[libdong]

It's produced by bacteria in the intestines, but not absorbed during the first pass through.

[lisper]

Why would it be absorbed any better the second time around?

(This thread is making me feel queasy.)

[libdong]

Like others have said, it takes time for B12 to be produced in significant quantities, and by that time it has passed the small intestine where it can be absorbed.

[sndean]

I'm not sure, but dogs have significantly shorter (looking quickly, 1/4 the length?) digestive tracts than we do. Less surface to absorb things.

[londons_explore]

probably produced too late in the process...

Rabbits for example re-eat every piece of faeces because the digestion of grasses take so long it takes two passes to get the energy out...