Fat Burning and Exercise Intensity
In general, the higher the exercise intensity, the greater the reliance on carbohydrates vs fat. in other words, you burn a greater % of calories from fat during low intensity exercise vs high intensity exercise.
The image above shows the fuel crossover effect as exercise intensity increases. So….high intensity exercise burns almost entirely carbohydrates.
However, high intensity exercise results in an elevated rate of caloric expenditure following cessation of the exercise. This is called Excess Post Exercise Oxygen Consumption (EPOC).
EPOC is the blue area above. Energy expenditure (metabolic rate) slowly returns to normal. EPOC can have a long tail that lasts for a couple of days after strenuous exercise. Some studies have shown elevated metabolism for up to 2 days following strenuous barbell training.
Finally, high intensity exercise, in addition to producing EPOC, can shift fuel utilization toward fat. Below is a graph from a study looking at fat utilization during EPOC after strenuous exercise.
Notice the increase in fat oxidation in the RE Day part of graph B. This is fat oxidation rate after resistance exercise, compared to the control day with no exercise.
So, while high intensity exercise burns almost entirely carbohydrates, it can produce EPOC and shift the body to a fat burning mode, for up to 2 days in some cases.