Tag Archives: energy systems

Fact or Fable: Fat Metabolism starts after 20 minutes of exercise

To improve body composition and weight loss it is often recommended by fitness experts to perform cardio training for a duration of 30 minutes or more. It is said that fat metabolism starts after 20 minutes of continuous training. After 20 minutes glycogen stores should be depleted and the body is forced to burn fat instead. Is this true or not and is this the most effective way of losing weight and improving body composition?

Firstly a large body of research has shown that exercising for longer durations is not necessarily more effective than short high intensity workouts in improving body composition. Moreover, the latest research has shown that low volume high intensity interval workouts are far more effective in stimulating fat loss than high volume moderate intensity exercise.

Secondly, the body will always try to burn fat and save the limited carbohydrate store for when it is necessary to work at higher intensities. The body of an average human only has enough carbohydrates to exercise at higher intensities for 20-30 minutes, which explains the source of this fable, but these stores can be increased by proper exercise as well.

It is true that when exercise intensity increases, the anaerobic energy systems are the first to supply the necessary extra energy and it takes a while before the aerobic systems, among which fat oxidation, have increased activity enough to supply the required energy. However, this process only takes a few minutes depending on training status. This means that if exercise intensity allows it, someone exercising for 20 minutes will be burning fat for at least 17 minutes.

Fat does not burn as easily as carbohydrates and therefore energy for performing higher intensity exercise will be provided by carbohydrates. However, when the body exercises regularly at these higher intensities, which do not allow fat metabolism or limits it, the body gets a signal to improve the efficiency and capacity of fat metabolism so it can sustain higher exercise intensities while using primarily fat.

In conclusion, fat metabolism is one of the most important energy systems in the human body. The body always strives to burn fat to produce ATP if possible or will adapt to allow for more efficient fat metabolism, making it easier to lose weight and improve body composition. However training duration does not have to be longer than 20-30 minutes to use fat or to be effective in weight loss if exercise intensity is high enough.


Adenosine triphosphate (ATP) is the fuel that provides energy for all energy demanding cellular processes. ATP is the fuel that allows contraction of the muscles. Below is the equation which depicts the reaction occuring within the muscles and makes contraction possible. This reaction is reversible so that ATP supply can be resynthesized.