Here is what I read in the book The Phoenix Protocol
-Water fasting forces the breakdown of vital body mass, putting your life at risk and any health benefits take far longer to achieve. It keeps you aloft for a while but you risk a crash landing
​
Is it true or is it hype?
TL;DR – No, water fasting does not cause your body to start breaking down “vital body mass.”
Here’s the thing–most of us that do water fasting have plenty of body fat to spare. Your body stores fat because it’s fairly easy for it to break down and use in the event that calories are no longer incoming from the digestive system. Now, once those fat stores start getting low, THEN your body will have no choice but to start breaking down what you’re calling vital body mass. But that takes a lot more work and it doesn’t get back nearly as much energy. Fat breakdown is much easier and provides a much greater “bang for the buck” which is why water fasting is so effective at lowering unwanted body fat.