The hypothalamus has a set point for electrolytes, fluid balance, body temp, blood pressure, and weight. The body sends information to the hypothalamus and it sends out changes the body needs to make to maintain the set point. The set point can be changed, but stays the same with statuesque.
This is why we are starving when we drastically cut our calories or why it is so easy to gain weight after we lose it. This is also why HCG is different. The HCG tricks your system into believing your body is getting more calories than it needs by using calories from adipose fat. This is also why the maintenance (no sugar not starch) portion of the diet is so important. With out maintenance your set point is not reset and you are not able to maintain your new hcg weight. This is truly how one boost their metabolism.