In one pass B12 is found almost only in animal foods, because only bacteria and archaea can make it.
Educational content, not medical advice — consult a clinician.
HCl releases B12 Stomach acid and pepsin break down food protein to release the B12 bound to it, and in people short of stomach acid this step collapses.
Homocysteine accumulates After methionine gives up its methyl group it becomes homocysteine, a potentially toxic intermediate, and its build-up in blood is linked to cardiovascular events, stroke and dementia.
B12's second enzyme · MUT B12 is also the cofactor of a second enzyme, MUT, which in mitochondria turns methylmalonyl-CoA into succinyl-CoA so special fuels such as odd-chain fatty acids can enter the TCA cycle.
B12 is found almost only in animal foods, because only bacteria and archaea can make it. Plants, fungi and animals themselves lack the enzymes; animals get it by eating forage that carries bacteria, or from microbes living in their gut, and then store it in liver, meat, eggs and milk.
Plants need to be taken one by one. Dried nori (purple laver) contains real, active B12, and one review judges it the most suitable natural source currently available to vegetarians, but the amount varies widely with species and processing, so you cannot count on it for a steady supply. What is measured in spirulina is mainly "pseudo-B12", which has no B12 activity in people. Chlorella contains real B12, but products range from almost none to a great deal.
So for vegans, the reliable sources are fortified foods and supplements.
Mechanism · Why B12 is found almost only in animal foods
B12 is the vitamin vegetarians are most likely to run short of, because it is found almost only in animal foods. The reason lies in where it comes from.
B12, also called cobalamin, is the largest and most complex molecule among the vitamins. At its center is a cobalt (Co) ion held inside a corrin ring, a close relative of the rings in heme (which holds iron) and chlorophyll (which holds magnesium).
One key fact: only prokaryotes (bacteria and archaea) have the full set of enzymes that make B12. Plants, fungi and animals cannot make it. Animals take it in from forage that carries bacteria, or from microbes living in their gut, and then accumulate it in liver, meat, eggs and milk. So dietary B12 comes almost entirely from animal foods:
Clams: about 99 µg per 100 gBeef liver: about 70 µg per 100 g, among the highest of all (see Organ Meat)Salmon and tuna: about 4-7 µg per 100 g (see Salmon · see Tuna)Eggs: about 0.6 µg per egg (see Eggs)Milk: about 1.3 µg per cup (see Milk) The US Recommended Dietary Allowance () is only 2.4 µg a day, a small number. But plant sources are almost all unreliable:
Dried nori (see Nori (Laver)) is the exception. It contains real, active B12, and the Watanabe 2014 review concludes that it is the most suitable natural source currently available to vegetarians. But the amount varies widely with species and processing, so relying on it for a steady intake is not realistic.Spirulina: the compounds measured are mainly "pseudo-B12", similar in structure but with no B12 activity in people.Chlorella: it contains real B12, but products range from almost none to a great deal, so it is not a reliable supply.Unfermented legumes and nuts: none.Fortified foods (fortified breakfast cereals, fortified plant milks) contain real B12. For vegans, this is the only reliable plant-based dietary source. So vegans and near-vegans usually need fortified foods plus a supplement.
watanabe-2014-b12-plant-sources
Myth · Which plant foods really contain B12
A common claim online is that "this plant contains B12, so vegans don't need to worry". Let's check the evidence item by item:
Nori (purple laver): this one has to be separated from the other algae it usually gets lumped with. Watanabe 2014 concludes that dried nori contains real B12 and is the most suitable natural source currently available to vegetarians, and that it also supplies iron and omega-3 fats. But "most suitable" is not the same as "enough": content varies severalfold with species, drying and storage, and in some samples the B12 breaks down into inactive analogues. So it is a bonus, not a plan that lets you stop a supplement.Spirulina: this is where "pseudo-B12" really comes from. The analogues measured in it are overwhelmingly pseudovitamin B12, which has no B12 activity in people. A "high in B12" label that counts these inactive analogues is misleading.Chlorella: unlike spirulina, it contains real, active B12, but products range from almost none to a great deal, so it is not a reliable supply.Whether pseudo-B12 gets in the way of absorbing real B12, or makes a blood B12 test read high, has not been measured directly in people.Fermented soy (miso, natto, tempeh): contains real B12, but only in trace amounts (< 0.1 µg per serving), far below the 2.4 µg , so it cannot be a main source.Mushrooms (especially shiitake): contain a little real B12, about 5 µg per 100 g of dried shiitake, but a real meal portion supplies < 0.5 µg.Bacteria on unwashed organic vegetables: a little in theory, unreliable in practice, and mostly washed away under modern hygiene. There are only two reliable non-animal sources of B12:
1. Fortified foods: fortified breakfast cereal (one serving of a cereal labeled with 25% of the Daily Value has about 0.6 µg), fortified plant milks (about 1-3 µg per cup), fortified nutritional yeast (about a quarter cup has 8.3–24 µg). Amounts vary widely between products, so go by the label 2. B12 supplements: cyanocobalamin is the most common, and methylcobalamin also works
Vegans can pick one of three approaches and stick with it (the UK Vegan Society's advice): eat fortified foods at least twice a day, at least four hours apart, for a total of at least 3 µg a day; or take one oral supplement of at least 10 µg a day (the smaller the dose, the more it has to be taken every day without missing); or take one high-dose 2000 µg tablet once a week, which covers the week in one go. Check blood B12, (MMA) and (Hcy) once a year.
This is not "a limitation of being vegan". Once you know the mechanism, you know how to close the gap.
watanabe-2014-b12-plant-sourcesvegan-society-b12
Chapter 2
Absorption needs a stomach protein
Absorbing B12 is a three-step handoff, and the intrinsic factor step cannot be skipped. Intrinsic factor (IF) is a protein made by the parietal cells of the stomach lining, and its job is to escort B12.
Step one happens in the stomach: acid and the enzyme pepsin strip B12 off the proteins in food, and another binding protein that arrives with saliva (R-protein) catches it first. Step two happens in the duodenum: enzymes from the pancreas cut the R-protein apart, and B12 changes hands and binds to intrinsic factor. Step three happens at the very end of the small intestine (the terminal ileum): receptors there recognize only the pair of intrinsic factor plus B12, never B12 on its own, and take it into the body.
Too little stomach acid, or long-term acid-suppressing drugs, loosens step one, so B12 from food is absorbed less well. If the stomach is removed, or autoimmunity destroys intrinsic factor, the whole route is cut.
A CLOSER LOOK
B12 changes escorts in the duodenum
Intrinsic factor comes from the stomach, but its handoff with B12 happens in the duodenum; the resulting complex then travels to the terminal ileum.
1Release from food first
Stomach acid and pepsin release B12 from food protein, after which R-protein catches it.
2Break the carrier, then hand off
Pancreatic enzymes break down R-protein, and released B12 binds intrinsic factor. The pancreas does not make intrinsic factor.
Illustration for understanding; not to scale. Saved figures include explanations and sources.
Mechanism · The three-step handoff of absorption
B12 absorption is one of the most complex pathways in nutrition, and it runs through three handoffs.
Stage one, the stomach:
Stomach acid (HCl) and pepsin strip B12 off the proteins in food.A carrier protein made by the salivary glands, called R-protein (R-binder, also known as haptocorrin), reaches the stomach when you swallow and binds B12 first, protecting it.The parietal cells of the stomach's glands secrete intrinsic factor (IF), but in the stomach intrinsic factor does not bind B12 yet. Stage two, the duodenum: protein-cutting enzymes from the pancreas break down the R-protein and B12 is released. In the mildly alkaline setting, B12 binds intrinsic factor to form the intrinsic factor-B12 complex.
Stage three, the terminal ileum: the intrinsic factor-B12 complex travels along the small intestine to its last stretch, where dedicated receptors take it into the body (the chapter on the entry point at the end of the small intestine covers this).
The step most often overlooked on this path is too little stomach acid. When this step fails, B12 cannot be stripped off food:
Atrophic gastritis affects about 15-20% of people over 60, and stomach acid falls as it progresses.Long-term proton pump inhibitors (, such as omeprazole) or H2 blockers suppress acid directly.Long-term metformin interferes with B12 absorption (the mechanism is not fully understood; it is generally thought to act at the last stage).Gastric bypass surgery or removal of the stomach cuts out or bypasses the cells that make intrinsic factor, and usually means B12 must be replaced for life, often by injection.
Clinical · Long-term metformin and B12
Metformin is the first-line drug for type 2 diabetes and one of the most widely used prescription drugs in the world. The longer people take it, the higher their risk of low B12.
The evidence comes from the Diabetes Prevention Program Outcomes Study (DPPOS; Aroda 2016, JCEM), a follow-up analysis of a randomized trial. People with prediabetes were randomly assigned to metformin or placebo, and the metformin group then kept taking the drug. At 5 years, low B12 (≤ 203 pg/mL) was found in about 4.3% of the metformin group and about 2.3% of the placebo group. Counting low plus borderline-low B12 together (≤ 298 pg/mL), the figures were 19.1% versus 9.5%. Each extra year of metformin raised the odds of B12 deficiency by about 13% ( 1.13). In the same study, people on metformin with low B12 also had more peripheral nerve damage.
The mechanism is not fully settled. One explanation with experimental support is that the drug interferes with the calcium-dependent uptake step at the receptors in the terminal ileum. Other studies suggest an indirect effect through changes in the gut microbiome. The idea that it directly destroys B12 by oxidation has weak support.
One clinical trap deserves care: diabetic peripheral neuropathy is often blamed on diabetes itself, but part of it is actually nerve damage from B12 deficiency, which looks almost the same (numb feet, tingling, lost sense of where the feet are). If it is misdiagnosed and only blood sugar is treated, without replacing B12, the nerve damage continues. B12 deficiency also raises .
On guidelines, the 2019 update from the American Diabetes Association and the European Association for the Study of Diabetes (ADA, EASD) recommends that people on long-term metformin have B12 checked regularly (every 1-2 years, ideally together with and homocysteine), and the American Association of Clinical Endocrinologists (AACE) recommends the same. In practice, few people actually get tested.
In practice: if you take metformin long term, ask your doctor to check B12 at least once a year (ideally with MMA), and get tested early if you notice unexplained numbness, worse balance, fatigue or memory decline. Preventive B12 is not needed, but a deficiency that is found should be treated promptly, with your doctor. An oral dose of 1000 µg of cyanocobalamin a day is often enough: even when the intrinsic factor route works poorly, about 1% passive diffusion still brings in about 10 µg, roughly 4 times the . When the deficiency is severe or has already reached the nerves, doctors usually start with intramuscular injections, and the doctor decides how many, how often and how to maintain afterwards.
The cells lining the last stretch of the small intestine (the terminal ileum) carry a receptor called cubilin. It recognizes only the pair of intrinsic factor plus B12 (intrinsic factor is the escort protein made by the stomach), never B12 on its own, and it is one of the most specific lock-and-key pairs in nutrient absorption.
Cubilin pulls the pair into the cell. Inside, B12 separates from intrinsic factor and transfers to another carrier protein, transcobalamin (TCII), then enters the blood through the portal vein and travels around the body.
This dedicated channel carries at most about 1.5-2 µg per meal or per dose. Swallow a 1000 µg tablet, and only 1.5-2 µg goes through it. Fortunately, at very high doses there is also passive diffusion that does not need intrinsic factor, about 1%, so such a tablet brings in roughly another 10 µg, more than 4 times the . That is why people whose intrinsic factor route works poorly (older adults, long-term users of acid-suppressing drugs, mild atrophic gastritis) often do well on a 1000 µg oral tablet, while people who have had the whole stomach taken out, or who have severe pernicious anemia, usually start with injections.
A CLOSER LOOK
Inside the gut cell, B12 changes carriers
This enlarges the main intrinsic factor–mediated route. Some absorption also occurs without intrinsic factor, so this is not the only entry route.
1The receptor recognizes the pair
The ileal cubilin receptor recognizes the intrinsic factor–B12 complex and takes it into the cell.
2A different carrier in blood
Inside the cell, B12 separates from intrinsic factor and joins transcobalamin (TCII) for transport around the body.
Illustration for understanding; not to scale. Saved figures include explanations and sources.
Clinical · Oral, under the tongue, or injection
There are three ways to take B12, and the evidence for each is clear enough to sort.
Ordinary oral tablets are entirely adequate for people whose intrinsic factor route is intact, and cyanocobalamin or methylcobalamin both work; B12-only tablets on the market typically contain 500-1000 µg each. Even when the dedicated channel is saturated (about 1.5-2 µg per dose), about 1% passive diffusion lets a 1000 µg tablet bring in roughly another 10 µg. It is the cheapest option and the easiest to keep up long term.
Under-the-tongue (sublingual) tablets are, in theory, absorbed through the lining of the mouth, bypassing the intrinsic factor route. But the Sharabi 2003 randomized trial (30 people with B12 deficiency) compared the same dose taken under the tongue or swallowed, and after 4 weeks blood B12 had risen by the same amount. There is no evidence that sublingual is clearly better than ordinary oral: the mouth lining absorbs very little B12, and most of what a sublingual tablet does probably comes from the part that is swallowed and absorbed in the gut. Unless swallowing is difficult, it is not worth paying more.
Intramuscular injection (IM) has well-defined uses: severe pernicious anemia (autoimmune destruction of the cells that make intrinsic factor), removal of the whole stomach, severe atrophic gastritis combined with several gut diseases, inability to take pills (persistent vomiting, severe malabsorption), and severe nerve symptoms that call for rapid replacement. The classic plan is 1000 µg by injection once a week for 8 weeks to correct the deficiency, then 1000 µg a month for long-term maintenance.
High-dose oral B12, 1000-2000 µg a day, is an alternative. Several randomized trials in people with B12 deficiency (including megaloblastic anemia) compared high-dose oral B12 with injections and found similar results: Kuzminski 1998 and Bolaman 2003 compared oral with injection, Castelli 2011 also compared oral with injection, and Bensky 2019 was a retrospective analysis of sublingual versus injection. The condition is that the patient really does take it every day.
As for "energy shots" and "B12 for a boost": the B12 injections sold by trendy clinics do not raise energy in people with no evidence of deficiency. They confuse "symptoms fading after a real deficiency is corrected" with "topping up when you already have enough". The placebo effect plus the clinic experience is what is really being sold.
To sum up: most deficiencies are handled with 1000 µg a day by mouth plus finding out why the deficiency happened; only a small group with clear indications needs injections; sublingual tablets show no advantage and are not worth the extra money; and taking more B12 when you are not short will not give you more energy.
The body holds about 2-5 mg of B12, most of it in the liver. At the of 2.4 µg a day, that is 3-5 years' worth. It is the largest store of any water-soluble vitamin; the others are barely stored at all.
There is also a recycling loop, called enterohepatic circulation: each day the liver releases about 1.4 µg of B12 into bile, and after the bile returns to the small intestine, most of that B12 is absorbed again through the intrinsic factor route. So even if you stop taking in B12 entirely, the store leaks slowly rather than emptying all at once.
This has three consequences. Someone who has just gone vegan will not run short right away; most people can last 2-3 years. But waiting until the store is empty is too late: nerve damage starts slowly, and if it goes on long enough it can become permanent. So it is best to set up a B12 habit at the moment you switch to a vegan diet. For vegans, a yearly check of and (MMA) is more sensitive than measuring B12 alone and can catch an early shortfall before symptoms appear.
Myth · Do new vegans really have years in reserve?
"The B12 in your liver lasts 3-5 years" is a figure that is often misunderstood, and it leads new vegans to put off starting B12. Here is why you should not rely on it.
The 3-5 year reserve is estimated for someone who has eaten animal foods for decades and has a healthy liver. Several situations shorten it considerably.
First, the recycling loop itself depends on intrinsic factor. The liver releases about 1.4 µg of B12 into bile each day, most of which is reabsorbed in the ileum, and that loop needs a working intrinsic factor route. If you also have atrophic gastritis, or take proton pump inhibitors () or metformin long term, the loop leaks faster even while the store still has stock.
Second, the starting store may not be full. Many people's stores are below the theoretical maximum, perhaps only 1-2 mg rather than the textbook 5 mg; people who were underweight, ate a narrow diet or had chronic illness before going vegan may have < 1 mg.
Third, early subclinical deficiency is not the same as symptoms. The rough sequence is this: 6-12 months after going vegan, serum B12 may start to fall; at 12-24 months, (MMA) and start to rise; numbness and anemia, the clinical symptoms, are what the "3-5 years" refers to. Yet the biochemical changes are already under way during the subclinical stage, and by the time symptoms appear, some of the nerve damage may be permanent.
So strict vegans should not wait for the store to run out. Set up a B12 habit from the day you go vegan, choosing one of three approaches:
Eat fortified foods (fortified breakfast cereal, plant milk, nutritional yeast) at least twice a day, at least four hours apart, for a total of at least 3 µg a day.Take one oral supplement (cyanocobalamin) of at least 10 µg a day; the smaller the dose, the more it has to be taken every day.Take one high-dose 2000 µg tablet once a week, which covers the week in one go and suits people who forget. In addition, have a blood test for B12, MMA and homocysteine in the first year and every year after that.
One situation needs special attention: babies breastfed by strictly vegan mothers can develop severe B12 deficiency within 6 months, with permanent nerve damage. The mother must take B12. Whether the baby needs its own supplement, and how much, should be settled with a pediatrician after birth (the adequate intake for babies under one year old is 0.4-0.5 µg a day). If a baby loses skills it had gained, becomes unusually sleepy or feeds poorly, get medical care promptly.
The claim that "nutritional yeast or nori is enough" is checked item by item in the chapter on food. It does not hold up.
vegan-society-b12
Chapter 5
Passing on methyl groups
Many reactions in the body work by hanging a methyl group on a molecule: flipping gene switches, repairing the outer layer of nerves, making several signaling molecules in the brain. They all draw their supply from here. B12 assists methionine synthase, which moves a methyl group from folate onto to make methionine, which then becomes , the body's general methyl donor.
When B12 is short, that move jams, SAM runs low, and the homocysteine that was never converted piles up in the blood. Folate can still keep blood-making going on its own, so the anemia is hidden while nerve damage carries on. That is why doctors check B12 first and only then decide whether to give folate.
Mechanism · Methionine synthase and SAM
Start with something small that runs through the whole body: many reactions work by hanging a methyl group (a tiny chemical tag with just one carbon) on a molecule. Once it is hung, a gene switch flips, the outer sheath of a nerve gets repaired, and several mood-related signaling molecules in the brain can be made. B12 is an irreplaceable screw in this machinery; without it, the whole tagging system jams. Here is exactly where it jams.
B12 is a helper (cofactor) for two enzymes. The first is methionine synthase (abbreviated MTR), which does a moving job in the cytoplasm: it carries a methyl group from folate onto a molecule called (Hcy), turning it into methionine (Met). Written as a reaction:
Homocysteine (Hcy) + 5-methyl-THF → methionine (Met) + THF (B12 is the helper at this step; folate is the one handing over the methyl)
Methionine then becomes (S-adenosylmethionine), the body's central methyl warehouse: anywhere a methyl tag is needed, it is collected from SAM. SAM supplies:
Methyl tags on DNA and histones, which decide which stretches of genes are on or offPhospholipids for cell membranes (phosphatidylcholine)Methyl groups for making and breaking down several brain signaling molecules (dopamine and serotonin themselves are built from tyrosine and tryptophan; the methionine cycle, through SAM, supplies the methyl groups that some of those steps need)Repair of the myelin sheath around nerve fibersCreatine synthesis So once B12 is short, the first enzyme (MTR) stalls, the SAM warehouse runs low, and every job on that list is done at a discount. At the same time, the homocysteine that should have been converted has no one to take it, and it builds up in the blood: hyperhomocysteinemia. In observational studies, it is linked with cardiovascular disease, stroke and dementia.
Folate is B12's partner at this step. The trouble is that even when B12 is short, as long as folate is adequate it can keep another blood-making job running, so the anemia is hidden and the lab results look acceptable while the damage on the nerve side carries on day after day. That is the biochemical reason why folate can mask B12 deficiency. It is also why doctors usually check B12 first and only then decide whether to give folate, never the other way around.
Myth · Must MTHFR carriers take methylcobalamin?
"If you have an variant you must take methylcobalamin, not cyanocobalamin" has been one of the biggest marketing lines in direct selling and functional medicine over the past decade. Let's check the evidence point by point.
Start with MTHFR C677T. The T allele is common worldwide, but how often it lands as TT homozygosity varies sharply by ancestry: NIH ODS gives about 25% of Hispanics, about 10% of Whites and of Asians, and about 1% of African Americans. TT homozygotes have markedly lower MTHFR enzyme activity. The main effect is less 5-methyltetrahydrofolate (), slightly higher , and a somewhat higher folate requirement in some people.
Next, the main forms of B12 supplement:
Cyanocobalamin: the most common form in industry, stabilized by a -CN group; everyone's body removes the -CN and converts it into the working forms.Methylcobalamin: already in the methyl-carrying form, going straight into the methionine synthase route.Adenosylcobalamin: going straight into the route of MUT, an enzyme in the mitochondria.Hydroxocobalamin: the preferred form for injection, staying in the body a long time. In a two-page commentary, Thakkar 2015 points out that the body needs two coenzyme forms. Methylcobalamin is only one of them; the other, adenosylcobalamin, works in the mitochondria, so taking methylcobalamin alone is actually incomplete, while cyanocobalamin and hydroxocobalamin can both be converted into both forms in the body. Whether methylcobalamin corrects a deficiency better than cyanocobalamin has no support from reliable human trials. The exception is very rare inherited metabolic disorders such as CblC disease, where treatment is mainly hydroxocobalamin and is decided by specialists.
So "MTHFR positive means you must take methylcobalamin" is a marketing line. MTHFR affects the folate cycle, not the conversion of B12. Even TT homozygotes remove the -CN from ordinary cyanocobalamin and put it to use normally. "Skipping the conversion step" means nothing for the great majority of people.
When might methylcobalamin make sense? People with severe liver failure may convert less efficiently. Heavy smokers carry a slightly higher cyanide load, so in theory methylcobalamin means a little less cyanide exposure, but the absolute amounts are tiny: one cigarette delivers far more cyanide than one cyanocobalamin tablet.
In practice: most people can simply take cyanocobalamin tablets at a common strength, which are cheap, stable and effective. A methylation gene test does no harm, but its result cannot be read as "you must take methylcobalamin and methylfolate". What really decides your B12 status is intake, absorption and medicines, not the chemical form.
B12's second enzyme works in the mitochondria: methylmalonyl-CoA mutase (MUT). It takes an intermediate left over from breaking down odd-chain fatty acids and several amino acids and turns it into succinyl-CoA, which can enter the citric acid cycle to make energy. Without B12 this step jams and (MMA) builds up in the blood, which makes it a very sensitive lab marker of B12 deficiency.
B12 deficiency damages the insulating layer of nerve fibers (the myelin), and the spinal cord pathways for position sense and movement are hit first. This is called subacute combined degeneration: numb feet, a staggering walk, and trouble standing still with the eyes closed. Exactly how the myelin is damaged is not fully settled; one explanation is that the piled-up intermediate gets built into the fats of the myelin by mistake. Folate can hide the anemia, but it cannot stop this nerve damage. Unexplained numb feet and an unsteady walk call for an early B12 test.
Mechanism · Myelin damage and the time window
B12's second enzyme is methylmalonyl-CoA mutase (MUT), which works in the mitochondria. It converts methylmalonyl-CoA, an intermediate left over from odd-chain fatty acids and certain amino acids, into succinyl-CoA, so it can enter the citric acid cycle to make energy.
When B12 is short, MUT stalls and (MMA) builds up in the blood, which makes it a very sensitive biochemical marker of B12 deficiency. At the same time, trouble starts in the myelin (the coat around nerve fibers, like the insulation on a wire), and demyelination sets in. How the myelin is damaged has two main explanations, neither fully proven: one holds that the piled-up odd-chain fatty acids are wrongly built into the fats of the myelin; the other holds that a shortage of methylation (supplied by ) means the myelin is not maintained.
The clinical picture is subacute combined degeneration: symmetric loss of myelin in the lateral and posterior columns of the spinal cord. The usual order is numb feet first, then a staggering walk, then being unable to stand steady with the eyes closed (loss of position sense), and in severe cases cognitive decline and even dementia. Timing matters: it is generally held that if it is found and treated within 6 months, most of it can recover; after more than 12 months, lasting damage is common.
This is the highest-risk part of the B12 story: folate alone can hide the anemia, but it cannot stop the nerve damage. Anyone with unexplained numbness, worsening balance or cognitive decline should have B12 checked early.
Clinical · A B12 test is more than one number
Looking at serum B12 alone misses a sizable share of real deficiencies: the lab method, recent supplements and the liver's stores can all skew the result. A complete workup usually has three parts.
Serum total B12 (the routine test):
< 200 pg/mL: clear deficiency200-350 pg/mL: a gray zone, in which a sizable share of people are in fact already functionally deficient> 350 pg/mL: usually enough, though a functional deficiency can still be missed (MMA) is the most specific: when the MUT enzyme stalls, MMA builds up, and in people with normal kidney function almost nothing else disturbs it. A value > 0.4 µmol/L strongly suggests B12 deficiency. When B12 falls in the gray zone, this is the most useful next test.
(Hcy) is sensitive but not specific: a shortage of B12, folate or B6 raises it, and so do reduced kidney function, an underactive thyroid and other conditions. A value > 15 µmol/L suggests one of these B vitamins may be low, and MMA is needed to tell which.
Holotranscobalamin (holoTC, often called "active B12") reflects the part cells can actually use; < 35 pmol/L suggests functional deficiency. It is widely used in European clinics but not common in US labs.
Two common patterns need telling apart. Raised MMA and raised Hcy together point to B12 deficiency. Normal MMA with raised Hcy points mostly to folate (or B6) deficiency, not B12. That is why testing B12 alone is not enough: a gray-zone result can be missed, and in people who take folate, B12 deficiency can be hidden.
Groups for whom a yearly check is recommended: vegans and near-vegans; people over 65; long-term users of , H2 blockers or metformin; people who have had gastric bypass or stomach surgery; people with autoimmune pernicious anemia (positive for antibodies against intrinsic factor); and anyone with long-standing unexplained fatigue, numbness, poor balance or cognitive decline.
A B12 test usually costs less than buying supplements at random for years: find out first, then decide what to take and how.
National Institutes of Health, Office of Dietary Supplements. (2024). Vitamin B12 — Fact Sheet for Health Professionals. Fact sheet (updated July 2, 2025; Wayback snapshot 20 September 2026): multivitamin/mineral supplements typically contain 5 to 25 mcg B12, B-complex products 50 to 500 mcg, B12-only supplements typically 500 to 1,000 mcg; absorption is only about 2% at 500 mcg and 1.3% at 1,000 mcg; a 2018 Cochrane review of 3 RCTs (153 participants) compared very high oral doses (1,000-2,000 mcg) with intramuscular B12; high oral doses (e.g. 1,000 mcg/day) might be equally effective in Crohn's disease and appear as effective as hydroxocobalamin injections after Roux-en-Y bypass. These are product contents and trial doses; the sheet gives no recommended daily supplement range (fact sheet). ods.od.nih.gov/factsheets/VitaminB12-HealthProfessional
EFSA Panel on Dietetic Products, Nutrition and Allergies. (2015). Scientific opinion on dietary reference values for cobalamin (vitamin B12). EFSA Journal, 13(7), 4150. Dietary reference values only: Adequate Intake 4 micrograms/day for adults (4.5 in pregnancy, 5 in lactation); no Average Requirement could be set; observed mean adult intakes in several EU countries 4.2-8.6 micrograms/day. The opinion gives no supplement dose (abstract, via Crossref). 10.2903/j.efsa.2015.4150
Stabler, S. P. (2013). Clinical practice. Vitamin B12 deficiency. The New England Journal of Medicine, 368(2), 149–160. 10.1056/NEJMcp1113996
Carmel, R. (2008). How I treat cobalamin (vitamin B12) deficiency. Blood, 112(6), 2214–2221. 10.1182/blood-2008-03-040253