Remember that initial weight loss experienced in crash dieting is mostly fluid. So though you may see a weight loss on the scales, you probably won't have affected your fat stores very much.

Further weight loss in a crash diet is generally half fat and half lean tissue. In extreme cases, losing lean tissue can be dangerous - the body may break down muscle to release energy, which leaves you feeling weak and tired and in really severe cases the heart muscle is affected. Because you feel so low it's easy to eat, to feel better again. Weight goes on easily and you're back to square one. So you shouldn't aim to lose weight overnight.

Two pounds a week is about the most that should come off if it's going to stay off.