At the beginning it's possible, mainly because overweight people are getting the extra exercise of moving around the mounds of fat. Unfortunately, after we loose some of the extra weight it starts to get more difficult, partially because the body goes into "famine" mode, meaning that we start to protect our calories since the body is starting to prepare for famine, and partially because we start to loose muscle mass along with the fat. That's why severe diets aren't good for you. You need to make sure you have enough protein to let you keep your muscle mass and exercise to burn the calories. Any other program doesn't work.