The information in the book is fantastic. I would highly suggest doing the whole juice fast, it is a very important part of the detox and without it I would be afraid that you may not get as clear as you want. The only weight that you will lose is water weight and as soon as you are off the fast you will gain all of that back. I am also underweight but I did not lose any more the normal. I normally range from 115-105 and haven't gone under 105 during the entire detox. The only thing I lost was those pesty little love handles I tend get and through this detox I have realize that is where I retain water. During the fasts and Liver Flushes those little love handles melted away. Only one day on the juice fast will not do anything for you. It actually takes a minimum of 3 days for your body to start detoxing. I really hope you decide to try it. Good luck and happy detoxing!!