It will slow down your metabolism a little bit (during and after the famine), to what extent I don't know. Depending on your body build, and fat %, the effects will vary. Though my view of going on a fast is kind of mixed - what Ghandi did was great, and I support his attempt. But people who fast to 'prove' themselfs or their religious belief is just wrong. But that's my view - if you google around you will find some pre-fasting facts, such as what you should eat, types of food, etc...
Just my 2 cents