Well, what's interesting about it is there's no real understanding of why.  The study seems to show a continued decrease in risk from dementia as the bmi increases.  I wonder what's going on that being morbidly obese puts a person at a lower risk than obese.  It does make me wonder if the underlying causes are something similar to them dying off.  I'm only speculating but perhaps the risk of dementia is correlated with a lack of physical activity and is being reflected by the fatter the fuck the less activity they generally do?