The only reason the life span for anorexics is shorter is because the average sufferer is younger.  Plus, you can lose 50 pounds pretty quickly and get to an underweight BMI. It can take years to balloon up to obese BMI and it is a pretty degenerative disease. You may not die right away but you you'll get joint pains, gangrene, shakiness,  amputations, blindness, organ failure, bypass surgery to unclog arteries and then somehow live to at least mid fifties.