pretty sure sooner or later you're going to have to figure out what to do with defunct colonists anyway and disassembling them is not the worst option...

Depends on what kind of colonists, tbh.

Half the books I've read about lunar colonies would probably disagree. :D

Unless they live forever this will solve itself. At a guess most of the carbon and water that you are consuming has been re-cycled a couple of times already, and I'm pretty sure that at least some of that was part of a human configuration at some point in its past.