Vitamin B12/ja: Difference between revisions

=== Gastric bypass surgery ===

Various methods of gastric bypass or gastric restriction surgery are used to treat morbid obesity. Roux-en-Y gastric bypass surgery (RYGB) but not sleeve gastric bypass surgery or gastric banding, increases the risk of vitamin B₁₂ deficiency and requires preventive post-operative treatment with either injected or high-dose oral supplementation. For post-operative oral supplementation, {{value|1000|u=μg/day}} may be needed to prevent vitamin deficiency.

===Diagnosis===

According to one review: "At present, no 'gold standard' test exists for the diagnosis of vitamin B₁₂ deficiency and as a consequence the diagnosis requires consideration of both the clinical state of the patient and the results of investigations." The vitamin deficiency is typically suspected when a routine complete blood count shows anemia with an elevated [[mean corpuscular volume]] (MCV). In addition, on the [[peripheral blood smear]], [[macrocyte]]s and hypersegmented [[polymorphonuclear leukocyte]]s may be seen. Diagnosis is supported based on vitamin B₁₂ blood levels below 150–180 [[Molar concentration#Units|pmol/L]] (200–250 [[Orders of magnitude (mass)#Picogram|pg/mL]]) in adults. However, serum values can be maintained while tissue B₁₂ stores are becoming depleted. Therefore, serum B₁₂ values above the cut-off point of deficiency do not necessarily confirm adequate B₁₂ status. For this reason, elevated serum [[homocysteine]] over 15 micromol/L and [[methylmalonic acid]] (MMA) over 0.271 micromol/L are considered better indicators of B₁₂ deficiency, rather than relying only on the concentration of B₁₂ in blood. However, elevated MMA is not conclusive, as it is seen in people with B₁₂ deficiency, but also in elderly people who have renal insufficiency, and elevated homocysteine is not conclusive, as it is also seen in people with folate deficiency. In addition, elevated methylmalonic acid levels may also be related to metabolic disorders such as [[methylmalonic acidemia]]. If nervous system damage is present and blood testing is inconclusive, a [[lumbar puncture]] may be carried out to measure [[cerebrospinal fluid]] B₁₂ levels.

Serum [[haptocorrin]] binds 80-90% of circulating B₁₂, rendering it unavailable for cellular delivery by [[Transcobalamin|transcobalamin II]]. This is conjectured to be a circulating storage function. Several serious, even life-threatening diseases cause elevated serum HC, measured as abnormally high serum vitamin B₁₂, while at the same time potentially manifesting as a symptomatic vitamin deficiency because of insufficent vitamin bound to transcobalamin II which transfers the vitamin to cells.

<div lang="en" dir="ltr" class="mw-content-ltr">

==Medical uses==

[[File:Hydroxocobalamin Injection.jpg|thumb|A vitamin B₁₂ solution (hydroxocobalamin) in a multi-dose bottle, with a single dose drawn up into a syringe for injection. Preparations are usually bright red.]]

===Treatment of deficiency===

Severe vitamin B₁₂ deficiency is initially corrected with daily intramuscular injections of {{value|1000|u=μg}} of the vitamin, followed by maintenance via monthly injections of the same amount or daily oral dosing of {{value|1000|u=μg}}. The daily dose is far in excess of the vitamin requirement because the normal transporter protein mediated absorption is absent, leaving only very inefficient intestinal passive absorption. Injection side effects include skin rash, itching, chills, fever, hot flushes, nausea and dizziness. Oral maintenance treatment avoids this problem and significantly reduces cost of treatment.

<div lang="en" dir="ltr" class="mw-content-ltr">

===Cyanide poisoning===

For [[cyanide]] poisoning, a large amount of hydroxocobalamin may be given [[intravenously]] and sometimes in combination with [[sodium thiosulfate]].

<div lang="en" dir="ltr" class="mw-content-ltr">

==Dietary recommendations==

Some research shows that most people in the United States and the United Kingdom consume sufficient vitamin B₁₂. However, other research suggests that the proportion of people with low or marginal levels of vitamin B₁₂ is up to 40% in the [[Western world]]. [[Grain]]-based foods can be [[food fortification|fortified]] by having the vitamin added to them. Vitamin B₁₂ supplements are available as single or multivitamin tablets. [[Pharmaceutical]] preparations of vitamin B₁₂ may be given by [[intramuscular injection]]. Since there are few non-animal sources of the vitamin, [[vegan]]s are advised to consume a [[dietary supplement]] or fortified foods for B₁₂ intake, or risk serious health consequences. Children in some regions of [[developing countries]] are at particular risk due to increased requirements during growth coupled with diets low in animal-sourced foods.

The US [[National Academy of Medicine]] updated estimated average requirements (EARs) and recommended dietary allowances (RDAs) for vitamin B{{sub|12}} in 1998. The EAR for vitamin B{{sub|12}} for women and men ages 14 and up is 2.0{{nbsp}}μg/day; the RDA is {{value|2.4|u=μg/day}}. RDA is higher than EAR so as to identify amounts that will cover people with higher than average requirements. RDA for pregnancy equals 2.6{{nbsp}}μg/day. RDA for lactation equals {{value|2.8|u=μg/day}}. For infants up to 12 months the adequate intake (AI) is 0.4–0.5{{nbsp}}μg/day. (AIs are established when there is insufficient information to determine EARs and RDAs.) For children ages 1–13 years the RDA increases with age from 0.9 to 1.8{{nbsp}}μg/day. Because 10 to 30 percent of older people may be unable to effectively absorb vitamin B{{sub|12}} naturally occurring in foods, it is advisable for those older than 50 years to meet their RDA mainly by consuming foods fortified with vitamin B{{sub|12}} or a supplement containing vitamin B{{sub|12}}. As for safety, [[tolerable upper intake level]]s (known as ULs) are set for vitamins and minerals when evidence is sufficient. In the case of vitamin B{{sub|12}} there is no UL, as there is no human data for adverse effects from high doses. Collectively the EARs, RDAs, AIs and ULs are referred to as [[dietary reference intake]]s (DRIs).

The [[European Food Safety Authority]] (EFSA) refers to the collective set of information as "dietary reference values", with population reference intake (PRI) instead of RDA, and average requirement instead of EAR. AI and UL are defined by EFSA the same as in the United States. For women and men over age 18 the adequate intake (AI) is set at 4.0{{nbsp}}μg/day. AI for pregnancy is 4.5 μg/day, for lactation 5.0{{nbsp}}μg/day. For children aged 1–14 years the AIs increase with age from 1.5 to 3.5{{nbsp}}μg/day. These AIs are higher than the U.S. RDAs. The EFSA also reviewed the safety question and reached the same conclusion as in the United States—that there was not sufficient evidence to set a UL for vitamin B{{sub|12}}.

The Japan National Institute of Health and Nutrition set the RDA for people ages 12 and older at 2.4{{nbsp}}μg/day. The [[World Health Organization]] also uses 2.4{{nbsp}}μg/day as the adult recommended nutrient intake for this vitamin.

For U.S. food and dietary supplement labeling purposes, the amount in a serving is expressed as a "percent of daily value" (%DV). For vitamin B{{sub|12}} labeling purposes, 100% of the daily value was 6.0{{nbsp}}μg, but on May 27, 2016, it was revised downward to 2.4{{nbsp}}μg. Compliance with the updated labeling regulations was required by 1 January 2020 for manufacturers with [[US$]]10&nbsp;million or more in annual food sales, and by 1 January 2021 for manufacturers with lower volume food sales. A table of the old and new adult daily values is provided at [[Reference Daily Intake]].

<div lang="en" dir="ltr" class="mw-content-ltr">

==Sources==

===Bacteria and archaea===

Vitamin B₁₂ is produced in nature by certain [[bacteria]], and [[archaea]]. It is synthesized by some bacteria in the [[gut microbiota]] in humans and other animals, but it has long been thought that humans cannot absorb this as it is made in the [[Large intestine|colon]], downstream from the [[small intestine]], where the absorption of most nutrients occurs. [[Ruminant]]s, such as cows and sheep, are foregut fermenters, meaning that plant food undergoes microbial fermentation in the [[rumen]] before entering the true stomach ([[abomasum]]), and thus they are absorbing vitamin B₁₂ produced by bacteria.

Other mammalian species (examples: [[rabbit]]s, [[pika]]s, [[beaver]], [[guinea pigs]]) consume high-fibre plants which pass through the gastrointestinal tract and undergo bacterial fermentation in the [[cecum]] and [[large intestine]]. In this [[hindgut fermentation]], the material from the cecum is expelled as "[[cecotrope]]s" and are re-ingested, a practice referred to as [[cecotrope|cecotrophy]]. Re-ingestion allows for absorption of nutrients made available by bacterial fermentation, and also of vitamins and other nutrients synthesized by the gut bacteria, including vitamin B₁₂.

Non-ruminant, non-hindgut herbivores may have an enlarged forestomach and/or small intestine to provide a place for bacterial fermentation and B-vitamin production, including B₁₂. For gut bacteria to produce vitamin B₁₂, the animal must consume sufficient amounts of [[Bush sickness|cobalt]]. Soil that is deficient in cobalt may result in B₁₂ deficiency, and B₁₂ injections or cobalt supplementation may be required for livestock.

=== Animal-derived foods ===

Animals store vitamin B₁₂ from their diets in their [[liver]]s and [[muscle]]s and some pass the vitamin into their [[Egg as food|eggs]] and [[milk]]. Meat, liver, eggs and milk are therefore sources of the vitamin for other animals, including humans. Insects are a source of B₁₂ for animals (including other insects and humans). Animal-derived food sources with a high concentration of vitamin B₁₂ include [[Liver (food)|liver]] and other [[organ meat]]s from [[Lamb and mutton|lamb]], [[veal]], [[beef]], and [[Turkey meat|turkey]]; also [[shellfish]] and [[crab meat]].

===Plants and algae===

There is some evidence that bacterial fermentation of plant foods and symbiotic relationships between algae and bacteria can provide vitamin B₁₂. However, the [[Academy of Nutrition and Dietetics]] considers plant and algae sources "unreliable", stating that [[Veganism|vegans]] should turn to fortified foods and supplements instead.

Natural plant and [[algae]] sources of vitamin B₁₂ include [[fermentation|fermented]] plant foods such as [[tempeh]] and seaweed-derived foods such as [[nori]] and [[laverbread]]. Methylcobalamin has been identified in ''[[Chlorella vulgaris]]''. Since only bacteria and some archea possess the genes and enzymes necessary to synthesize vitamin B₁₂, plant and algae sources all obtain the vitamin secondarily from symbiosis with various species of bacteria, or in the case of fermented plant foods, from bacterial fermentation.

<div lang="en" dir="ltr" class="mw-content-ltr">

===Fortified foods===

Foods for which vitamin B₁₂-fortified versions are available include [[breakfast cereals]], plant-derived [[milk substitute]]s such as [[soy milk]] and [[oat milk]], [[energy bar]]s, and [[nutritional yeast]]. The fortification ingredient is cyanocobalamin. Microbial fermentation yields adenosylcobalamin, which is then converted to cyanocobalamin by addition of potassium cyanide or thiocyanate in the presence of sodium nitrite and heat.

As of 2019, nineteen countries require food fortification of wheat flour, maize flour or rice with vitamin B₁₂. Most of these are in southeast Africa or Central America.

Vegan advocacy organizations, among others, recommend that every vegan consume B₁₂ from either fortified foods or supplements.

<div lang="en" dir="ltr" class="mw-content-ltr">

=== Supplements ===

[[File:Methylcobalamin tablets.jpg|thumb|200px|A blister pack of 500 µg methylcobalamin tablets]]

Vitamin B₁₂ is included in multivitamin pills; in some countries grain-based foods such as bread and pasta are fortified with B₁₂. In the US, non-prescription products can be purchased providing up to 5,000{{nbsp}}µg each, and it is a common ingredient in [[energy drink]]s and [[energy shot]]s, usually at many times the recommended dietary allowance of B₁₂. The vitamin can also be supplied on prescription and delivered via injection or other means.

[[Sublingual administration|Sublingual]] [[methylcobalamin]], which contains no [[cyanide]], is available in 5{{nbsp}}mg tablets. The metabolic fate and biological distribution of methylcobalamin are expected to be similar to that of other sources of vitamin B₁₂ in the diet. The amount of cyanide in cyanocobalamin is generally not a concern, even in the 1,000{{nbsp}}µg dose, since the amount of cyanide there (20{{nbsp}}µg in a 1,000{{nbsp}}µg cyanocobalamin tablet) is less than the daily consumption of cyanide from food, and therefore cyanocobalamin is not considered a health risk.

<div lang="en" dir="ltr" class="mw-content-ltr">

=== Intramuscular or intravenous injection ===

Injection of [[hydroxycobalamin]] is often used if digestive absorption is impaired, but this course of action may not be necessary with high-dose oral supplements (such as 0.5–1.0{{nbsp}}mg or more), because with large quantities of the vitamin taken orally, even the 1% to 5% of free crystalline B₁₂ that is absorbed along the entire intestine by passive diffusion may be sufficient to provide a necessary amount.

A person with cobalamin C disease (which results in combined [[methylmalonic aciduria]] and [[homocystinuria]]) may require treatment with intravenous or intramuscular hydroxocobalamin or transdermal B₁₂, because oral cyanocobalamin is inadequate in the treatment of cobalamin C disease.

=== Nanotechnologies used in vitamin B₁₂ supplementation ===

Conventional administration does not ensure specific distribution and controlled release of vitamin B₁₂. Moreover, therapeutic protocols involving injection require health care people and commuting of patients to the hospital thus increasing the cost of the treatment and impairing the lifestyle of patients. Targeted delivery of vitamin B₁₂ is a major focus of modern prescriptions. For example, conveying the vitamin to the bone marrow and nerve cells would help myelin recovery. Currently, several nanocarriers strategies are being developed for improving vitamin B₁₂ delivery with the aim to simplify administration, reduce costs, improve pharmacokinetics, and ameliorate the quality of patients' lives.

=== Pseudovitamin-B₁₂ ===

Pseudovitamin-B₁₂ refers to B₁₂-like analogues that are biologically inactive in humans. Most cyanobacteria, including ''[[Spirulina (dietary supplement)|Spirulina]]'', and some algae, such as ''[[Porphyra]] tenera'' (used to make a dried seaweed food called [[nori]] in Japan), have been found to contain mostly pseudovitamin-B₁₂ instead of biologically active B₁₂. These pseudo-vitamin compounds can be found in some types of shellfish, in edible insects, and at times as metabolic breakdown products of cyanocobalamin added to dietary supplements and fortified foods.

Pseudovitamin-B₁₂ can show up as biologically active vitamin B₁₂ when a microbiological assay with ''Lactobacillus delbrueckii'' subsp. lactis is used, as the bacteria can utilize the pseudovitamin despite it being unavailable to humans. To get a reliable reading of B₁₂ content, more advanced techniques are available. One such technique involves pre-separation by [[silica gel]] and then assessment with B₁₂-dependent ''E. coli'' bacteria.

A related concept is [[antivitamin]] B₁₂, compounds (often synthetic B₁₂ analogues) that not only have no vitamin action, but also actively interfere with the activity of true vitamin B₁₂. The design of these compounds mainly involve replacement of the metal ion with [[rhodium]], [[nickel]], or [[zinc]]; or the attachment of an inactive ligand such as 4-ethylphenyl. These compounds have the potential to be used for analyzing B₁₂ utilization pathways or even attacking B₁₂-dependent pathogens.

<div lang="en" dir="ltr" class="mw-content-ltr">

== Drug interactions ==

===H₂-receptor antagonists and proton-pump inhibitors===

Gastric acid is needed to release vitamin B₁₂ from protein for absorption. Reduced secretion of [[gastric acid]] and [[pepsin]], from the use of [[H2 antagonist|H₂ blocker]] or [[proton-pump inhibitor]] (PPI) drugs, can reduce absorption of protein-bound (dietary) vitamin B₁₂, although not of supplemental vitamin B₁₂. H₂-receptor antagonist examples include [[cimetidine]], [[famotidine]], [[nizatidine]], and [[ranitidine]]. PPIs examples include [[omeprazole]], [[lansoprazole]], [[rabeprazole]], [[pantoprazole]], and [[esomeprazole]]. Clinically significant vitamin B₁₂ deficiency and megaloblastic anemia are unlikely, unless these drug therapies are prolonged for two or more years, or if in addition the person's dietary intake is below recommended levels. Symptomatic vitamin deficiency is more likely if the person is rendered [[achlorhydria|achlorhydric]] (a complete absence of gastric acid secretion), which occurs more frequently with proton pump inhibitors than H₂ blockers.

<div lang="en" dir="ltr" class="mw-content-ltr">

===Metformin===

Reduced serum levels of vitamin B₁₂ occur in up to 30% of people taking long-term [[anti-diabetic medication|anti-diabetic]] [[metformin]]. Deficiency does not develop if dietary intake of vitamin B₁₂ is adequate or prophylactic B₁₂ supplementation is given. If the deficiency is detected, metformin can be continued while the deficiency is corrected with B₁₂ supplements.

===Other drugs===

Certain medications can decrease the absorption of orally consumed vitamin B₁₂, including [[colchicine]], extended-release [[potassium]] products, and antibiotics such as [[gentamicin]], [[neomycin]] and [[tobramycin]]. Anti-seizure medications [[phenobarbital]], [[pregabalin]], [[primidone]] and [[topiramate]] are associated with lower than normal serum vitamin concentration. However, serum levels were higher in people prescribed [[valproate]]. In addition, certain drugs may interfere with laboratory tests for the vitamin, such as [[amoxicillin]], [[erythromycin]], [[methotrexate]] and [[pyrimethamine]].

<div lang="en" dir="ltr" class="mw-content-ltr">

==Chemistry==

[[File:B12 methylcobalamin.jpg|thumb|200px|Methylcobalamin (shown) is a form of vitamin B₁₂. Physically it resembles the other forms of vitamin B₁₂, occurring as dark red crystals that freely form cherry-colored transparent solutions in water.]]

Vitamin B₁₂ is the most chemically complex of all the vitamins. The structure of B₁₂ is based on a [[corrin]] ring, which is similar to the [[porphyrin]] ring found in [[heme]]. The central metal ion is [[cobalt]].  As isolated as an air-stable solid and available commercially, cobalt in vitamin B₁₂ (cyanocobalamin and other vitamers) is present in its +3 oxidation state.  Biochemically, the cobalt center can take part in both two-electron and one-electron reductive processes to access the "reduced" (B_12r, +2 oxidation state) and "super-reduced" (B_12s, +1 oxidation state) forms.  The ability to shuttle between the +1, +2, and +3 oxidation states is responsible for the versatile chemistry of vitamin B₁₂, allowing it to serve as a donor of deoxyadenosyl radical (radical alkyl source) and as a methyl cation equivalent (electrophilic alkyl source).

Four of the six coordination sites are provided by the corrin ring, and a fifth by a [[dimethylbenzimidazole]] group. The sixth coordination site, the [[reactive center]], is variable, being a [[Cyanide|cyano group]] (–CN), a [[hydroxyl]] group (–OH), a [[methyl]] group (–CH₃) or a 5′-deoxy[[Adenosine|adenosyl]] group.  Historically, the covalent carbon–cobalt bond is one of the first examples of carbon–metal bonds to be discovered in biology. The [[hydrogenase]]s and, by necessity, enzymes associated with cobalt utilization, involve metal–carbon bonds. Animals have the ability to convert cyanocobalamin and hydroxocobalamin to the bioactive forms adenosylcobalamin and methylcobalamin by means of enzymatically replacing the cyano or hydroxyl groups.

[[File:Cobalamin-general-structure-color.png|thumb|The structures of the four most common vitamers of cobalamin, together with some synonyms. The structure of the 5'-deoxyadenosyl group, which forms the R group of adenosylcobalamin is also shown.|400px]]

<div lang="en" dir="ltr" class="mw-content-ltr">

=== Methods for the analysis of vitamin B₁₂ in food ===

Several methods have been used to determine the vitamin B₁₂ content in foods including microbiological assays, chemiluminescence assays, polarographic, spectrophotometric and high-performance liquid chromatography processes. The microbiological assay has been the most commonly used assay technique for foods, utilizing certain vitamin B₁₂-requiring microorganisms, such as [[Lactobacillus delbrueckii subsp. lactis|''Lactobacillus delbrueckii'' subsp. ''lactis'']] ATCC7830. However, it is no longer the reference method due to the high measurement uncertainty of vitamin B₁₂.

Furthermore, this assay requires overnight incubation and may give false results if any inactive vitamin B₁₂ analogues are present in the foods. Currently, radioisotope dilution assay (RIDA) with labelled vitamin B₁₂ and hog IF (pigs) have been used to determine vitamin B₁₂ content in food. Previous reports have suggested that the RIDA method is able to detect higher concentrations of vitamin B₁₂ in foods compared to the microbiological assay method.

<div lang="en" dir="ltr" class="mw-content-ltr">

==Biochemistry==

<div lang="en" dir="ltr" class="mw-content-ltr">

===Coenzyme function===

Vitamin B₁₂ functions as a [[coenzyme]], meaning that its presence is required in some enzyme-catalyzed reactions. Listed here are the three classes of enzymes that sometimes require B₁₂ to function (in animals):

# [[Isomerase]]s

#: Rearrangements in which a hydrogen atom is directly transferred between two adjacent atoms with concomitant exchange of the second substituent, X, which may be a carbon atom with substituents, an oxygen atom of an alcohol, or an amine. These use the adoB₁₂ (adenosylcobalamin) form of the vitamin.

# [[Methyltransferase]]s

#: Methyl (–CH₃) group transfers between two molecules. These use the MeB₁₂ (methylcobalamin) form of the vitamin.

# [[Dehalogenase]]s

#: Some species of anaerobic bacteria synthesize B₁₂-dependent dehalogenases, which have potential commercial applications for degrading chlorinated pollutants. The microorganisms may either be capable of ''de novo'' corrinoid biosynthesis or are dependent on exogenous vitamin B₁₂.

In humans, two major coenzyme B₁₂-dependent enzyme families corresponding to the first two reaction types, are known. These are typified by the following two enzymes:

[[File:Folate methionine cycle.svg|thumb|Simplified schematic diagram of the folate methionine cycle. Methionine synthase transfers the methyl group to the vitamin and then transfers the methyl group to homocysteine, converting that to methionine.|400px]]

[[Methylmalonyl-CoA mutase|Methylmalonyl coenzyme A mutase]] (MUT) is an isomerase enzyme which uses the AdoB₁₂ form and reaction type 1 to convert [[L-methylmalonyl-CoA]] to [[succinyl-CoA]], an important step in the catabolic breakdown of some [[amino acid]]s into succinyl-CoA, which then enters energy production via the [[citric acid cycle]]. This functionality is lost in [[vitamin B12 deficiency|vitamin B₁₂ deficiency]], and can be measured clinically as an increased serum [[methylmalonic acid]] (MMA) concentration. The MUT function is necessary for proper [[myelin]] synthesis. Based on animal research, it is thought that the increased methylmalonyl-CoA hydrolyzes to form methylmalonate (methylmalonic acid), a neurotoxic dicarboxylic acid, causing neurological deterioration.

[[Methionine synthase]], coded by ''MTR'' gene, is a methyltransferase enzyme which uses the MeB₁₂ and reaction type 2 to transfer a methyl group from [[5-methyltetrahydrofolate]] to [[homocysteine]], thereby generating [[tetrahydrofolate]] (THF) and [[methionine]]. This functionality is lost in [[vitamin B12 deficiency|vitamin B₁₂ deficiency]], resulting in an increased [[homocysteine]] level and the trapping of [[folate]] as 5-methyl-tetrahydrofolate, from which THF (the active form of folate) cannot be recovered. THF plays an important role in DNA synthesis, so reduced availability of THF results in ineffective production of cells with rapid turnover, in particular red blood cells, and also intestinal wall cells which are responsible for absorption. THF may be regenerated via MTR or may be obtained from fresh folate in the diet. Thus all of the DNA synthetic effects of B₁₂ deficiency, including the [[megaloblastic anemia]] of [[pernicious anemia]], resolve if sufficient dietary folate is present. Thus the best-known "function" of B₁₂ (that which is involved with DNA synthesis, cell-division, and anemia) is actually a [[:wikt:facultative|facultative]] function which is mediated by B₁₂-conservation of an active form of folate which is needed for efficient DNA production. Other cobalamin-requiring methyltransferase enzymes are also known in bacteria, such as Me-H₄-MPT, coenzyme M methyltransferase.

<div lang="en" dir="ltr" class="mw-content-ltr">

==Physiology==

===Absorption===

Vitamin B₁₂ is absorbed by a B₁₂-specific transport proteins or via passive diffusion. Transport-mediated absorption and tissue delivery is a complex process involving three transport proteins: [[haptocorrin]] (HC), [[intrinsic factor]] (IF) and [[transcobalamin II]] (TC2), and respective membrane receptor proteins. HC is present in saliva. As vitamin-containing food is digested by [[hydrochloric acid]] and [[pepsin]] secreted into the stomach, HC binds the vitamin and protected it from acidic degradation. Upon leaving the stomach the hydrochloric acid of the [[chyme]] is neutralized in the [[duodenum]] by [[sodium bicarbonate|bicarbonate]], and pancreatic proteases release the vitamin from HC, making it available to be bound by IF, which is a protein secreted by gastric [[parietal cell]]s in response to the presence of food in the stomach. IF delivers the vitamin to receptor proteins [[cubilin]] and [[amnionless]], which together form the [[cubam]] receptor in the distal [[ileum]]. The receptor is specific to the IF-B₁₂ complex, and so will not bind to any vitamin content that is not bound to IF.

Investigations into the intestinal absorption of B₁₂ confirm that the upper limit of absorption per single oral dose is about 1.5{{nbsp}}µg, with 50% efficiency. In contrast, the passive diffusion process of B₁₂ absorption — normally a very small portion of total absorption of the vitamin from food consumption — may exceed the haptocorrin- and IF-mediated absorption when oral doses of B₁₂ are very large, with roughly 1% efficiency. Thus, dietary supplement B₁₂ supplementation at 500 to 1000{{nbsp}}µg per day allows [[pernicious anemia]] and certain other defects in B₁₂ absorption to be treated with daily oral megadoses of B₁₂ without any correction of the underlying absorption defects.

After the IF/B₁₂ complex binds to cubam the complex is disassociated and the free vitamin is transported into the [[portal circulation]]. The vitamin is then transferred to TC2, which serves as the circulating plasma transporter,  Hereditary defects in production of TC2 and its receptor may produce functional deficiencies in B₁₂ and infantile [[megaloblastic anemia]], and abnormal B₁₂ related biochemistry, even in some cases with normal blood B₁₂ levels. For the vitamin to serve inside cells, the TC2-B₁₂ complex must bind to a cell receptor protein and be [[endocytosis|endocytosed]]. TC2 is degraded within a [[lysosome]], and free B₁₂ is released into the cytoplasm, where it is transformed into the bioactive coenzyme by cellular enzymes.

[[Antacid]] drugs that neutralize stomach acid and drugs that block acid production (such as [[proton-pump inhibitor]]s) will inhibit absorption of B₁₂ by preventing release from food in the stomach. Other causes of B12 malabsorption include [[intrinsic factor]] deficiency, [[pernicious anemia]], [[bariatric surgery]] pancreatic insufficiency, obstructive jaundice, tropical sprue and celiac disease, and radiation enteritis of the distal ileum. Age can be a factor. Elderly people are often [[achlorhydria|achlorhydric]] due to reduced stomach parietal cell function, and thus have an increased risk of B₁₂ deficiency.

<div lang="en" dir="ltr" class="mw-content-ltr">

===Storage and excretion===

How fast B₁₂ levels change depends on the balance between how much B₁₂ is obtained from the diet, how much is secreted and how much is absorbed. The total amount of vitamin B₁₂ stored in the body is about 2–5{{nbsp}}mg in adults. Around 50% of this is stored in the liver. Approximately 0.1% of this is lost per day by secretions into the gut, as not all these secretions are reabsorbed. [[Bile]] is the main form of B₁₂ excretion; most of the B₁₂ secreted in the bile is recycled via [[enterohepatic circulation]]. Excess B₁₂ beyond the blood's binding capacity is typically excreted in urine. Owing to the extremely efficient enterohepatic circulation of B₁₂, the liver can store 3 to 5 years' worth of vitamin B₁₂; therefore, nutritional deficiency of this vitamin is rare in adults in the absence of malabsorption disorders. In the absence of enterohepatic reabsorption, only months to a year of vitamin B₁₂ are stored.

=== Cellular reprogramming ===

Vitamin B₁₂ through its involvement in one-carbon metabolism plays a key role in [[Induced pluripotent stem cell|cellular reprogramming]] and tissue regeneration and epigenetic regulation. Cellular reprogramming is the process by which somatic cells can be converted to a pluripotent state. Vitamin B₁₂ levels affect the histone modification [[H3K36me3]], which suppresses illegitimate transcription outside of [[Promoter (genetics)|gene promoters]]. Mice undergoing in vivo reprogramming were found to become depleted in B₁₂ and show signs of [[methionine]] starvation while supplementing reprogramming mice and cells with B₁₂ increased reprogramming efficiency, indicating a cell-intrinsic effect.

<div lang="en" dir="ltr" class="mw-content-ltr">

==Synthesis==

===Biosynthesis===

{{Main|Cobalamin biosynthesis}}

Vitamin B₁₂ is derived from a [[tetrapyrrole|tetrapyrrolic structural framework]] created by the enzymes [[Porphobilinogen deaminase|deaminase]] and [[uroporphyrinogen III synthase|cosynthetase]] which transform [[aminolevulinic acid]] via [[porphobilinogen]] and [[hydroxymethylbilane]] to [[uroporphyrinogen III]]. The latter is the first [[macrocycle|macrocyclic]] intermediate common to [[heme]], [[chlorophyll]], [[siroheme]] and B₁₂ itself. Later steps, especially the incorporation of the additional methyl groups of its structure, were investigated using ¹³C [[isotopic labelling|methyl-labelled]] [[S-adenosyl methionine]]. It was not until a [[genetic engineering|genetically engineered]] strain of ''[[Pseudomonas denitrificans]]'' was used, in which eight of the genes involved in the biosynthesis of the vitamin had been [[gene expression|overexpressed]], that the complete sequence of [[methylation]] and other steps could be determined, thus fully establishing all the intermediates in the pathway.

Species from the following [[Genus|genera]] and the following individual species are known to synthesize B₁₂: ''[[Propionibacterium]] shermanii'', ''[[Pseudomonas]]'' ''denitrificans'', ''[[Streptomyces]]'' ''griseus'', ''[[Acetobacterium]]'', ''[[Aerobacter]]'', ''[[Agrobacterium]]'', ''[[Alcaligenes]]'', ''[[Azotobacter]]'', ''[[Bacillus]]'', ''[[Clostridium]]'', ''[[Corynebacterium]]'', ''[[Flavobacterium]]'', ''[[Lactobacillus]]'', ''[[Micromonospora]]'', ''[[Mycobacterium]]'', ''[[Nocardia]]'', ''[[Proteus (bacterium)|Proteus]]'',

''[[Rhizobium]]'', ''[[Salmonella]]'', ''[[Serratia]]'', ''[[Streptococcus]]'' and ''[[Xanthomonas]]''.

<div lang="en" dir="ltr" class="mw-content-ltr">

===Industrial===

Industrial production of B₁₂ is achieved through [[Fermentation (biochemistry)|fermentation]] of selected microorganisms. ''[[Streptomyces griseus]]'', a bacterium once thought to be a [[fungus]], was the commercial source of vitamin B₁₂ for many years. The species ''[[Pseudomonas denitrificans]]'' and ''[[Propionibacterium freudenreichii]]'' subsp. ''shermanii'' are more commonly used today. These are grown under special conditions to enhance yield. [[Rhone-Poulenc]] improved yield via genetic engineering ''P. denitrificans''. ''[[Propionibacterium]]'', the other commonly used bacteria, produce no [[exotoxin]]s or [[endotoxin]]s and are generally recognized as safe (have been granted [[GRAS]] status) by the [[Food and Drug Administration]] of the United States.

</div>

The total world production of vitamin B₁₂ in 2008 was 35,000&nbsp;kg (77,175&nbsp;lb).

<div lang="en" dir="ltr" class="mw-content-ltr">

===Laboratory===

{{Main|Vitamin B12 total synthesis|l1=Vitamin B₁₂ total synthesis}}

The complete laboratory [[vitamin B12 total synthesis|synthesis of B₁₂]] was achieved by [[Robert Burns Woodward]] and [[Albert Eschenmoser]] in 1972. The work required the effort of 91 postdoctoral fellows (mostly at Harvard) and 12 PhD students (at [[ETH Zurich]]) from 19 nations. The synthesis constitutes a formal total synthesis, since the research groups only prepared the known intermediate cobyric acid, whose chemical conversion to vitamin B₁₂ was previously reported. This synthesis of vitamin B₁₂ is of no practical consequence due to its length, taking 72 chemical steps and giving an overall chemical yield well under 0.01%. Although there have been sporadic synthetic efforts since 1972, the Eschenmoser–Woodward synthesis remains the only completed (formal) total synthesis.

<div lang="en" dir="ltr" class="mw-content-ltr">

==History==

{{Further|Vitamin#History}}

===Descriptions of deficiency effects===

Between 1849 and 1887, [[Thomas Addison]] described a case of [[pernicious anemia]], [[William Osler]] and William Gardner first described a case of neuropathy, Hayem described large red cells in the peripheral blood in this condition, which he called "giant blood corpuscles" (now called [[macrocyte]]s), [[Paul Ehrlich]] identified [[megaloblasts]] in the bone marrow, and [[Ludwig Lichtheim]] described a case of [[myelopathy]].

===Identification of liver as an anti-anemia food===

During the 1920s, [[George Whipple]] discovered that ingesting large amounts of raw [[liver]] seemed to most rapidly cure the anemia of blood loss in dogs, and hypothesized that eating liver might treat pernicious anemia. [[Edwin Cohn]] prepared a liver extract that was 50 to 100 times more potent in treating pernicious anemia than the natural liver products. [[William Bosworth Castle|William Castle]] demonstrated that gastric juice contained an "intrinsic factor" which when combined with meat ingestion resulted in absorption of the vitamin in this condition. In 1934, George Whipple shared the 1934 [[Nobel Prize in Physiology or Medicine]] with [[William P. Murphy]] and [[George Minot]] for discovery of an effective treatment for pernicious anemia using liver concentrate, later found to contain a large amount of vitamin B₁₂.

<div lang="en" dir="ltr" class="mw-content-ltr">

===Identification of the active compound===

While working at the Bureau of Dairy Industry, U.S. Department of Agriculture, [[Mary Shaw Shorb]] was assigned work on the bacterial strain ''Lactobacillus lactis'' Dorner (LLD), which was used to make yogurt and other cultured dairy products. The culture medium for LLD required liver extract. Shorb knew that the same liver extract was used to treat pernicious anemia (her father-in-law had died from the disease), and concluded that LLD could be developed as an assay method to identify the active compound. While at the University of Maryland she received a small grant from [[Merck & Co.|Merck]], and in collaboration with [[Karl Folkers]] from that company, developed the LLD assay. This identified "LLD factor" as essential for the bacteria's growth. Shorb, Folker and [[Alexander R. Todd]], at the [[University of Cambridge]], used the LLD assay to extract the anti-pernicious anemia factor from liver extracts, purify it, and name it vitamin B₁₂. In 1955, Todd helped elucidate the structure of the vitamin. The complete [[Analytical chemistry|chemical structure]] of the molecule was determined by [[Dorothy Hodgkin]] based on [[Crystallography|crystallographic]] data and published in 1955 for which, and for other crystallographic analyses, she was awarded the Nobel Prize in Chemistry in 1964. Hodgkin went on to decipher the structure of [[insulin]].