Its nothing complex, its simple actually. Body senses food shortage = adapts to environment by increasing fat storage to cope with future uncertainty. Thats what biology does you know. It ADAPTS. I think they call it evolution.
Literally every animal study ive seen shows weight regain and weight rebound after a period of calorie restriction. If the calorie restriction is severe it will force temporary decreases in fat mass while the calorie restriction persists, thats just more physics than biology. However many people pretending to be intelligent will think that is how to treat obesity.
Ofcourse calorie restriction is really incretin restriction. Thats whats really going on. As I have said many times, the incretin affect is the only way your body is able to determine current food availability in the environment. Leptin has a similar role, it tells the body about current reserves of fat stores that are already in the body. In a normal human the incretin response is suppose to be directly proprtional to calories ingested, however as we have seen it is broken in both situations of very low adiposity and high adiposity. In anorexia, the incretin response is over-exaggerated while in obesity the incretrin response is depressed.
As we know Roux-en-Y gastric bypass (RYGB) is one of the most effective "treatments" for obesity as far as producing weight loss goes, and there is considerable evidence to suggest it is only affective because it goes some way to restoring the incretin response. Up to 20% of patients cannot sustain their weight loss beyond 2 to 3 years after RYGB surgery. Weight regain-promoting consequences are attributed to a failure to sustain elevated plasma PYY concentrations ( 1 )