All of these are direct quotes that I hear day in and day out. However, anyone who has been on an extreme diet that created drastic weight loss in just a few weeks, knows that the minute they came off that diet all of the weight came right back. Why is this? Why can't you maintain weight loss?
Your body has been starved of nutrients for the past 10-14 days or however long you've been eating minimal calories (800-900/day). It has become very efficient at making the most of the calories it does get from food and drink. Once you resume to eating like a normal human being, your body stores the calories instead of burning them. Why?... because it fears you are going to start starving it again with another crazy fad diet. While you were dieting, the body used muscle mass for energy to protect its fat stores. This directly leads to a loss of muscle, which in turn lowers metabolic rate so that the body needs fewer calories to survive. Therefore, weight loss slows down. So when you start to eat a normal amount of calories, your body doesn't burn them like it used to.
Instead of throwing your body through the ringer, choose to lose weight through making healthy lifestyle changes. The Academy of Nutrition and Dietetics suggests 1/2 to 2lbs per week as a realistic weight loss goal. Choose to lose this weight in a way that is sustainable for you. By doing this, you can know that a pound lost, is gone for good!