You don't have to fast. I would try the PB shakes. Just make sure
you drink enough water. Some people get some constipation in the beginning.
The good thing is that you start slowly and build up.
It is also not costly.
Click on the FAQ at the top of this page and you will see a message from PTree with an explanation.
You can also go to "Ask Shelley" forum and under FAQ she has info
on bowel cleansing.