You shouldn't. It was revealed later that Morgan Spurlock, the star of the movie, was also secretly drinking himself to death while he was making the documentary. Not to shame an addiction OR defend McDonalds too much here, but being a raging alcoholic and blaming your health problems on hamburgers and french fries on a massive public stage is/was extraordinarily irresponsible.
He also ate nothing but McDonald's - three meals a day, even if he was already "full". In one scene, he literally vomits, then continues eating the food.
Literally zero people do what Spurlock did in that film.
But many alcoholics do.
I've seen two in my life vomit while walking without missing a step.
FWIW, there is some controversy around the “methodology” and honesty in that film. Not saying you should change your view of McDonald’s, but possibly of that movie.
The maker of that documentary was a massive alcoholic, that's what caused his liver problems, not eating McDonald's.