I discussed this a while back, but I tend to lose weight if I'm doing a significant chunk of fieldwork (say, more than 2 months) and gain weight when I'm back in the office. When I'm in the field, I'm constantly in motion - pacing while on the phone, scurrying back to the truck/trailer/storage unit to grab something, hauling stuff around. When I'm not doing fieldwork, I'm mostly sedentary at the office and then I come home and do maybe an hour of exercise one or twice a week.
I thought that most people lost weight in the field like I do, but when I asked around, I found that many of my coworkers gain weight. This is usually for a few reasons:
1. Regular exercise: unlike me, lots of other people who do fieldwork like physical activity/exercise, and when they're not in the field, they're on an adult rec league or they hike six mountains every weekend or they go on long bike rides/runs. When they're working long days, they don't have the time or facilities/equipment to exercise like they normally would.
2. Alcohol. This really deserves its own post, but briefly, we tend to drink more in the field than when we're at home.
3. Eating outside food: some people are diligent about cooking for themselves, either for their own enjoyment/interest in saving money or because they have dietary restrictions. The rest of us eat out. A lot.
I have no idea whether it's more common to gain or lose weight in the field. Readers, do you gain? lose? or can you actually maintain a steady weight regardless of where you're working?