Fasting is not good for everyone. I've been basically fasting for the last six days (just can't handle much food), and it's killing me. I'm light-headed and exhausted, and I've become pretty slow on the uptake thinking-wise. I'm already too thin and too tired. I have too much stress in my life to get the downtime I'd need if I were to do this on purpose.
Fasting should only be done carefully, with planning, and by people who are healthy and rested enough to handle it.