The Various Forms of Vitamin B12
Vitamin B12 is the most chemically complex of all the vitamins. Its chemical name is “cobalamin”, derived from its central cobalt atom. The impressive formula – C63H88N14O14PCo – gives an idea of the size of the molecule that constitutes vitamin B12. Nonetheless, cobalamin almost never occurs in its chemically pure form, but is usually bound to other molecules. These different binding partners determine the names of the resultant vitamin B12 forms.
Vitamin B12 Forms in Food
In foods the most common vitamin B12 forms are:
Methylcobalamin, Adenosylcobalamin, Hydroxocobalamin
Adenosylcobalamin and hydroxocobalamin are the most frequently occurring forms in meat, whilst methylcobalamin is especially found in dairy. Other forms of vitamin B12 are very rarely found in food products – and even then only in minor traces. Moreover, it is very rare that any forms of vitamin B12 are found in plant-based foods, making a dietary supply of B12 difficult for vegans to obtain
Vitamin B12 Forms in the Body
In the body, absorbed B12 works as a coenzyme (further info: Vitamin B12 Benefits), which supports the functions of a multitude of important enzymes. Only two forms of B12 are active as a coenzyme in the body:
Methylcobalamin and adenosylcobalamin are the active coenzyme forms of B12. Methylcobalamin works in the cell plasma, whilst adenosylcobalamin is only active in the mitochondria.
Hydroxocobalamin (a.k.a. hydroxycobalamin) is not itself a coenzyme form of B12, yet can easily be converted by the body into both methyl- and adenosylcobalamin, and is a common transitional form in the B12 metabolism. Furthermore, it binds particularly well to the body’s transport molecules and thus circulates for a long time in the blood – this makes it the best storage form of the vitamin.
In all tissues (muscles, organs – especially the liver) mainly adenosylcobalamin is found
In the blood and spinal cord methylcobalamin and hydroxocobalamin are found in equal measure (10)
In the cells both adenosylcobalamin and methylcobalamin are required, which can easily be transformed into each other
Vitamin B12 Active Ingredients in Supplements
Previously, in vitamin B12 supplements synthetic cyanocobalamin and hydroxocobalamin were mainly used in B12 injections. Since the benefits of hydroxocobalamin over cyanocobalamin have become more apparent, the former has become the main substance used in B12 injections in Europe. Some researchers even think that cyanocobalamin should be completely withdrawn from the market (1).
Likewise in oral supplements such as tablets and capsules, cyanocobalamin is still the most used active ingredient. Although methylcobalamin and adenosylcobalamin are the bioactive forms of B12, they are unfortunately very chemically unstable outside of the body – mainly due to their photosensitivity – and therefore more difficult to produce.
Recently, however, methylcobalamin and adenosylcobalamin have become increasingly used in supplements because of their clear therapeutic advantages (see below).
The Best Vitamin B12 Supplement is a Mixture
When it comes to supplements, the ideal content is a mixture of all the natural B12 forms, as the body urgently needs each one for different tasks. The two active forms are produced on separate metabolic pathways and perform completely different functions. Whilst for a long time an intake of methylcobalamin alone was considered sufficient, today this has been increasingly drawn into question.
Instead of taking a single active ingredient, a combination of all of the natural B12 forms is the optimal solution – as it mirrors the B12 content in foods.
The ideal B12 supplement contains a mix of methylcobalamin, hydroxocobalamin and adenosylcobalamin.