This is an excerpt from an New York Times article on weight loss originally published in The Scandinavian Journal of Medicine & Science in Sports. (A free registration/login is required to read the NYT article.)
"Researchers in Spain and Sweden had 15 healthy but overweight Swedish men restrict their calories to about 360 a day, a reduction of approximately 1,800 calories. What calories they did ingest came in liquid form: Some men drank mostly sugary carbohydrates, others a high-protein drink. The men also exercised — a lot. Their days began with 45 minutes of cranking an arm-pedaling machine for an upper-body workout. Then, as a group, the men strolled for eight hours across the Swedish countryside, with only a 10-minute break every hour. They were allowed as much of a low-calorie, sports-type beverage as they wanted during their walks.
"Most of the men “were surprised that it was easier than they thought it would be",.... Some of the subjects experienced “minor problems with pain in the joints” and blisters on their feet, according to Calbet, but none dropped out or complained of hunger."
"After four days, the men had each lost almost 11 pounds, with nearly half of that coming from body fat; the rest of the loss came primarily from muscle mass. The researchers had anticipated that the high-protein drink would protect people against muscle-mass loss. In fact, the losses were the same, whether the men had been given sugar or protein." [emphasis added]
Does this extrapolate to lean, fit, trained hikers?
Maybe not directly, but it does seem to imply that you need Real Food, and plenty of it, even if you don't feel hungry, if you want to avoid catabolism of lean muscle mass.