AAt first, the contestants are losing huge amounts of water weight. Many people, even in common, everyday life, lose their respective biggest numbers when they first starting working out and watching what they eat because their bodies are not used to the increased activity level. After a few weeks of working out, their bodies grow accustomed to the increased activity level and they don't lose quite as much weight.On the show, the contestants are on very restrictive calorie allowances (about 1200-1400 a day for women and 1600-1800 for men) and they work out like professional athletes (6-8 hours in the gym) every day. They have target calorie burns: 6000 for women, 8000 for men. Obviously, when there's that great of a difference between the calories consumed and the calories burned off, the excess calories have to come from somewhere and they come from the fat the contestants have accumulated. Just as obviously, it's not practically for someone in real life to be able to spend 6-8 hours in a gym every day, which is why it's unreasonable to expect to lose 15-30 pounds in a week. In addition, it should be noted that the contestants are constantly under the watchful eye of professional trainers (Bob and Jillian, on the days they are there and other professional trainers on days when they are away filming promotions for the show and such) and doctors, who provide cold packs and such at the end of workouts. It is not advisable to push oneself to these extreme limits without seeing a doctor first and periodically throughout the process.