Try doing a 100km bike ride on an up/down road circuit, with fat as the main fuel.
EDIT: lol, downvotes. This is not about your metabolism while you're sitting on a chair at work, typing on a keyboard. This is about extreme endurance. I suggest you actually go ahead and try it, before thinking you've formed any sort of informed opinion on the subject.
Try doing a 100km bike ride on an up/down road circuit, with fat as the main fuel.
EDIT: lol, downvotes. This is not about your metabolism while you're sitting on a chair at work, typing on a keyboard. This is about extreme endurance. I suggest you actually go ahead and try it, before thinking you've formed any sort of informed opinion on the subject.