Fasting followed by cardio is a thing. It's called "bonk training". Running out of glycogen is the whole point of it. It supposedly helps your body make the

Fasting followed by cardio is a thing. It's called "bonk training". Running out of glycogen is the whole point of it. It supposedly helps your body make the switch from glucose and glycogen to body fat and ketones more smoothly. It might have been useful for triathletes and marathoners, if they weren't eating glycogen gel during their races now. You'd carbo-load, hit the wall at mile 20 or so, and just keep going at 80% your normal athletic capacity instead of 70%, because you trained for it. But now, you'd just be left in the dust by someone who has been sucking on a tube of glycogen gel and thus never hits the wall at all. I suppose it would still be useful for weight loss, if exercising while bonked wasn't self-torture.