V i t a m i n B 12
Vitamin B12 has a key role in the normal functioning of the brain and nervous system, and for the formation of blood.
It is normally involved in the metabolism of every cell of the human body, especially affecting DNA synthesis and regulation, but also fatty acid synthesis and energy production.
Vitamin B12 is a the umbrella name term for a family of compounds called corrinoids. Structurally a very complicated vitamin, it contains a small amount of cobalt, hence referred to as Cobalamin.
Neither fungi, plants nor animals are capable of producing vitamin B12. Only bacteria have the enzymes required for its synthesis, although many foods are a natural source of B12 because of bacterial symbiosis. The B12 attaches itself to protein and is liberated during the digestion.
Vitamin B12 is an especially important vitamin for maintaining healthy nerve cells. Nerves are encased in an insulating sheath which is made of a protein known as myelin. Cobalamin is critical for maintaining this myelin sheath around the nerves, and protects against brain atrophy and impaired cognitive function.
Protein in food, required for growth and repair of cells, depends upon B12 for proper cycling through the body. Many of protein's key components, called amino acids, become unavailable for use in the absence of B12. Since one of the steps in carbohydrate and fat processing requires B12 for its completion, insufficiency of the vitamin can also affect the movement of carbohydrates and fats through the body.
Folate (Vitamin B9) and vitamin B12 help cells to multiply, a function that is particularly important to cells that are replaced rapidly, such as red blood cells and the cells lining the gastrointestinal tract. They also help iron to work better in the body and to produce a compound involved in immune function and mood.
Unlike the other water-soluble vitamins, vitamin B12 can be stored by the body. The body can store about 5 - 12 mg of vitamin B12, mostly in the kidneys and the liver. Excess quantities are disposed of via the kidneys or the bile.
Vitamin B12 has extremely low toxicity, and even taking enormous doses does not appear to be harmful to healthy individuals.
2.4 µg RDA - 500 µg UL
RDA (recommanded dietary allowances) per day 
UL (upper intake level) per day 
 Dietary Reference Intakes: Vitamins.
The National Academies, 2001
 Risk Factor: Vitamin Deficiency
Andreas Jopp, 2010