I fuel before I train. I typically fuel during workouts that are medium distances and always during log workouts (so I can finish them).

I have seen the effects of fueling on my ability to push myself harder and get a faster or a workout of higher quality in.

But why? In theory, couldn't I NOT fuel during my medium length workouts, ones I don't need it, and still vet just as quality of a workout in? If my body has to work at a certain level to hold let's say an 8min/mi pace, and I must fuel to keep it there, couldn't I just not fuel and get the equivalent workout with less fuel going at 8:30's?

I don't know how the various systems of the body work together, that's obvious. I've seen the results done the 'proper' way. I'm just curious if anyone can break down the relationship between my digestive track and my muscular system.