I have been underweight in the past, and I was never sure of the exact cause. I know that many factors were playing into it, including severe candida and a toxic body!! I know that I cannot do any kind of fasting because I need to eat every three hours, so I only plan on doing the cleanses that don't involve fasting. The P&B shakes are quite easy, but it does take a committment to timing and reading your body's responses. Just try it, and you won't be disappointed.
P.S. You may have some type of detox reaction, but it should be shortlived if you keep up with it!