I did 21 days once several years ago so I know from personal experience the benefits I expect. After years of responsible eating and consistent exercise I had reached a plateau I couldn't get past. The Water Fast did a great job.
Things I learned: Breaking the fast for a "this time only" snack or meal set me back tremendously and I had a difficult mental adjustment to get back on it. If I stay fasting it gets easier. I am a sucker for "free food" and have to avoid company parties and friends who want me to "just try this" because it's so delicous. Surprisingly I did just fine making food for my family, though I had to watch out for how easy it is to "sample" while I am cooking. Drinking enough water is vitally important. Staying with water was important for me. Substituting diet soda really started appetite cravings. Getting extra sleep keeps my energy up.
Well, me weight drifted back up over the last couple years, so I'm going to try this again. Starting a 30 day Water Fast today. Help, support and encouragement welcome. Maybe it will be easier together.