The fat burning zone (I prefer the term "fat burning bias" because it's not a switch but a gradient) would only begin to dominate as you use up all the energy i

The fat burning zone (I prefer the term "fat burning bias" because it's not a switch but a gradient) would only begin to dominate as you use up all the energy in the food you eat. Any food you eat (not just carbs!!!) will cause insulin to be released, which "smothers" the fat release signaling (just like you wouldnt dip into savings when you have cash on hand). Only when available food energy diminishes, and therefore insulin diminishes does the catecholamine (epinephrine, etc) signal dominate, driving body-fat release for use as metabolic fuel.