Water only fasting is as simple as it gets. Eat nothing. And when you’re thirsty, drink plain water.
The advantage of water only fasting is its severity. And you will lose weight quickly.
There are, however, several disadvantages to water only fasting. First of all, it can be less comfortable (you’ll probably feel hungrier and more sluggish), and you might feel foggy during the fast, so it’s not a good thing to do if you have to work or stay mentally sharp. And you may not feel very energetic.
I say “may” because sometimes you will feel energetic, and sometimes you will feel sharp.
You will definitely feel thirsty, and you should make sure you drink all the water you want. Your waves of hunger may occasionally feel intense but they will come and go, and often a drink of water will make the hunger pang subside.
One of the surprising results of water only fasting is you will usually sleep well — better than you’ve slept in a long time. You’ll feel unusually relaxed and your sleep is likely to be deep and restful.
The longest I’ve ever fasted with water only is three days. In the book, The Complete Idiot's Guide to Fasting, the authors recommend if you’re going to go longer than three days, you should find a doctor familiar with fasting to monitor you during the fast to make sure your body is handling it well.
You may feel cooler than usual while you’re fasting. Your body is trying to conserve energy. Just wear warmer clothing. You may not feel as energetic as you normally do. Take advantage of it, and really relax.
The most relaxed I’ve ever been was on the third day of a water only fast. It’s the only time I’ve ever been that calm. It has happened every time I got to the third day. I think it’s because the body tries to conserve energy and feeling agitated or tense is too wasteful of calories, so the body stays deeply calm. It was great. I felt impressively peaceful.
Try fasting for just one day the first time. Maybe try a one-day fast several times (with at least a week between fasts) before trying a two-day fast.
Some fasting clinics allow you to live on their premises while fasting, so they can monitor your vital signs. Two men, Terry Porter and Joseph Callahan, made an documentary (Fasting Can Save Your Life) during their fast at a clinic. They went thirty days on a water-only fast, and filmed themselves several times over the course of the fast. Porter lost 47 pounds and Callahan lost 59 pounds. Watching their transformation is fascinating.
Fasting can produce some amazing results. But water only fasting is not the only way to do it. There are advantages and disadvantages of water-only fasting. If you want to start out with something milder, the main alternative is juice fasting.
I've read a lot of books on fasting. I didn't think many of them were very good. But I found two I really liked, and I recommend them if you're interested in trying a fast:
Fasting: An Exceptional Human Experience
Principles of Fasting