Needless to say, “water fasting” is an extreme diet. It's extremely hard, but some people swear by it. Literally, you drink only water, no food, no juice even. Not surprisingly, some people report 35 pound drop in weight after going on the water diet for a few weeks.
Proponents of fasting (not just water fastening) assert that it's a natural human function. They cite the fact that many ancient societies resented to fasting when they were sick as a wail to treat disease and illness. Perhaps, this is the cause of loss of appetite during sickness. In other words, during illness, the body loses its appetite because it wants to focus on getting better, not digesting food. This is a plausible claim considering the amount of energy the body consumes simply digesting food.
Undeniably, fast gives your digestive system a chance to rest, allowing your body to focus on healing and growth. The body takes the time to rid itself of toxins and waste at the cellular level, so clearly fast is beneficial to long-term health. It also forces your body to tap into its energy reserves ie stored sugars and fat. If there are no high levels of fat, the body will burn muscle tissue, which is obviously less than ideal, so it's important to be deliberate when fasting. But if you can make it past the hunger pangs (which you will), you'll notice that you're more energetic.
Of course, it's not advisable to binge after a fast. Pigging out on your favorite junk foods is counter productive and may shock your body. However, prolonged fast causes the stomach to shrink, so you just will not be able to eat as much as before. So if done properly, fasting not only brings the immediate benefit of a healthy body, but it can help regulate eating habits in the future. In the case of an extreme fast like a water diet, it's smart to slowly transition to juice for a few days, and then into soup, vegetables, and often back to your regular diet.
Of course, the rapid weight loss from the water fast is not as sustained as a more balanced fast. People who go on juice fasts or fasts where they do not eat during the day see better results and sustained weightloss compared to those that do not, primarily because most of the weight lost is water.
Lastly, one of the greatest benefits of fast is that you become aware of how much you consume everyday, and how little you actually need. You realize that you do not need fancy food and that you'll survive if you bring simple meals from home, rather than going to expensive restaurants. The feeling of gratitude and appreciation for everything in your life has stress-relieving effects as well.