I'm not sure the causation works out here, it seems just as likely that the obese population may contain a heavily (da-DUM-dum!) disprportionate number of people with brain mass/degeneration issues in the first place, since it seems likely these folks would have much less impulse control than the fully functional ones. To make the causation link implied, it would have to be a life study with a baseline on the same people and taking genetics into account as well.