it can mess you up if you dont do it right.

you need to eat very lightly a month after you fast.
you cant eat like you did before your fast. if you still eat like you did that made you fat you will get fat again.

fasting detoxes you so you should clean up your diet anyway before doing it.
sometimes people give into cravings they get while fasting for "bad foods" and then they go on a bing post fast. DO NOT DO THIS. that is a sure fire way to get fat again if not fatter. plus you will just feel like crappo.

to fast and lose weight and keep it off you have to take steps

clean up your diet
fast
eat lightly a month after your fast with a little bit of exercise.
continue to eat healthy in a way that works for you witha little bit of exercise.

as a rule of thumb for foods:
avoid all things chemical and processed. in other words if it doesnt look like it did when it was harvested, dont eat it.