Vitamin B12 plays a key role in the normal functioning of the brain and nervous system, along with the formation of blood. It is normally involved in the metabolism of every cell of the body, especially affecting the DNA synthesis and regulation but also fatty acid synthesis and energy production.

Vitamin B12 is unique among the vitamins because it contains a metal ion (cobalt) located in the center of its activity. For this reason cobalamin is the term used when referring to compounds having B12 activity. Methylcobalamin and 5-dexoyadenosyl cobalamin (also known as adenosylcobalamin) are the forms of vitamin B12 used in the human body. Many commercial supplements on the market use cyanocobalamin; this form of vitamin B12 needs to be converted by the body to methylcobalamin or adenosylcobalamin.

Methylcobalamin is vital for the functioning of a folate-dependent enzyme known as methionine synthase. This enzyme is essential for the conversion of homocysteine to methionine (an area of great concern in autism and cardiovascular disease, among other conditions). Methionine is an amino acid which is required for the synthesis of S-adenosylmethionine (SAMe); a methyl group donor used in many biological methylation reactions.

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