Comment by plorkyeran

The obvious answer is that even if the body does become more efficient and reduce calories burned on other things, there's an upper limit to that. The Hazda walk 14 km per day, which is only 800 calories per day. By professional athlete standards, that's a relaxing offseason level of exercise. It's plausible that your body could cut its base calorie consumption from 2000 to 1200 and stay at a stable weight while only consuming 2000 calories, but it certainly can't cut it to -500 calories and stay at a stable weight while burning 2500 a day exercising and only eating 2000.