There is a common misconception that more exercise intensity burns more fat. It is true that more exercise intensity burns more calories but at some intensity level exercise burns more carbohydrates than fat. At even higher intensities there is no fat burned at all. Back to my chart of %VO2max (on the horizontal axis) vs calories of fat (blue) or carbs (brown).
The maximum amount of fat burning is at about 52% of VO2max. At around 78% of VO2max equal calories of fat and carbs are burned. After that point fat drops off quickly and carbs take over. At around 98% of VO2max all the energy comes from carbs. At this point the energy from carbs is over 20 kcals per minute which is nearly 2x the max energy that came from fat.
This is the reason that I have a hard time performing at high intensities. I just don’t have the carb stores to sustain longer high intensity efforts. This is why I changed my training to work more at the lower intensities where I can exclusively use fat as fuel.
It should be possible to push the blue curve over to the right farther through training. Zach Bitter’s numbers (Zach Bitter – Another FASTER participant) show that he uses fat for 98% of his energy at 75% of his VO2max. At that point I am nearly 50-50. I don’t think that this is just dietary fat adaptation but exercise adaptation. Zach can mobilize more fat energy than I can and training explains the difference.