If you go on a crash diet, you will of course lose some weight initially. But what you are doing when you are crash dieting, is you are sending your body into survival mode. Survival mode means your body will want to hold onto fat and slow down your metabolism in order to do this. For an average sized woman, when you drop you calories below 1200 a day, you will start to decrease your metabolic rate. So when you start to eat again, you won’t be able to eat much without piling on the pounds.