When they breach 50% there is no question: they are no longer human.  superfats can be desceptive with % tho, there is a lot of infrastructure tissues that your body needs to support the mass (like skin and blood vessels, and enlarged organs) and adipose tissue can hold up to 30% of its mass in additional water mass, with 250+ lbs of adipose tissue on a 450+ thats up to 75 lbs more (or more realistically 30-50 lbs).  its a real catastrophe for the body.