From (excerpt) :

"Why is a vitamin B12 deficiency rare in vegetarians?

The lack of a B12 deficiency amongst vegetarians is largely
attributed to the following:

- Vegetable and plant-based foods can contain small amounts of
B12 which are sufficient to cover requirements.

- The body's B12 reserves last longer than those of all other
known vitamins. The body, and in particular the liver, keeps a
reserve of around 2 - 5 mg of B12. This quantity is sufficient
to cover the body's demand for a period of 3 - 5 years. B12
is largely secreted into the small intestine together with the
bile juices. Most of it is then re-resorbed and restored again
for future needs (enterohepatic cycle). This in effect means
that, with a low or even no intake of B12, it can take up to
20 years before the reserves are depleted and clinical symptoms
of a deficiency start to show.

- The bacteria in the large intestine are capable of
manufacturing B12, although these are mainly the inactive B12-
types (Analoga) which the body cannot use. In addition, any
genuine B12 produced here can not be utilised due to the lack
of intrinsic factor needed to aid resorption. On the other
hand, the greater quantities of fiber consumed by vegetarians
tend to promote the settlement of bacteria in the lower areas
of the small intestine and hence also the production of B12 in
this region. As there is normally also a sufficient
concentration of intrinsic factor in this area, the body is
able to utilise any B12 that the bacteria may produce here

- The bacterial flora in the mouth and throat can also contribute
towards covering the demand.

- Another area of discussion is the apparent lack of any B12
deficiencies in countries with a less stricter sense of
hygiene. Microorganisms, insect pests and sometimes faecal
remains (from the use of animal manures) can all be found on
food plants. If such plants are not thoroughly washed, such
microorganisms can be consumed and also contribute towards
providing B12. There are recorded cases of Asians who emigrated
to western countries suffering from a B12 deficiency even
though they never changed their diets. Such cases have been
attributed to the generally higher standard of food hygiene in
the modern western countries."