Originally Posted by
jzbob81
Not quite true either.
That really depends in which "gear" your body is at the time. If you are doing low intensity workout your body will (after depletion of glycogen) use mainly fat as energy source.
But if you're doing high intensity workout your body will tear down muscle mass as it can obtain energy from muscle tissue much faster than from fat.
You are talking about extreme cases which rarely if ever affect anyone posting in this forum.
The likelihood is that at low intensity, a rider who normally posts here would stop exercising before s/he has depleted all glycogen stores as well as fat stores.