[grayclhn]

It does. Getting your stomach good at that (and figuring out what food you can personally tolerate best) is a nontrivial part of training for longer ultramarathons.

[idontwantthis]

I was aware of the eating but I still don’t quite understand it. You don’t completely run out of fat do you? Can a person not run on fat stores alone?

[doctor_phil]

Breaking down fat costs a lot of oxygen. The body can run on fat if you have a lot of oxygen to spare. If the effort is causing you to breath heavily, then you are probably mostly burning faster carbohydrates (glycogen, a sort of sugary fuel the body has already stored in the muscles).

Running out of fast carbohydrates is (at least in biking) called "bonking". You'll feel extremely tired, maybe be shaking, and feeling dizzy. It's a blood sugar dip: a very low and long dip. Your brain usually runs on sugar too, so you'll feel stupid and angry. Your body still has fat stores, so your body can keep running - but at a much lower effort intensity.

[avg_dev]

Really interesting. I wouldn’t have conceived of this. But it only makes sense.