We wanted to discuss vitamin B12 and dispel the wrong information we often see being repeated. First of all vitamin B12 does not come from animals at all. It is the only vitamin synthesized exclusively by microorganisms. It is found in soil, plants and others and would be able to eat about 143 gram of dirt per day, or 3.44 kg of spinach, or like our rabbit friends our own feces it would satisfy our daily requirements (Mozafar, 1994).
Just like many other animals we also make our own vitamin B12 in our guts. However, the place where we make it in our intestine is after the place that is able to absorb it, hence why we need an external source. These two excellent articles by the Vegan Society UK covers pretty much what everyone ought to know about it.
We will just add that since the original publication of this article there has been some evidence that regular anti-acid intake for heartburn may affect the absorption rate of B12 and therefore vegans taking various form of anti-acid medication might want to monitor their B12 levels closely until we know more.
Mozafar, A. “Enrichment of Some B-Vitamins in Plants with Application of Organic Fertilizers.” Plant and Soil, vol. 167, no. 2, 1994, pp. 305–311. JSTOR, JSTOR, www.jstor.org/stable/42939740.