Did a little further research and E is not completely wrong. We always burn some fat, but as the exercise intensity increases the percentage that comes from fat decreases and the percentage from muscle protein increases.
Still, those boys who are busting azz need to quickly replace the cals they are burning at a much higher than normal rate if they are going to be able to keep going.
I had one experience on a Goat hunt that taught me a lesson. We came back after 14hours of climbing under load and I was beat. The guide said we need to eat and I said I need to sleep I'll eat in the morning. He said eat and I said good night. The next day I was pretty much done in and stayed that way until I filled up the tank. You can't go far on empty.
