Not to argue McDonald's is health food, but his method was he had to eat McDonald's 3 times per day - even if he wasn't hungry. And additionally, if the cashier ever asked if he wanted to supersize, he had to say yes. So he was just forcing McDonald's down his throat to the point of throwing up. Name a food where you wouldn't gain weight eating it in excess.
And it was later revealed he was lying to doctors by saying he wasn't drinking when he in fact had a very bad alcohol addiction. You even see in the movie a doctor tell him he's baffled how his liver has seen such damage from McDonald's since that should really only happen from an alcohol addiction. Well, turned out years later he was secretly drinking the whole time.
The movie is a hoax. McDonald's even gave an official PR response and basically the gist was "we actually agree with the movie, you should not be eating our food like that."