Vitamin B12
<p ><b>Vitamin B<sub >12</sub></b>, <b>vitamin B12</b> or <b>vitamin B-12</b>, also called <b>cobalamin</b>, is a water-soluble <a href="http://en.wikipedia.org/wiki/Vitamin" title="Vitamin" >vitamin</a> with a key role in the normal functioning of the <a href="http://en.wikipedia.org/wiki/Brain" title="Brain" >brain</a> and <a href="http://en.wikipedia.org/wiki/Nervous_system" title="Nervous system" >nervous system</a>, and for the formation of <a href="http://en.wikipedia.org/wiki/Blood" title="Blood" >blood</a>.</p>
<p>
It is one of the eight <a href="http://en.wikipedia.org/wiki/B_vitamins" title="B vitamins" >B vitamins</a>. It is normally involved in the <a href="http://en.wikipedia.org/wiki/Metabolism" title="Metabolism" >metabolism</a> of every <a href="http://en.wikipedia.org/wiki/Cell_(biology)" title="Cell (biology)" >cell</a> of the human body, especially affecting <a href="http://en.wikipedia.org/wiki/DNA" title="DNA" >DNA</a> synthesis and regulation, but also <a href="http://en.wikipedia.org/wiki/Fatty_acid" title="Fatty acid" >fatty acid</a> synthesis and energy production. </p>
<p>
It is the largest and most structurally complicated vitamin and can be produced industrially only through bacterial fermentation-synthesis.</p>
<p>
Vitamin B<sub >12</sub> consists of a class of chemically related compounds (<a href="http://en.wikipedia.org/wiki/Vitamer" title="Vitamer" >vitamers</a>), all of which have
vitamin activity. It contains the biochemically rare element <a href="http://en.wikipedia.org/wiki/Cobalt" title="Cobalt" >cobalt</a>.</p>
<p>
<a href="http://en.wikipedia.org/wiki/Biosynthesis" title="Biosynthesis" >Biosynthesis</a> of the basic structure of the vitamin is accomplished only by <a href="http://en.wikipedia.org/wiki/Bacteria" title="Bacteria" >bacteria</a>, but conversion between different forms of the vitamin can be accomplished in the human body. </p>
<p>
A common synthetic form of the vitamin, <a href="http://en.wikipedia.org/wiki/Cyanocobalamin" title="Cyanocobalamin" >cyanocobalamin</a>, does not occur in nature, but is used in many pharmaceuticals and supplements, and as a food additive, because of its stability and lower cost. </p>
<p>
In the body it is converted to the physiological forms, <a href="http://en.wikipedia.org/wiki/Methylcobalamin" title="Methylcobalamin" >methylcobalamin</a> and <a href="http://en.wikipedia.org/wiki/Adenosylcobalamin" title="Adenosylcobalamin" class="mw-redirect" >adenosylcobalamin</a>, leaving behind the <a href="http://en.wikipedia.org/wiki/Cyanide" title="Cyanide" >cyanide</a>, albeit in minimal concentration. </p>
<p>
More recently, <a href="http://en.wikipedia.org/wiki/Hydroxocobalamin" title="Hydroxocobalamin" >hydroxocobalamin</a>, methylcobalamin, and adenosylcobalamin can also be found in more expensive pharmacological products and food supplements. The extra utility of these is currently debated</p>
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Vitamin B<sub >12</sub> was discovered from its relationship to the disease <a href="http://en.wikipedia.org/wiki/Pernicious_anemia" title="Pernicious anemia" >pernicious anemia</a>, which is an <a href="http://en.wikipedia.org/wiki/Autoimmune_disease" title="Autoimmune disease" >autoimmune disease</a> in which <a href="http://en.wikipedia.org/wiki/Parietal_cell" title="Parietal cell" >parietal cells</a> of the stomach
responsible for secreting <a href="http://en.wikipedia.org/wiki/Intrinsic_factor" title="Intrinsic factor" >intrinsic factor</a> are destroyed. </p>
<p>
Intrinsic factor is crucial for the normal absorption of B<sub >12</sub>, so a lack of intrinsic factor, as seen in pernicious anemia, causes a <a href="http://en.wikipedia.org/wiki/Vitamin_B12_deficiency" title="Vitamin B12 deficiency" >vitamin B<sub >12</sub> deficiency</a>. </p>
<p>
Many other subtler kinds of vitamin B<sub >12 </sub>deficiency and their biochemical effects have since been elucidated.<sup id="cite_ref-nih_0-0" class="reference" ><a href="http://en.wikipedia.org/wiki/Cobalamin#cite_note-nih-0" >[1]</a></sup></p>