>That's true of a lot of foods. Definitely true of meat, especially red meat.

I'd definitely be interested in a citation if you have one! I find this really interesting, especially since I've accidentally cut red meat out of my diet recently (despite enjoying it quite a lot, so I don't want to be ill next time I have it!), so I'm wondering on what sort of timescales these effects happen (both losing and gaining the ability to digest a certain food group).

I don't have a research paper to cite, but it seems fairly common knowledge that people who go vegetarian for a while --either by choice or by moving somewhere meat is scarce-- often have a period of re-adjustment when they return to eating meat; especially red meat. This is on a time scale of a few months in each direction.

This is not a universal truth -- I was raised vegetarian and began eating red meat in college; I never had an adjustment period. In fact, I suspect the incidence of this is greatly overstated and largely psychological.