Vitamin B12 is one of the essential nutrients in our diet. It  is required for the development of red blood Cells and also for the covering sheath around the nerves. There are many chemical forms of this vitamin .  All the forms  have a cobalt atom at the centre of a ring. The commonest naturally occurring form of vitamin B12 is methylcobalamin, which is found in human body. It is required for methionine synthesis.It is also needed for the enzyme methylmalonyl CoA mutase. Another form is hydroxocobalamine, which exist in minor amounts as compared to methylcobalamine. Hydroxycobalamine is formed from methylcobalamine on exposure to light. Only microorganisms can synthesize vit  B12. Humans get this vitamin by consumption of  food rich in vitamin b12, which can be obtained from animal sources. The food items such as Vegetables, and fruits are devoid of  B12

There are two mechanisms for absorption of  B12. The first mechanism is by direct absorption of the vitamin via the oral mucosa. This is a rapid process, but the amount absorbed is very less. The other mechanism is the main route of absorption which is an active process i.e. it requires energy. This active form of absorption occurs in  the ileum of the gastrointestinal tract with the help of gastric intrinsic factor.It is then bound to protein complexes like haptocorrins , which are released in to the gastrointestinal tract by enzymes. Haptocorrins are glycoprotein present in the saliva for binding with b12. The haptocorrin is digested by trypsin, a pancreatic enzyme and the Vitamin B12 is released, which then binds with the intrinsic factor. The B12 –intrinsic factor complex reaches the ileum and the vitamin  is absorbed into the circulation leaving behind the intrinsic factor.  The  vitamin B12 in the bile and those derived from sloughed off intestine cells are usually reabsorbed by a process called enterohepatic reabsorption.

Now the absorbed vitamins are transported to the target sites by transport proteins. These include transcobalamine Iand II. These are proteins synthesized in the liver.  It transports the vitamin to bone marrow, placenta, and nerve tissues.

Vitamin B12 deficiency can cause megaloblastic anemia. The main symptoms include disturbances in walking and balancing,  loss of vibration sense, confusion, and dementia. The deficiency of the vitamin can cause deragement of nerves as the protective layer is lost. The main reasons for the deficiency are malabsorption, decreased intake and pernicious anemia.