I do a fair amount of cycling over what some would consider long distance - ~40 mile rides every 2-3 weeks and shorter rides more frequently.
I have to consume a fair quantity of high GI carbs while on the bike or I will most certainly Bonk somewhere round the 120 minute mark.
What I would like to understand is how this consumption affects fat burning. How does the body "select" what energy sources to draw on while exercising. Does the exercise trigger the body to continually burn fat (and everything else it can find), or is low blood glucose needed to force this?
Please note this is NOT a generic fat loss question. I'm not looking for "how to maximise fat loss". I'm looking to understand the science.
From what I've read the heart rate during exercises is linked to the proportion of calories drawn from glycogen and fat. I've seen significant disagreement about weather this means fat burning drops off at high heart rates or not. But lets presume for the sake of argument that I'm exercising at a rate which does burn significant quantities of fat.
It's also understood that taking in carbs during very long rides will increase endurance by allowing your body to burn the carbs you're eating / drinking before burning the glycogen and thus increase the time before glycogen is depleted.
What I'm really asking here is whether the carbs I eat during the ride affect fat burned and glycogen burned evenly (leaving the proportions unchanged). Or does or is this affect asymmetric? Does it significantly affect one more than the other and thus change the proportions?