I have an aunt that fasted three days a week for about 4 years, as part of a vow she made to God when her dad was really sick one time.
On the other days she would, of course, eat whatever she wanted. Within about 18 months or so, she had lost about 70 (?) pounds and was down to about 105 lbs. This was a healthy weight for her b/c she is 4' 9"
ANYWAY, after that her weight leveled and maintained for the rest of the time she did it.
Of course the down side was when she lost the weight, she started getting alot of attention from men, and the she backslid . . .
I don't think the purpose of religious fasts are to backslide . . she didn't read the rules very well.
And when she did stop the fasting, she slowly gained all the weight back (though it did take a few years)