We've stumbled into an episode of "Aussie folk beliefs" or he got that shit from
Beverly Hills Cop. It's a scene in the movie where the Judge Reinhold character is reading an article in the newspaper.
So that's his source: an actor reading a newspaper
in a movie.
imdb has the quote:
(officers are on stakeout of Axel Foley)
Detective Rosewood: Wow. You know, it says here that by the time the average American is fifty, he has five pounds of undigested red meat in his bowels.
Sergeant Taggart: Why are you telling me this? What makes you think I have any interest in that at all?
Detective Rosewood: Well, you eat a lot of red meat.
Those nutty colonic-crazed Californians.