I've fasted for religous purpose. For Ramadan, 30 days of no food and no water from sunrise to sunset.
Usually i've lost size and strength, but there is one year when I was so meticulous, and would wake up like 4 times at night and load up on protein, steak, casein protein, etc. That year I actually got stronger during the month of Ramadan.
I don't practice it anymore, as I no longer believe in it.