Answer:
Fasting cleanses your body. My pastor told me that fasting for a week will cleanse your body of the preservatives you've put in it over the past year. Your body eats up the fat you've been storing. You have to fast for at least three days to see any changes. One or two days isn't enough.
You can physically go 40 days without eating. I once heard of a man who fasted 70 straight days and lived. My pastor met a woman who fasted 40 days three times in one year.
That said, fasting is about devoting more time to prayer, not dieting. If you skip a meal, you should go pray. Fasting without prayer is called anorexia.
Fasting is simply not eating. Most people can go several days without food and suffer no serious or long lasting effects.
However it is not healthy to do it for prolonged periods of time, with or without prayer. Praying may sustain the soul, if such a thing exists, but adds nothing to the requirements of a healthy body.