If we want to understand how our body utilises the different energy substrates during a different state of activity, you need to get some notions about our energy metabolism.
In the post-absorption period, at rest, skeletal muscle does not use glucose as the main energy substrate: more precisely, about 10% of the energy generated in resting muscle derives from the oxidation of glucose, 85 -90% from the oxidation of free fatty acids (FFA) and only 1-4% from amino acids ( Protein ).
In the early stages of any physical activity, muscle glycogen and ATP-CP ( Adenosine triphosphate – Creatine Phosphate ) are used through anaerobic ( without oxygen ) metabolic pathways.
As the exercise continues, while the high intensity continues, glycogen becomes the main energy source of reference.
In medium-high intensity exercise, the use of hepatic glycogen is such as to result in a halving of stocks in less than an hour.
Instead, FFA represents the energy source of greatest reference for the body, being their prevalent use for basic metabolism (MB) and during moderate-intensity physical activities (20-50% of VO2max).
For fats, depletion situations practically never occur.
During exercise, the main limiting factor in the use of FFA is the intensity of the exercise itself.
During intense and protracted work, the high concentration of lactate results in little or no use of fats.
The graph below expresses the energy expended at a different percentage of our VO2Max ( Maximal Oxygen Uptake ) that represent the intensity of the exercise.
As you can see from the graph, the use of FFA and Muscle Triglyceride progressively decreases with the increase in oxygen uptake and therefore with the intensity of exercise.
The reduced use of FFA occurs mainly for two reasons:
- FFA need more oxygen to be broken down to produce energy.
- FFA need more time to be broken down to produce energy.
Due to these dynamics of energy metabolism, a high-intensity exercise cannot have a long duration ( glucose is the main substrate that depletes rapidly ), just as a long endurance exercise cannot be very intense ( FFA are the main substrate ).
This graph below explains the percentage contribution of the different substrates as an energy source during physical exercise.
As we can see from the graph the contribution of the FFA as an energy source during the first part ( 30 minutes ) of all kind of exercises is very low.
Instead, in the second part ( after 30 minutes ), the contribution of the FFA as an energy source starts to increase rapidly.
By summarizing all the notions described above about energy metabolism, we can identify 3 fundamental points for burning fat during physical exercise:
- Physical Exercise should be longer than 30 minutes.
- The intensity of exercise should be around 50-60% of your VO2Max ( It corresponds to approximately 90-120 Heart Beats per minute.
- You can change the type of exercise to avoid both physical and mental fatigue that could reduce the duration of the exercise.