Vitamin B12

Vitamin B-12 is naturally found in meat (especially liver and shellfish), milk and eggs. Animals, in turn, must obtain it directly or indirectly from bacteria, and these bacteria may inhabit a section of the gut which is posterior to the section where B-12 is absorbed. Thus, herbivorous animals must either obtain B-12 from bacteria in their rumens, or (if fermenting plant material in the hindgut) by reingestion of cecotrope fæces. Eggs are often mentioned as a good B-12 source, but they also contain a factor that blocks absorption.Doscherholmen, A; et al. (1975) Proc Soc Exp Biol Med, Sep;149(4):987-90 Certain insects such as termites contain B-12 produced by their gut bacteria, in a manner analogous to ruminant animals. An NIH Fact Sheet lists a variety of food sources of vitamin B-12.
According to the U.K. Vegan Society, the present consensus is that any B-12 present in plant foods is likely to be unavailable to humans and so these foods should not be relied upon as safe sources, as the B-12 analogues can compete with B-12 and inhibit metabolism. Also, vegan humans who eat only plant based foods must ordinarily take special care to supplement their diets accordingly. the only reliable vegan sources of B-12 are foods fortified with B-12 (including some soy products and some breakfast cereals), and B-12 supplements. 
While lacto-ovo vegetarians usually get enough B-12 through consuming dairy products, vitamin B-12 may be found to be lacking in those practicing vegan diets who do not use multivitamin supplements or eat B-12 fortified foods. Examples of fortified foods often consumed include fortified breakfast cereals, fortified soy-based products, and fortified energy bars. Claimed sources of B-12 that have been shown through direct studies of vegans to be inadequate or unreliable include, laver (a seaweed), barley grass, and human gut bacteria. People on a vegan raw food diet are also susceptible to B-12 deficiency if no supplementation is used.
==References==
Category:Nutrition