As a religious matter, I think fasting is a good thing to do. Anyway, I don't fast much---I love food, but I do think fasting is a very good, powerful spiritual discipline. I have some thoughts on the spiritual side of fasting I could share with people by PM, but I won't go into them here.
Physically, fasting is a mixed bag. Now, just fasting for a few hours or even a day isn't going to really do much long term to one's body, whether good or bad. But if one fasts for several days, consuming only juices or perhaps only water, obviously, this is going to hurt muscle bulk. Now, for someone who has a lot of fat and not much muscle to lose, this sort of fast might do them more good than harm. (Besides reducing BF, fasting also supposedly helps to rid the body of toxins.) I certainly imagine though that it would hurt someone with lots of muscle and low BF. Also, certain people, such as diabetics, pregnant women, and people with cancer, should not fast. To do so would be dangerous.
As a matter of discipline, whether spiritual or merely personal in nature, instead of fasting, one could consider giving up certain foods for, say, a week or longer. Obviously, things like candy and soda pop would be good things to abstain from. Ditto alcohol. One could also give up certain healthy foods, like lean meats, while still getting protein from eggs, whey, or yogurt. By giving up certain foods but not others, one could deny oneself certain pleasures and be disciplined in that way while still not denying oneself the things one actually needs, like protein. I recently did this for a while. My energy levels were still up, my body felt great, and I didn't worry about losing any muscle. But at the same time, I often found myself wanting the dark chocolate in the fridge or some beef or some other enjoyable food, and denying myself those foods was also good as a matter of discipline and self-reflection.
