Consume Product

This is true, but if someone only sit on their ass there is no reason for their body to lose mass by JUST getting rid of fat.
The more muscle mass you have, the more energy you consume by simply existing. So a good tactic(for a human body) during a calorie deficit is to not only to burn fat, but also muscles.

So to keep your lean mass(everything that is not fat) you have to "tell" the body to not get rid of it(since it costs energy to keep it alive). The best and simplest thing is to lift weights! GH helps in this process, but it wont help if the only thing is starvation/fasting(can't build anything in a deficit(except ex pet for for noobs).