- Aug 2020
It appears therefore that the labeled vitamin was absorbed from the large intestine, perhaps by non-specific passive diffusion. Similarly, vitamin B12 synthesized in the large intestine by microbial flora could gain access to the tissues by such a mechanism. The possible role of intestinal vitamin B12 of microbial origin in rats, and perhaps in man, and factors affecting the utilization of this vitamin are discussed.
This provides some reason to be agnostic as to whether vegans can obtain B12 naturally. Of course that shouldn't matter to any rational person (who'd just take a supplement). Note that I was searching for this info, and this is the first piece of evidence I found. Thus, I suspect there is more evidence since 1965. I'll try to tag updates with "nativeB12"