Without a doubt, water fasting is just starving your body, there's no way it can be a good thing!
But... I have been looking into other types of fasting, the "lemonade diet" (called "master cleanser" in the US, "lemon detox diet" here in Australia) in particular. And, I have to say, I'm not entirely opposed to the idea. Sure, your calorie intake is very low, and your metabolism will slow down as a result. But I think there are great benefits to giving your body a rest from digesting solid food for a period of time - both mental and physical. I did try it on and off during December (ok, so I'm not the world's best faster, I broke the fast on numerous occasions and had to restart
), but for me it gave by body the rest that it had been craving for so long!! I was to the stage of overeating on a daily basis, excercising 3 hours a day to keep up with it all... and my body was breaking down. The fast was the best thing for me at the time
I think that the body has a great ability to heal itself, it's always trying to get itself back to a state of homeostasis, where it feels comfortable. So, I don't believe that you can permanently slow down your metabolism through fasting. It will slow down for sure, but once you start eating again, it will recover... eventually. Someone might be able to prove me wrong, but that's just what I think...
Do a google search for Stanley Burroughs, who wrote the original Master Cleanser text back in the dark ages lol