All fasting slows down the metabolism, this is a fact of life and the body was created to do this. This is one of the reasons that any kind of fast is not the best way to lose weight without the commitment to a healthy Diet after.
If you ate the SAD (standard american diet) before the Master-Cleanse and go back to it. Your chances of gaining all the weight back and perhaps even more are quite high if not a certainty. The Master-Cleanse is not a weightloss diet primarily. It is as the name implies a "cleanse".
Try to look at this as a cleanse, not a weight loss plan. When it is over you can enjoy the benefits of a healthier body while you lose weight safely and effectively on a healthy diet.