I once lost 15 lbs in two weeks AND made significant gains in the weight room working out every day with energy to spare on ~1500 calories a day. I'm 6'1" and heavy-set, so theoretically I need about 2500 cal a day if I just sit on the couch. Having energy and building muscle on this diet astounded me. It was a simple diet of raw fruits and vegetables, tuna canned in water, and a protein supplement. I wasn't even that hungry! Too bad I let stress get the better of me
but I'm going to try it again this summer.