I've done The Master Cleanse about 5 times, and it has never been to loose weight. I am on the low end of what my weight should be (6'3" at 168) and have always been concerned about loosing too much weight.
From reading the book and personal experience, the book states that you will only loose what your body doesn't need. I weigh myself every day and night, and am on day 9. I've lost 6 pounds. 2 of that 6 has do to with the food you loose at the beginning of the fast that will be immediately gained.
Other friends I have had who did fast to loose weight found they did not loose as much as quickly as they thought.
The primary reason for fasting or cleansing is not to loose weight!
If you are overweight, then you will loose weight. The primary reason for fasting is to cleanse your body of toxins.