An interesting review published this week in the Journal of Agricultural and Food Chemistry delves into the subject of vitamin B12 in food and how to prevent deficiencies.
Vitamin B12 is a water soluble vitamin that is important for metabolism, affecting energy production and synthesis of DNA and fatty acids. It is only produced by certain bacteria and microbes called Archaea, animals cannot produce it. Ruminants such as cows rely on bacteria in the gut to produce for them and then the meat eaters amoung us can get our fix from eating beef. It is also present in shell fish (that eat bacteria that produce it) and some fermented foods (fermented in bacteria that produce it).
One point in the publication was that the vitamin B12 derived from the cyanobacteria Spirulina, sold as a dietary supplement is 83% pseudo-B12, which our bodies barely absorb.
For the vegetarians amoung us, although not exclusively of course, fermented food such as tempe were recommended along with edible mushrooms and algae.