Vitamin B12
Vitamin B12, also called cobalamin, is a vitamin most often sold with a trace of cyanide attached. The usual supplement form, cyanocobalamin, carries a cyano group because cyanide keeps the molecule from degrading. In a 1,000 microgram dose, that adds up to 20 micrograms of cyanide, less than a person eats from ordinary food in a day. A close relative, hydroxocobalamin, is even used to treat cyanide poisoning.
At the center of every B12 molecule sits one atom of cobalt. Four of its six bonding sites are held by a ring called a corrin, which resembles the porphyrin ring that holds iron in heme. A dimethylbenzimidazole group supplies a fifth. The sixth site is the reactive one, and whatever hangs there decides which version of the vitamin the molecule is.
The options are a cyano group, a hydroxyl group, a methyl group or a deoxyadenosyl group. Chemists call these versions vitamers, and the family is sometimes simply named the cobalamins. B12 is the most chemically complex of all the vitamins, and its cobalt explains much of why.
As an air-stable solid, the cobalt sits in a +3 oxidation state. Inside living systems it can drop to +2 or even +1, and that shuttling gives the vitamin its versatile chemistry. Depending on the form, it can hand off a deoxyadenosyl radical or behave like a methyl cation. Its carbon-cobalt bond was among the first carbon-metal bonds ever discovered in biology.
Only two forms do the work inside cells. Methylcobalamin operates in the cytosol and adenosylcobalamin in the mitochondria. Animals can build both from cyanocobalamin or hydroxocobalamin by swapping out the cyano or hydroxyl group.
Spirulina, an algal supplement, shows why structure alone is not enough. Its main corrinoid is pseudovitamin B12, which looks similar to the real vitamin but has no vitamin activity, even though spirulina is sometimes wrongly claimed to provide it.
Worse are the antivitamins, usually synthetic analogues that actively interfere with true B12. Designers make them by replacing the cobalt with rhodium, nickel or zinc, or by attaching an inactive ligand such as 4-ethylphenyl. Researchers see potential in them for analyzing B12 pathways and even for attacking pathogens that depend on the vitamin.
Cows and sheep get their B12 from microbes living in their first stomach. Only certain bacteria and archaea carry the genes and enzymes needed to build the vitamin, so every other creature borrows it. Ruminants ferment plant food in the rumen before it reaches the true stomach, called the abomasum. That order lets them absorb the B12 the bacteria have already made.
Rabbits, pikas, beavers and guinea pigs take a different route. Their fermentation happens in the cecum and large intestine, so they expel material called cecotropes and eat it again. This habit, cecotrophy, lets them absorb the vitamins their gut bacteria synthesized. Humans host B12-making bacteria too, but those live in the colon, downstream from the small intestine where most absorption happens.
Cobalt is the hidden requirement. Gut bacteria can make B12 only if the animal eats enough cobalt, and soil poor in cobalt can leave livestock deficient. Farmers then turn to B12 injections or cobalt supplements. In intensive farming, swine and poultry get B12 through fortified feed.
For people, the usual answer is animal food. Animals store the vitamin in their livers and muscles, and some pass it into eggs and milk. Liver and other organ meats from lamb, veal, beef and turkey are rich sources, as are shellfish and crab meat. Absorption varies: less than 9 percent from eggs, against 40 to 60 percent from fish, fowl and meat. Insects supply B12 to animals too.
Tempeh, nori, laverbread and Chlorella vulgaris all offer some B12, but each gets it secondhand from bacteria. The Academy of Nutrition and Dietetics calls plant and algae sources unreliable and points vegans to fortified foods and supplements.
Breakfast cereals, soy milk, oat milk, energy bars and nutritional yeast come in fortified versions. As of 2019, nineteen countries required B12 fortification of wheat flour, maize flour or rice, mostly in southeast Africa or Central America. The fortification ingredient is cyanocobalamin, made when fermentation yields adenosylcobalamin that is then treated with potassium cyanide or thiocyanate, sodium nitrite and heat.
Industry grows the bacteria in bulk. Streptomyces griseus, a bacterium once thought to be a fungus, was the commercial source for many years. Pseudomonas denitrificans and Propionibacterium freudenreichii subsp. shermanii are more common now. Rhone-Poulenc improved yields by genetically engineering P. denitrificans. Propionibacterium makes no exotoxins or endotoxins and has generally recognized as safe status from the United States Food and Drug Administration. World production in 2008 was 35,000 kilograms.
In the United States, nonprescription products can carry up to 1,000 micrograms each. Energy drinks and energy shots often contain many times the recommended allowance, a habit that sets up the claims examined in the final chapter.
The upper limit of B12 absorption from a single oral dose is about 1.5 micrograms, at roughly 50 percent efficiency. That ceiling reflects a hand-off between three transport proteins. They are haptocorrin, intrinsic factor and transcobalamin II.
Haptocorrin starts in saliva. In the stomach, hydrochloric acid and pepsin free the vitamin from food, and haptocorrin grabs it and shields it from acid damage. Past the stomach, bicarbonate neutralizes the acid in the duodenum, and pancreatic proteases release the vitamin from haptocorrin. Intrinsic factor, a protein secreted by the stomach's parietal cells when food arrives, then takes over.
Intrinsic factor carries the vitamin to the distal ileum, where cubilin and amnionless together form the cubam receptor. The receptor recognizes only the intrinsic factor and B12 complex, so loose vitamin is ignored. Once the complex comes apart, the free vitamin enters the portal circulation and is handed to transcobalamin II, the transporter in plasma. Cells take up that complex by endocytosis, break down the transporter in a lysosome and release free B12 into the cytoplasm, where enzymes convert it into its active coenzyme forms.
There is a side door. Passive diffusion normally accounts for a very small share of absorption, at roughly 1 percent efficiency. With very large oral doses, however, it can exceed the protein-mediated route. That is why daily megadoses of 500 to 1,000 micrograms can treat pernicious anemia without repairing the defect behind it.
The body holds about 2 to 5 milligrams of B12 in total, around half of it in the liver. About 0.1 percent is lost each day through secretions into the gut. Bile is the main route of loss, yet most of it is recycled through enterohepatic circulation, and surplus beyond the blood's binding capacity leaves in urine. Thanks to that recycling, the liver can hold 3 to 5 years' worth of the vitamin. Without intrinsic factor or ileal receptors, stores last only months to a year.
Two human enzymes show what the vitamin does with its cargo. Methylmalonyl-CoA mutase uses the adenosyl form to convert L-methylmalonyl-CoA into succinyl-CoA, a step in breaking down some amino acids that feeds the citric acid cycle. When B12 runs short, this step fails and methylmalonic acid builds up, which clinicians can measure. Animal research suggests this neurotoxic acid contributes to nerve deterioration, and the enzyme is needed for proper myelin.
Methionine synthase, coded by the MTR gene, uses the methyl form to move a methyl group from 5-methyltetrahydrofolate to homocysteine. The products are methionine and tetrahydrofolate, or THF. Without B12, homocysteine rises and folate becomes trapped in the 5-methyl form, from which THF cannot be recovered. THF is needed for DNA synthesis, so fast-turnover cells such as red blood cells and intestinal wall cells suffer first.
That means the vitamin's best-known role, its link to DNA synthesis and anemia, is a facultative one. Enough fresh dietary folate resolves those effects, because B12 only conserves an active form of folate.
B12 also appears in the study of cellular reprogramming, which converts ordinary cells to a pluripotent state. Its levels affect the histone modification H3K36me3, which suppresses illegitimate transcription outside gene promoters. Mice undergoing in vivo reprogramming became depleted in B12 and showed signs of methionine starvation. Giving B12 to reprogramming mice and cells raised reprogramming efficiency, pointing to an effect inside the cell itself.
Between 40 and 80 percent of vegetarians who take no supplement and eat no fortified food are deficient in B12. In Hong Kong and India, roughly 80 percent of vegans have been found deficient. Plant foods do not contain enough of the vitamin to prevent a shortage. Fortified cereal, plant-based milks and nutritional yeast can close the gap.
Diet is not the usual culprit, though. The most common cause is malabsorption. Autoimmune gastritis attacks the parietal cells that make intrinsic factor, and without it no B12 can be absorbed. That condition is pernicious anemia.
Other causes include bariatric surgery, pancreatic insufficiency, obstructive jaundice, tropical sprue, celiac disease, inherited intrinsic factor deficiency and radiation enteritis in the distal ileum. Roux-en-Y gastric bypass raises the risk and requires preventive injected or high-dose oral supplements afterward. Sleeve gastric surgery and gastric banding do not carry that extra risk.
Age matters because stomach acid declines. Older adults are often achlorhydric, meaning they make little gastric acid, and absorption falls particularly after 60.
Drugs can add to the problem. Cimetidine, famotidine, omeprazole and pantoprazole reduce the acid that frees dietary B12 from protein, though they do not affect supplemental B12. Clinically significant deficiency is unlikely unless such treatment runs for two or more years or the diet is also poor. Up to 30 percent of long-term users of the diabetes drug metformin show lower serum B12, yet adequate intake or supplements prevents deficiency.
Colchicine, extended-release potassium products and the antibiotics gentamicin, neomycin and tobramycin can also reduce absorption. Nitrous oxide overuse converts the active vitamin to an inactive form.
At levels only slightly below normal, symptoms can include fatigue, headaches, numbness or tingling, poor balance, blurred vision, memory problems and depression. Severe or prolonged deficiency can lead to dementia, psychosis and irreversible damage to the brain and nervous system. Optic nerve atrophy and neuritis have been linked to it as well.
Pernicious anemia is characterized by a triad of problems. The first is megaloblastic anemia, caused by inhibited DNA synthesis. The second is gastrointestinal trouble, including diarrhea, constipation and loss of bladder or bowel control. The third is neurological, with sensory or motor deficits and subacute combined degeneration of the spinal cord. There is also an association with gastric antral vascular ectasia, nicknamed watermelon stomach.
A serum concentration below 148 picomoles per liter in a pregnant woman counts as low maternal B12. At that level, the risks include miscarriage, preterm birth and low birth weight. The placenta concentrates the vitamin, so newborns have higher serum levels than their mothers. Recently absorbed vitamin reaches the placenta and breast milk more effectively than what sits in the mother's liver.
The World Health Organization does not list B12 among supplements for healthy pregnant women. It is often suggested in a multivitamin with folic acid, especially for vegetarian or vegan mothers. Breast milk B12 declines over months of nursing. Exclusive or near-exclusive breastfeeding beyond six months is a strong indicator of low vitamin status in the infant, especially when the pregnancy was already low in B12.
Infants and young children with low B12 can show anemia, poor physical growth and neurodevelopmental delays. Children diagnosed with low serum B12 can receive intramuscular injections, then move to an oral supplement. Folate levels can change how the symptoms look, which complicates diagnosis.
In 1849, Thomas Addison described a case of what became known as pernicious anemia. Over the next four decades, other physicians added pieces. William Osler and William Gardner described a case of neuropathy. Hayem saw enlarged red cells that he called giant blood corpuscles, now called macrocytes. Paul Ehrlich identified megaloblasts in bone marrow, and Ludwig Lichtheim described a case of myelopathy. By 1887, the symptoms were catalogued, but the cause remained unknown.
The breakthrough came from dogs and dinner. In the 1920s, George Whipple found that eating large amounts of raw liver seemed to cure blood-loss anemia in dogs fastest, and he guessed that liver might help with pernicious anemia. Edwin Cohn then prepared a liver extract 50 to 100 times more potent than natural liver products. William Castle showed that gastric juice holds an intrinsic factor that, combined with eaten meat, allows absorption of the vitamin. In 1934, Whipple shared the Nobel Prize in Physiology or Medicine with William P. Murphy and George Minot for a treatment using liver concentrate.
Mary Shaw Shorb, working at the Bureau of Dairy Industry in the United States Department of Agriculture, supplied the next piece. She had been assigned a bacterial strain called Lactobacillus lactis Dorner, or LLD, which was used to make yogurt and other cultured dairy products. Its culture medium needed liver extract. Shorb knew the same extract treated pernicious anemia, and her father-in-law had died of the disease. She reasoned that LLD could serve as an assay to hunt for the active compound.
At the University of Maryland she won a small grant from Merck and teamed with Karl Folkers of that company. Together they developed the LLD assay, which showed that an LLD factor was essential for the bacteria's growth. Shorb, Folkers and Alexander R. Todd of the University of Cambridge used the assay to extract the anti-pernicious anemia factor from liver, purify it and name it vitamin B12.
Structure came next. In 1955, Todd helped elucidate it. Dorothy Hodgkin determined the complete chemical structure from crystallographic data, publishing in 1955 and 1956, and received the Nobel Prize in Chemistry in 1964 for that work and other crystallographic analyses. She went on to decipher the structure of insulin. Todd won a Nobel in 1957 and Robert Burns Woodward in 1965, each with important contributions to B12 study.
Woodward later joined Albert Eschenmoser in building the vitamin from scratch. They finished in 1972 with 91 postdoctoral fellows, mostly at Harvard, and 12 PhD students at ETH Zurich, drawn from 19 nations. Technically it was a formal total synthesis, since the teams prepared a known intermediate, cobyric acid, whose conversion to B12 had already been reported. It took 72 chemical steps and an overall yield well under 0.01 percent, so it has no practical use. It remains the only completed formal total synthesis.
How bacteria build the molecule stayed unclear far longer. Researchers finally mapped the full sequence of methylation and other steps using a genetically engineered Pseudomonas denitrificans in which eight of the relevant genes were overexpressed. The pathway begins with a compound called uroporphyrinogen III, the first large ring intermediate shared by heme, chlorophyll, siroheme and B12.
One review concluded that no "gold standard" test exists for diagnosing B12 deficiency. Doctors therefore weigh the patient's condition alongside test results. A routine blood count showing anemia with an elevated mean corpuscular volume often raises suspicion, and a blood smear may show macrocytes and hypersegmented polymorphonuclear leukocytes. Blood levels below 150 to 180 picomoles per liter, or 200 to 250 picograms per milliliter, in adults support the diagnosis.
Blood levels can mislead, because serum values may hold steady while tissue stores drain. Homocysteine above 15 micromoles per liter and methylmalonic acid above 0.271 micromoles per liter are considered better indicators. Neither is conclusive alone. Methylmalonic acid also rises with kidney insufficiency and in methylmalonic acidemia, and homocysteine rises with folate deficiency. If nerve damage is present and blood tests are inconclusive, a lumbar puncture can measure B12 in cerebrospinal fluid.
Haptocorrin binds 80 to 90 percent of circulating B12, making it unavailable for delivery by transcobalamin II. Several serious diseases raise haptocorrin, so a patient can show abnormally high serum B12 while starving at the cellular level.
Severe deficiency is first corrected with daily intramuscular injections, then maintained with monthly injections or daily oral doses. The oral dose far exceeds the body's need, because the normal transporter-mediated absorption is absent in these patients. Injection side effects include rash, itching, chills, fever, hot flushes, nausea and dizziness. There are not enough studies to say whether pills work as well as injections.
Hydroxocobalamin is the injected form. High-dose oral supplements of 0.5 to 1.0 milligram or more may do the job too, since passive diffusion absorbs 1 to 5 percent of free crystalline B12. There is no advantage to the adenosyl or methyl forms in treating deficiency. Hydroxocobalamin can also treat cobalamin C disease, a rare inherited disorder.
For cyanide poisoning, a large intravenous dose of hydroxocobalamin is given, sometimes with sodium thiosulfate. The toxic cyanide ion displaces the hydroxide group, forming harmless cyanocobalamin that leaves the body in urine.
The United States National Academy of Medicine updated its recommendations in 1998. The estimated average requirement is 2.0 micrograms per day for people aged 14 and up, and the recommended allowance in pregnancy is 2.6 micrograms. Infants up to 12 months have an adequate intake of 0.4 to 0.5 micrograms, and children aged 1 to 13 have an allowance rising from 0.9 to 1.8 micrograms. No upper limit exists, because there is no human data on harm from high doses.
Other authorities disagree. The European Food Safety Authority sets adequate intake at 4.0 micrograms per day for adults, 4.5 in pregnancy and 5.0 in lactation. Japan and the World Health Organization both use 2.4 micrograms for adults.
On the 27th of May 2016, the United States cut the daily value used on food labels from 6.0 micrograms to 2.4 micrograms. Manufacturers with US$10 million or more in annual food sales had to comply by the 1st of January 2020, and smaller ones by the 1st of January 2021.
About 10 to 30 percent of older people may be unable to absorb the naturally occurring vitamin in food. Anyone over 50 should therefore get most of their allowance from fortified foods or a supplement. Some research puts the share of people in the Western world with low or marginal levels at up to 40 percent.
In the 1970s, John A. Myers, a physician in Baltimore, developed a program of intravenous vitamins and minerals for various conditions. His formula included cyanocobalamin and became known as the Myers' cocktail. After Myers died in 1984, other physicians and naturopaths took up what they called intravenous micronutrient therapy. They made unsubstantiated claims about fatigue, low energy, stress, anxiety, migraine, depression, weakened immunity and weight loss.
Other than a report on case studies, no benefits are confirmed in the scientific literature. Clinics and spas still prescribe versions of these mixes, along with intramuscular shots of B12 alone. A Mayo Clinic review found no solid evidence that B12 injections boost energy or help with weight loss.
The habit reaches into ordinary medicine too. One large study found that physicians often repeatedly gave older patients cyanocobalamin injections without good reason. Most subjects either had normal serum levels or had not been tested before the shots.
Conventional delivery has a deeper limit. It does not guarantee specific distribution or controlled release of the vitamin, for instance to bone marrow and nerve cells. Nanocarrier strategies aimed at fixing that remained embryonic as of 2021.
Updated
Common questions
What is vitamin B12 and what does it do in the body?
Vitamin B12, also called cobalamin, is a water-soluble vitamin and one of eight B vitamins. It acts as a cofactor in DNA synthesis and in fatty acid and amino acid metabolism, and it supports myelin synthesis and red blood cell maturation.
Where does vitamin B12 come from?
Vitamin B12 is made only by certain bacteria and archaea. Animals obtain it from gut bacteria or from animal foods such as meat, liver, fish, eggs and dairy, and people also get it from fortified foods and supplements.
Who is at risk of vitamin B12 deficiency?
Vegans, vegetarians who take no supplement, older adults, pregnant and breastfeeding women on low-animal-food diets, and people with malabsorption are at risk. Long-term metformin or acid-reducing drugs and Roux-en-Y gastric bypass surgery also raise the risk.
What are the symptoms of vitamin B12 deficiency?
Symptoms include fatigue, numbness or tingling, poor balance, memory problems and depression. Severe or prolonged deficiency can cause megaloblastic anemia, pernicious anemia, dementia and irreversible nerve damage.
Who discovered vitamin B12?
Mary Shaw Shorb, Karl Folkers and Alexander R. Todd extracted, purified and named vitamin B12 using the LLD assay. Dorothy Hodgkin determined its full structure from crystallographic data, published in 1955 and 1956.
How is a vitamin B12 deficiency treated?
Severe deficiency is first corrected with daily intramuscular injections, then maintained with monthly injections or daily oral doses. High-dose oral supplements can work because about 1 percent of a very large dose is absorbed by passive diffusion.
Do vitamin B12 injections give you more energy?
A Mayo Clinic review found no solid evidence that vitamin B12 injections provide an energy boost or aid weight loss. Benefits of intravenous micronutrient mixes such as the Myers' cocktail remain unconfirmed in the scientific literature.
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