What exactly is B12 and where does it come from?
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What exactly is B12?
Cobalamin, which can be found in many variants or forms, is what is meant by "B12". The variants or forms include both co-enzymes in humans (i.e., the physiologically active forms in humans*) and the variants that can be converted into co-enzymes via human metabolism. The expression vitamin B12 itself, strictly speaking, refers only to cyanocobalamin.
B12 is, structurally, the largest and most complex vitamin for humans. It has cobalt as a central atom, which is bound to the organic part via six ligands (i.e., binding partners). Four of the ligands stem from the so-called corrin-ring system that forms a plane with the cobalt. The other two ligands are vertically aligned to each side of the plane. One of the vertical ligands is a 5,6‑dimethylbenzimidazole group. The sixth ligand is (bio)chemically easily replaceable and gives its name to the individual variants.
Cobalamin exists in different forms
The most important cobalamins are:
(the "R" in the figure stands for the corresponding sixth ligand.)
(1) cyanocobalamin
(2) methylcobalamin*
(3) 5′-desoxyadenosylcobalamin*
(4) hydroxycobalamin
(5) aquacobalamin
In addition, there are also cobalamin derivatives and analogs, which, in the human metabolism, are not converted into the co-enzyme forms of cobalamin and also cannot replace it. Often, these are forms that, instead of having the 5,6-dimethyl-benzimidazol group, have either no group or another group.
All animals, including human beings, need B12 for their metabolism; plants, however, do not. This is the reason why B12 is usually not found in plant-based foods.
In nature, only bacteria are capable of producing B12. Such bacteria are in, for instance, the digestive system of ruminants, who can absorb B12 produced from these bacteria. In the large intestine of humans, B12-producing bacteria are also present, but humans are not capable of absorbing B12 via the large intestine.
Since B12-producing bacteria live also in the soil and in water bodies, B12 is enriched also over the food chain by animals.