I've been doing water only fasting once a week for the last 12 years or so. Everyone thinnks that it is some sort of weight loss technique but it's not about weight loss. I'm a large guy and I have lost alot of weight through time but I've been doing it to give my body a chance to maintain itself. One good thing about water only fasting is that I used to have problems with asthma. While I rarely have breathing problems now (sometimes I have too much milk or I laugh too much and it can impair my breathing a bit), it's great to be able to breathe clearly every day.
The one things you should consider is that, if you do take on a habit of fasting once a week, make sure you do it randomly (IE not the same day every week or the same time). If you body acclimatises itself to the fasting, it doesn't become as effective.
If you do any 3 or 5 day fasts, never over-exert yourself physically. Just do your normal everyday activities and rest when necessary. It gets weird the first few days living only on water but once you break the barrier, you will find your mind much clearer.
Well, that's my input on the subject.
Revanto
PS: In addition, I'd like to add that fasting and starvation are two different things. Fasting is a state-of-mind as well. If your brain thinks 'ah, he's taking a break so I can function better', your body will adapt better to the situation and will act on a more functional level. It's a fact that mental attitudes affect how the body functions.
Imagine you had a car and you never turned off the engine or took it in for complete maintenance. Fasting would be the equivalent of turning off the engine and taking it to bet maintained often. Not only would your car run better but longer, too.