I always thought on a scientific level that the flesh, organs, bones, etc etc they consumed was absorbed by the "virus" inside them and repurposed towards regeneration/energy of the body. Otherwise if not then just wait a year and let the zombies rot out through natural/unnatural decomposition heh. On a less serious note, I love the L4D games and the banter that occurs between the 4 main characters of both games is a hoot... I could see Ellis saying something along those lines. Top shelf Joey-Darkmeat, top shelf indeed.
The way I see it, Zombies don't have much by way of bodily functions. Sure they eat, but I don't think they pee or poop. Or even throw up. I think they just keep eating until their gut bursts, like so: [link] But, being Zombies, they don't die when that happens. So they just keep on eating and having food fall right through them, leaving a trail of yuck wherever they go.