There are intestinal bacteria that may produce B-12, but I haven't seen any proof that this is a reliable source. If we all started our lives with 4 years of breastfeeding (which gave as a good start and a pool of B12 that could be recirculated), never drank chlorinated water or plants that had been in touch with chlorinated water etc, we could possibly rely on our bodies' ability to recycle and manufacture B12, but most of us are exposed to a lot of B12-unfriendly substances. In some people, B-12 producing bacteria exist in the small intestine where the vitamin theoretically can in be absorbed, but there are several reasons that should not believe that we should get enough B12 from recycling or manufacturing B12.
IMO people living 'normal' life (normal as in how most people live, not normal as in 'natural') life can't rely on the body's ability to manufacture B12. We may both manufacture and recycle B12 in our intestines, but I haven't seen any scientific studies showing that we don't need an external intake of B12.
Here are some quotes from other sites discussing the same topic:
From http://www.healthy.net/scr/Article.asp?Id=1924In the large intestine of a healthy person, friendly flora such as lactobacillus acidophilus manufacture vitamins (B12, biotin and vitamin K), degrade toxins, prevent colonization by pathogens, crowd out other less beneficial species, stimulate the immune system and produce short chain fatty acids (SCFAs) from fiber, which are the primary energy souce for the cells lining the colon.
From http://community.channel4.com/eve/fo...9/m/5590009793B12 is also manufactured by bacteria in the human intestines, but it is not known how much we can naturally absorb and utilize from that source. In general, digestion and absorption must be good for adequate B12 to be obtained. Many laxatives and overuse of antacids can reduce absorption and deplete stores of B12.
Gastric, intestinal, liver organic substances of both primates animals fed for more than three years in a diet without source of vitamin B12 and vegan human had been extracted and showed high content of vitamin B12 and in vitro the extracted organisms showed to manufacture vitamin B12
Both the men and the primate animals had high/normal level of B12.Experiments carried out on vegans showed that B12 manufactured by bacterial flora in the stomach and intestines (not colon) are highly absorbed in the ileum