Juxtapositions

I'm fascinated with this British tv show, SuperSize vs. SuperSkinny.  (Click here for some clips)
People's brains are wired so differently, producing such visible, dramatic results.  How much of it is nature, how much is nurture.  Is a person born anorexic or an overeater, or are they created?

(I personally love the quote from one of the obese americans they profile when she says "You have to believe that the voice telling you that you will start your diet tomorrow, that that person is out to kill you.")