Why you don’t burn much fat during HIT and why it does not matter.
As exercise intensity increases, you release more “fight or flight” and catabolic hormones. Insulin production decreases but many other hormones increase. Note that HIT is performed at or near 100% VO2 max typically.
Epinehprine is a strong activator of hormone sensitive lipase, which is the enzyme in the adipocyte (fat cell) that breaks down triglycerides into fatty acids and glycerol. So….you would think that at higher exercise intensities that the proportion of fat burned would increase. However, this is not the case. See below:
So, why this paradox? It turns out that lactate and hydrogen ions play a key role in suppressing fat oxidation. Hydrogen ions inhibit hormone sensitive lipase and lactate stimulates the resynthesis of fatty acids. Therefore, even though high intensity exercise increases epinephrine production (a stimulator of hormone sensitive lipase), fat oxidation is reduced. But here is why it does not matter. The higher the intensity of exercise, the greater the shift in respiratory exchange ratio (R) after the exercise session. R is a measure that tells us what fuel we are using. See below:
What we see is after high intensity exercise, there is a prolonged shift (up to 48 hours!) towards a lower R, which means more fat oxidation during recovery, after exercise, even though you burned little during exercise.