the food goes into zero-space, just like everything else inside your body that doesn't have your DNA (like a yeerk). If you eat something when you're morphed (like say the spider that Jake ate while in lizard morph) it goes into zero-space when you demorph. Now whether the food comes back, I don't know. I imagine it doesn't because of something Ax said in #42...
Now to directly answer your question, I think how hungry a morph is resets. Like say you morph a tiger, and after a while you get hungry in it, you demorph, then remorph, and you're not hungry anymore...
that's what I think anyway.