I can't remember if I've written a post about this already, I've done a lot of vitamin posts... but here goes.

Vitamin B-12 is a water soluble vitamin. It is needed for various cellular processes including folate metabolism and the maintenance of myelin sheaths. The RDA for this vitamin is about 2.4 mcg for a healthy adult. Deficiency symptoms of this vitamin include pernicious anemia, megaloblastic anemia and nerve damage resulting in paralysis and death. There is no upper limit/toxicity levels established with this vitamin as of yet.

Popular sources of this vitamin include fortified foods, which allow for easy consumption of the vitamin.

Natural vitamin b-12 which is bound to animal protein prevents it's absorption. This is not the case when taken in supplement form or plant form.

Only bacteria, fungi (for example mushrooms and molds) and algae can synthesize vitamin b-12. ANIMALS OBTAIN THE VITAMINS FROM THESE SOURCES. Plants do not make vitamin b-12 either. Good plant sources have been soy, tempeh, miso, seaweed, spinach, mushrooms, etc.