I recently finished a forty day, forty night water fast and while I wouldn't recommend anyone do it, it was effective. At the end I was down roughly 40lbs/18kg, around five of which came back relatively quickly as my body began retaining water.
I supplemented sodium and magnesium during the fast (which is essential), but otherwise only consumed water. During my first few days of re-feeding, I also supplemented sodium diphosphate to avoid hypophosphatemia - which is really only a problem when slamming carbs after an extended fast, but I wanted to be careful.
The most interesting thing for me was that after the first couple days (I've done shorter duration fasts ranging from one to five days intermittently for a few years), my desire to eat was entirely mental - the physical pangs had gone. After about the tenth day I was simply not hungry, at all.
I supplemented sodium and magnesium during the fast (which is essential), but otherwise only consumed water. During my first few days of re-feeding, I also supplemented sodium diphosphate to avoid hypophosphatemia - which is really only a problem when slamming carbs after an extended fast, but I wanted to be careful.
The most interesting thing for me was that after the first couple days (I've done shorter duration fasts ranging from one to five days intermittently for a few years), my desire to eat was entirely mental - the physical pangs had gone. After about the tenth day I was simply not hungry, at all.