The crash diet gives nothing more and nothing less than the yo-yo effect, where nothing wins. Why yo-yo effect The yo-yo effect occurs because the body receives very little food during a period. The body receives a signal that food has become scarce and therefore needs to be very economical with energy. As a result, little will be burned, and the body will even break muscle tissue to consume this as energy. In the meantime, the body stores as much food as possible. When the crash diet has ended, and people start eating normally again, the body will store as much of this food as fat.