Advice

I would suggest that anyone Water Fasting read "Eating and Fasting for Health" by Joel Fuhrman. Many of the responses below are inaccurate. Fuhrman is an M.D. who uses therapuetic Water Fasting and diet for treatment of diseases and weight loss. By the way, there is no other fast than Water Fasting (unless you do not drink anything which is not recommended by very many practitioners, and then only for a short time). The Master Cleanse is a Lemonade Diet of 1000 calories a day of maple syrup. By the way, a quart of water is the minimum to drink, but excessive drinking of water should be avoided because it will wash out (deplete) your electrolytes and can cause death. Furhman's book recommends supervised fasting, but many have done their fasting unsupervised, including me. There is nothing in the medical literature to suggest that one should avoid water fasting and use juice or some other liquid initially, or that any Bowel Cleanse is necessary. If you want that kind of system go to The Master Cleanse or Juice Fasting (diet really) boards. There is a chapter in Fuhrman's book answering questions about what to expect on a water fast. He addresses the issues of juice, etc. so called fasts. I've done them all, and in my humble opinion, water fasting is far more powerful than partial diets of juice or syrup drinks. Also, read the story on metabolism and water fasting, and get direct references to the journal articles on which Fuhrman's book is based.
If you do The Master Cleanse in the middle of a Water Fast you will take your body out of ketosis and protein sparing and have to start all over for 48-72 hours to reenter that physiological state.
Your metabolism will slow down and stay slowed down for 4-6 weeks after you break your fast. Furhman explains all that and answers nearly every question you may have about water fasting.
I hope everyone undertaking a fast reads this book. It will alleviate fears and provide good Science from a doctor who specializes in this area of treatment.
I am not affiliated in anyway with Fuhrman.