it is possible. when you dont eat, your body starts feeding off of the fat deposits that you have around your body. after thats all used up, your body will start eating your muscle. it has happened to me, and i have lost around 30 pounds from that.
For those here unfamiliar with Ensure - its a drink which substitutes for a meal. You can live off them, but I don't recommend it because of the high sugar content. They come in different flavours (chocolate, vanilla, etc) and are loaded with vitamins. Imagine drinking chocolate milk with a multivitamin. They are approximately 250 calories, so depending on your weight.... I would say if you drank less than 6 per day, yes you would probably lose weight. Brush your teeth after drinking them though - you don't want to lose any more teeth :P