Wmax, subjects used muscle glycogen and other fat sources (e.g. fat stored in the muscle)
– With an exercise intensity of 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
– 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 relatively quickly during high-intensity exercise (60-90 min)
– This provides the rationale for ingesting carbs 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? No.
– 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
– Study: https://www.ncbi.nlm.nih.gov/pubmed/11579177

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