I don't believe this diet ruins your metabolism - you find out what foods cause you to gain. You may be one of the lucky ones who can eat starches after doing this diet and not gain, but then maybe not. You may be able to eat 1 piece of pizza but not two, or your may only be able to eat 1/2 slice, or none at all. It is all based on your body.
As you go through the phases, you will learn what you can eat, or how much of it you can eat. It is a learning curve