I was a mountaineer as a youngster. It was brutal because you did it from dawn to dusk, sometimes with no rest periods at all. I could burn 6000 calories in a day. I couldn't eat enough to restore it because I couldn't carry enough food. I came back from a long climb in Peru once weighing 118 lbs. (5' 9") Not only was there no body fat, there wasn't much of anything left.