Vitamin B12 is the largest and most complex of all the vitamins. Vitamin B12 comprises the only cobalt-containing molecules (so-called ‘cobalamins’) associated with biological activity in humans. (The cobalt gives this water-soluble vitamin its distinctive red color.)
Vitamin B12 is produced exclusively by microbial synthesis in the digestive tract of animals. Therefore, animal protein products, in particular organ meats (eg liver, kidney), are the primary source of vitamin B12 in the human diet. Other good sources are fish, eggs and dairy products.