Besides just gaining weight, the movie also claimed that Spurlock got major liver damage and started suffering from a range of mental health issues, all as a result of eating fast food. Notably, none of the people who replicated the experiment suffered from these problems.
Years after the movie’s release Spurlock admitted to being a lifelong alcoholic, despite claiming otherwise in the documentary. Alcohol abuse can easily explain the liver damage, and alcohol withdrawal during the filming of the documentary also explains the sudden mental health problems he was experiencing.
Hmm, I'm not sure what to think with this new information.
I really enjoyed the documentary when I watched it as a kid. But I did remember thinking it was weird he puked so early in the film. I brushed it off as "everyone is different", and forgot about it.