Vitamin B12 deficiency in humans may be slow to develop because of large liver stores. Typical American diets contain about 5-15 micrograms per day, allowing substantial liver storage to take place. It is estimated that about 3000 micrograms are stored in an adult, and 30-50 micrograms are stored in an infant or child. Persons who completely give up animal products may go for years before showing any signs and symptoms of a B12 deficiency or before any nervous system disorders are manifest. Vitamin B12 is made by bacteria and fungi, but not by yeasts or higher plants.
Friendly bacteria resides in large quantities in the gastrointestinal tract of animals and humans.
Since the manufacturing site of B12 in humans is not located where absorption occurs, humans can not rely on its availability. The total vegetarian must find vitamin B12 from other reliable sources. For example, one cup of milk contains 0.9 micrograms of vitamin B12 while 8 ounces of yogurt has 1.5 micrograms. Sterilized, boiled, or canned milk destroys about one-half of the vitamin B12. Vegetarians who use a B12 supplement should ensure that it contains an active form of the vitamin-namely, cyanocobalamin or hydroxocobalamin.