Deficiencies usually occur when something is destroying the B12 in your body such as cigarettes, or there are parasites competing for the B12, or if B12 is not properly absorbed because of inadequate production of the intrinsic factor.

That being said it is important to have an adequate amount of B12. A deficiency can cause nerve damage and pernicious anemia, a serious type of anemia where red blood cells are improperly formed and the white blood cell count can also be very low. B12 also helps build your immune system and is extremely important if you are pregnant.

Many people believe that B12 comes only from animal sources. This is a myth. Vitamin B12 comes from bacteria. Neither plants or animals can synthesize it. The reason that we find more B12 in meat is that animals have more parasites and bacteria than does vegetable matter. Vegetables grown in manure will also have some B12 although the amounts are small. Our intestines and mouth also contain bacteria that produce B12, however, it is unlikely that the amounts produced are adequate so the balance comes in with food that you eat.

There are a variety of sources of B12 other than meat products and one of the best is nutritional yeast fortified with B12. You can also find it in products fortified with B12 such as soymilk, cereals, grain burger etc. as well as B12 supplements. Tempeh, miso and seaweed are not reliable sources of B12 because the amounts they contain are dependent on the type of processing used. Amounts on labels cannot always be trusted because the current method for measuring B12 measures both active and inactive forms of B12. The inactive analogue form is not absorbed by the body.

Finally, it is important to remember that a B12 deficiency is very rare in the US. It is more commonly found in third world countries. A vegan or vegetarian diet incorporating a variety of foods will provide an adequate amount of B12.