What is Vitamin B12?
[[File:Cobalamin skeletal.svg|thumb|right|250px|The cobalamine molecule. Its half-life in the blood is only 6 days, but its half-life in the [[liver]] is 400 days]] [[File:B12 methylcobalamin.jpg|thumb|270px|Methylcobalamin (shown) is a form of vitamin B<sub>12</sub>. The dark red crystals form cherry-colored transparent solutions in water]] '''Vitamin B<sub>12</sub>''' (or vitamin B-12) is also called '''cobalamin'''. It has a key role in the normal functioning of the [[brain]] and [[nervous system]], and in the formation of [[blood]]. It is one of the eight [[B vitamins]]. B<sub>12</sub> is normally involved in the [[metabolism]] of every [[cell (biology)|cell]] of the [[human body]], especially [[DNA]] synthesis and regulation, but also [[fatty acid]] and [[amino acid]] metabolism.<ref>{{cite book|title=Interrelations between essential metal ions and human diseases|last1=Yamada|first1=Kazuhiro|publisher=Springer|year=2013|editor=Astrid, Helmut|series=Metal Ions in Life Sciences|volume=13|pages=295–320|chapter=Chapter 9. Cobalt: its role in health and disease|doi=10.1007/978-94-007-7500-8_9|pmid=24470095 |editor-last2=Roland Sigel}}</ref> [[Fungi]], plants and animals (including humans) cannot produce vitamin B<sub>12</sub>. Only [[bacteria]] and [[archaea]] have [[enzymes]] for its [[synthesis]], although animals can convert it to the version they need. Many foods are a natural source of B<sub>12</sub> because of bacterial [[symbiosis]]. It is produced by some of the [[gut flora]] of herbivores. Carnivores, of course, eat herbivores. Also [[algae]] and plants get it from [[symbiosis]]. So, by one route or another, animals must get vitamin B<sub>12</sub> from bacteria (and archaea). The vitamin is the largest and most structurally complicated vitamin. It can be produced industrially only by bacterial [[fermentation]]-synthesis. Vitamin B<sub>12</sub> is a group of chemically related compounds, all of which have vitamin activity. It contains the biochemically rare element [[cobalt]] sitting in the cente