I've never not lost weight on any outdoor trip I've taken, backpacking in my early years or canoe tripping now.
Even when I was working in commercial construction and my job was more physically demanding than most, I still lost weight.
Just the daily demands of a trip, walking/paddling. Setting camp, cutting firewood/hauling water, the lack of easily accessible/unlimited junk food is why most people will see a weight loss.
  We should really call it fat loss, becasue if your taking in the proper fuel, the " weight" loss is body fat, not muscle loss, which isn't what we want.
The more you move, the higher your metabolism becomes which means your going to burn more calories even when your just sitting around the campfire.
Our modern life has its advantages but all that convenience and lack of physical effort to accomplish tasks, is coming at a terrible price health-wise for people our age and especially then younger generation.
I can remember vividly that in my high school days ( late 60's) you could count the fat kids on both hands.
Now it's just the opposite. When the average kid spends 7 hours a day either watching TV or playing electronic games, it's no wonder the current generation may be the first one in history to have a shorter lifespan than the preceding one.
  Getting the kids off the couch and into a active lifestyle should be a national priority.
Everyone hates " diets" and exercising is a habit that may have a hard time establishing.
But enjoyable activities that burn calories and give your body the exercise it needs, kill two birds with one stone and the people who participate in them are much more likely to do it for a long time.
I don't think many of us think of tripping as primary exercise, we think of it as a fun activity that we look forward to doing. The side benefits are fat loss and maintaining/building muscle mass.