I camp for 6 weeks every summer and also lose weight.
The reason in my case is that my activity level goes way up (swimming, walking, chopping wood) and my food intake drops (too lazy to leave the beach to go make lunch).
Plus it takes concentrated effort even to make a cup of coffee: walk to water pump, pump water, carry pail, chop wood, make fire, heat water.
In other words, live like our ancestors. Write that book!