I really believe that's very unsafe. Fasting is best done IMO for short periods and I would ALWAYS keep myself well hydrated as it's dangerous not to.
Best way to lose weight? Do colon cleansing, eat fewer carbs and less fat, and don't overdo the healthy, light food you are left with. If you don't overdo carbs and have lots of light veg and fish, your body will BURN the fat off naturally. Don't cut all carbs, just reduce them.