Yep this. It's why crash diets "don't work" They lose weight during the crash diet and then EXPLODE when they revert to hobgoblin diet. But I doubt that a body with 50kg fat would feel a need to budget it's limitless resources. Theory is pretty sound for normal humans (and verified with tests on humans during ww2) If you use 2200kcal a day but only eat 1200kcal and have only 10 days of fat stores, it's not such a bad idea from your metabolism to budget. If it can reduce your bmr by about 500kcal to 1700kcal a day you would get 20 days from the same fat stores. (I think the experiments found a bottom limit your body could go)