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This question came about from reading the comments of this (very unclear) question, which the author did not properly clarify.

Vegans are often recommended to take vitamin B12 supplements, as the vitamin is not present in sufficient quantities within vegan food. This implies that plants do not produce (or need) vitamin B12 in their metabolic processes.

However, herbivorous animals do not require any vitamin B12 supplementation, yet contain significant amounts of vitamin B12 in their food products.

What, then, is so different about the digestive tracts of humans and other herbivores that prevent humans from obtaining sufficient vitamin B12 from bacterial sources? Can humans be supplemented with gut bacteria from herbivores, allowing them to obtain sufficient vitamin B12 while on a vegan diet without requiring supplementation?

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    $\begingroup$ Could be due to 1. non-human herbivoure animals have large cecums in their digestive tracts, located between the small and large intestine where bacterial fermentation takes place. 2. They are mainly depending on bacteria for B12 synthesis. 3. They eat soil which provides cobalt for synthesis :P 4. Humans are hindgut fermenters. It could also be a good reason to observe the dietary habits of these animals in the wild - which are naturally way different from human vegans. Also the metabolism and related biochemical pathways. I am curious to know the answer myself. $\endgroup$ – bonCodigo Jul 5 '15 at 15:30
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The bacteria in the foregut of cud chewing animals (e.g. cows) provide enough B12 and other B vitamins. Uptake of B12 happens mainly in the small intestine. There are lots of bacteria in the human intestine but most fermentation of plant matter happens in the hindgut.

Hindgut fermentation also produces B12 but since this happens past the small intestine it is not sufficient to provide enough B12. Therefore non foregut fermenting herbivores (e.g. rabbits) get their B12 when they consume their own feces, which they do anyway to obtain higher digestion efficiency of plant matter. I would not recommend this process to any vegan human though.

As mentioned by bonCodigo, these animals need to take up enough cobalt, and their requirements for this element are higher than our own.

Source: Contributions of Microbes in Vertebrate Gastrointestinal Tract to Production and Conservation of Nutrients C. EDWARD STEVENS , IAN D. HUME Physiological Reviews Published 1 April 1998 Vol. 78 no. 2, 393-427

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    $\begingroup$ +1, I highly recommend that one read this paper, it includes a large amount of information about other species of animals and their nutrient uptake strategies. $\endgroup$ – March Ho Jul 5 '15 at 23:23
  • $\begingroup$ Primates are especially interesting as even the most herbivorous of them eat animal sources high in b12 like insects, eggs, or meat. Even gorilla the most herbivorous primate eats insects, and sometimes even have to reprocess their feces to get b12. $\endgroup$ – John Jun 1 '17 at 20:52

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