Calories in v. calories out is fat loss or gain. Or maybe Mass loss or gain. People get real crazy about simple addition and subtraction.
My understanding is a “normal” adult male has like 40,000 calories in stored fat in their bodies. Not a 5’7” 340 dude. A typical adult man.
Hypothetically if a person trains their bodies’ ability to access and use their stores of fat without needing to introduce sugar to kick start metabolism it reduces lactate and increases the time an athlete can maintain steady state aerobic activity (60% MHR area) for many many hours without food.
I believe the concept arose from the idea Humans probably walked their prey to death hundreds of years ago prior to developing advanced weapons and hunting tactics. Not many animals can walk for 4 days without food and just a little water.
Just questions…. I majored in Exercise Physiology and am still interested in how science applies. That said, I am most certainly not an endurance athlete.