Vitamin B12 – Cobalamin

Vitamin B12 — also called cobalamin — is unlike any other vitamin in several ways. It’s the only vitamin that contains a metal: cobalt, held at the center of a ring-shaped structure, which is where the name cobalamin comes from. It’s the only water-soluble vitamin the body stores in large amounts — mostly in the liver, often enough to last several years — which is why deficiency takes a long time to develop, and also why it can be well advanced before anyone notices. And it has the most elaborate absorption system of any nutrient: a relay involving saliva, stomach acid, pancreatic enzymes, a special protein made by the stomach lining, and dedicated receptors at the end of the small intestine. When any step fails, vitamin B12 deficiency follows no matter how much is in the diet — and the nerve damage from undetected deficiency can be severe and only partly reversible.

Vitamin B12 – Key Mechanisms

Only two enzyme reactions in the human body are known to need vitamin B12. Both are essential, and both fail in deficiency.

Methionine synthase (the methylation and folate cycles): this enzyme converts homocysteine into methionine, using methylcobalamin as its cofactor and 5-MTHF (the active form of folate) as the methyl donor. This one reaction does three things at once: it regenerates methionine for making SAM (S-adenosylmethionine), the body’s main methyl donor; it frees folate from the 5-MTHF form so it can return to the DNA-making cycle; and it clears homocysteine from the blood. When B12 is deficient, homocysteine builds up and folate gets “trapped” as 5-MTHF — producing a megaloblastic anemia that looks identical to folate deficiency but is actually caused by lack of B12. This is the folate–B12 link described on the folate page.

Methylmalonyl-CoA mutase (mitochondrial energy metabolism): this enzyme converts methylmalonyl-CoA into succinyl-CoA, which enters the citric acid (TCA) cycle. Its cofactor is adenosylcobalamin — the mitochondrial form of vitamin B12. Methylmalonyl-CoA comes from breaking down odd-chain fatty acids and the amino acids valine, isoleucine, methionine, and threonine. When this enzyme fails, methylmalonic acid (MMA) builds up — and raised MMA in the blood or urine is one of the most sensitive markers of B12 deficiency, often rising before blood B12 levels fall below normal.

B12 and the nerves: nerve damage — loss of the myelin sheath around nerves and degeneration of parts of the spinal cord — is the defining feature of severe B12 deficiency. Exactly why it happens isn’t fully understood. The leading explanations are that reduced SAM production impairs the methylation reactions needed to maintain myelin, and that the methylmalonyl-CoA buildup leads to abnormal fatty acids being built into nerve membranes.

The Intrinsic Factor Cascade — The Most Complex Absorption Story in Nutrition

Absorbing vitamin B12 takes a multi-step relay that no other vitamin comes close to:

1. In the mouth and stomach: stomach acid and the enzyme pepsin release B12 from food proteins, and the freed B12 binds to haptocorrin (also called R-protein), a protein from saliva. This release depends on stomach acid — which is why acid-reducing medicines (PPIs and H2 blockers) reduce B12 absorption from food.

2. In the duodenum: pancreatic enzymes break down haptocorrin, releasing B12, which then binds to intrinsic factor (IF) — a protein made by the parietal cells of the stomach lining. The IF–B12 complex is very stable and protects B12 on its way to the end of the small intestine.

3. In the terminal ileum: the IF–B12 complex binds to dedicated receptors (cubilin) on the cells lining the final part of the small intestine, which take it in. B12 is then passed to transcobalamin, a transport protein, and carried in the blood to the tissues.

4. In the liver: excess vitamin B12 is stored — typically around 1–5mg in total, enough to last several years. No other water-soluble vitamin is stored like this.

Pernicious anemia is the most important condition caused by a failure of this relay. It’s an autoimmune disease in which the immune system attacks the stomach’s parietal cells and intrinsic factor, so little or no intrinsic factor is available. Without it, only a backup route remains: about 1% of any oral B12 dose is absorbed by passive diffusion, without intrinsic factor. At normal dietary amounts that’s negligible, but at high supplement doses (1,000µg a day), passive diffusion absorbs about 10µg — enough to maintain B12 status. That’s why high-dose oral B12 can replace injections for many people with pernicious anemia. Sublingual tablets (dissolved under the tongue) work about as well as swallowed tablets — mostly through the same passive absorption in the gut — but not better.

Vitamin B12 – RDA (Recommended Dietary Allowance)

* Adequate Intake (AI) for infants. Source: US National Academies, via the NIH Office of Dietary Supplements. The daily value used on food labels is 2.4µg. Because so many older adults absorb B12 poorly from food, the National Academies advise people over 50 to get most of their B12 from fortified foods or supplements. In Europe, EFSA sets a higher adequate intake of 4µg a day for adults.

Vitamin B12 – ODA (Optimal Daily Allowance)

Get at least the RDA — 2.4µg a day for adults, or the 4µg Europe recommends — from animal foods, fortified foods, or supplements. For people who absorb B12 normally, there’s no evidence that more improves energy or health. The people who genuinely need more are those whose absorption or intake is limited — vegans, adults over 50, and people taking metformin or acid-reducing medicines, or with pernicious anemia — and for them, supplement doses are usually much higher than the RDA because only a small fraction is absorbed (see “How to Take” below).

Vitamin B12 Is Found In (per 100g)

Note: USDA’s figure for cooked clams is unusually high — raw clams are listed at 11.3µg per 100g — but clams are an outstanding source either way.

Vitamin B12 is found almost only in animal foods — meat, poultry, fish, shellfish, eggs, and dairy — with shellfish and organ meats far ahead of everything else. Oily fish such as mackerel, anchovies, and sardines are excellent everyday sources. Plant foods aren’t reliable sources: the B12 in some algae (such as nori) and in tempeh varies widely in amount and may not be the active form. Fortified foods — plant-based milks, some breakfast cereals, and fortified nutritional yeast — are the main reliable source for people who avoid animal products.

Vitamin B12 Helps With

• Red blood cell production, and preventing megaloblastic anemia
• Maintaining the myelin sheath around nerves
• Regenerating methionine and making SAM for the methylation cycle
• Clearing homocysteine, alongside B6 and folate
• Making DNA, by keeping folate in circulation (B12 frees “trapped” folate)
• Mitochondrial energy production, through methylmalonyl-CoA mutase and the TCA cycle
• Brain function and memory — deficiency can cause cognitive problems
• Mood — deficiency is linked with depression, partly through methylation reactions that affect neurotransmitter production
• Normal development in babies and children — the developing nervous system depends heavily on B12

Vitamin B12 for Athletes and Active People

Vitamin B12 matters most for performance through red blood cell production and nerve health — and the athletes most at risk are those who eat little or no animal food.

Oxygen-carrying capacity: the anemia caused by B12 deficiency reduces red blood cells and haemoglobin, cutting oxygen delivery to working muscles and limiting endurance. In athletes, it can show up as unexplained fatigue and declining performance — particularly in vegan and vegetarian athletes, whose B12 status may be borderline.

Nerve function: fast, reliable signals from nerves to muscles depend on healthy myelin, which needs adequate B12. Clear deficiency causes nerve damage; whether milder, borderline B12 status affects athletic performance hasn’t been well studied.

Energy metabolism: the B12-dependent enzyme methylmalonyl-CoA mutase feeds succinyl-CoA into the TCA cycle, as part of breaking down odd-chain fatty acids and several amino acids.

Vegan and vegetarian athletes: this is the group for whom B12 genuinely matters. Deficiency develops slowly — sometimes over years — and its early stages (falling blood B12, rising homocysteine and MMA) can come before any obvious symptoms. Reliable supplementation or fortified foods, plus periodic blood tests, are essential, not optional.

A note on B12 injections for energy: high-dose B12 injections have a reputation in some athletic circles as an energy booster. They help only when there’s an actual deficiency. In people with adequate B12, extra B12 doesn’t increase energy — the enzymes that need it are already fully supplied.

Vitamin B12 Deficiency

Deficiency develops slowly because of the liver’s large stores. After intake or absorption stops, symptoms often take several years to appear — so by the time nerve symptoms show up, depletion has been going on for a long time, and damage may already be significant.

Deficiency risk is higher in:

• Vegans and strict vegetarians: without supplements or reliable fortified foods, deficiency is very likely over time. B12 from algae and fermented foods isn’t reliable.
• Older adults: many have atrophic gastritis — thinning of the stomach lining that reduces stomach acid (needed to release B12 from food) and sometimes intrinsic factor. An estimated 10–30% of older adults absorb B12 from food poorly.
• People with pernicious anemia: the autoimmune loss of intrinsic factor leaves only passive absorption (about 1% of a dose), so high-dose oral supplements or injections are needed.
• People taking metformin: long-term use reduces B12 absorption, likely by interfering with the calcium-dependent uptake in the ileum, and a significant minority of long-term users develop low B12. Diabetes guidelines recommend checking B12 periodically in people taking metformin, especially if they develop nerve symptoms.
• People taking acid-reducing medicines (PPIs and H2 blockers): these reduce the stomach acid needed to release B12 from food. Use for two years or more has been linked with a higher risk of deficiency.
• People who’ve had weight-loss surgery: particularly gastric bypass, which routes food past most of the stomach and its intrinsic factor-producing cells — lifelong B12 supplementation is standard afterwards.
• People with Crohn’s disease affecting the terminal ileum, or who’ve had it removed: the main absorption site.
• People with heavy, long-term alcohol use: alcohol can impair B12 absorption.
• People with HIV.
• People exposed to nitrous oxide (N₂O), including recreational use: nitrous oxide inactivates vitamin B12 by oxidizing its cobalt, disabling methionine synthase. In someone with borderline B12, even a single anaesthetic exposure can trigger nerve symptoms, and heavy recreational use (“laughing gas” or “whippets”) has caused a rise in cases of B12-related spinal cord damage in young people — one reason the UK made possession of nitrous oxide for recreational use illegal in 2023.

Subacute combined degeneration of the spinal cord (SCD) — the neurological syndrome:

Severe vitamin B12 deficiency can cause SCD — loss of myelin in several spinal cord pathways and in the peripheral nerves:

• Dorsal columns: loss of vibration sense and position sense, causing an unsteady walk, a positive Romberg’s sign, and difficulty walking in the dark
• Corticospinal (lateral) tracts: stiff weakness in the legs, brisk reflexes, and a positive Babinski sign
• Peripheral nerves: symmetrical tingling, numbness, and burning in the hands and feet
• Advanced cases: confusion, memory loss, depression, irritability, and psychosis — sometimes called “megaloblastic madness”

The combination of lost vibration sense alongside weakness and brisk reflexes is characteristic, and a tuning fork test is a useful bedside screen for the dorsal column involvement.

SCD is only partly reversible — early treatment stops it progressing, but established damage may not fully recover. That’s why B12 deficiency needs to be caught early, and why the masking effect of high-dose folic acid is so dangerous.

A note on blood testing: the standard serum B12 test has real limitations. Normal ranges typically start around 200pg/mL, but some people with results in the low-normal range (about 200–400pg/mL) have functional deficiency. More informative tests include:

• Methylmalonic acid (MMA): the most sensitive marker, often rising before serum B12 falls below normal — though it can also be raised by kidney disease
• Holotranscobalamin (“active B12”): measures the B12 that’s actually available to cells, rather than total B12
• Homocysteine: raised in B12 deficiency, though also raised by low folate or B6

General symptoms of vitamin B12 deficiency:

• Fatigue and weakness
• Megaloblastic anemia — large, immature red blood cells, causing tiredness, paleness, and breathlessness
• Tingling and numbness in the hands and feet
• Difficulty walking and loss of balance
• A sore, smooth, glossy tongue (glossitis)
• Mouth ulcers
• Memory problems, brain fog, mood changes, and depression
• In severe cases: confusion, psychosis, and spinal cord damage

Excessive Doses of Vitamin B12

No upper limit (UL) has been set for vitamin B12, because no toxicity has been found from oral intake, even at high doses. Daily doses of 1,000µg or more, used to treat pernicious anemia, are generally well tolerated.

Rare side effects include acne-like skin eruptions (mainly reported with high-dose injections) and mild digestive upset. A few other findings are worth knowing:

• High blood B12 and cancer: unusually high blood B12 levels are linked with some cancers in population studies, but this is generally thought to be because the disease raises B12 levels, not because supplements cause cancer.
• Long-term high-dose supplements in male smokers: in one large observational study, men — especially smokers — who took high-dose B12 supplements (more than 55µg a day from single-vitamin products) for about 10 years had a higher risk of lung cancer. This doesn’t prove cause and effect, but it’s a reason not to take high doses long-term without a reason to.
• Kidney disease: in a trial in people with diabetic kidney disease, high-dose B vitamins (including 1mg of cyanocobalamin) were linked with faster decline in kidney function, so people with significant kidney disease should take high-dose B vitamins only on medical advice.

Different Types of Vitamin B12

• Cyanocobalamin: the most stable and least expensive supplement form. It’s synthetic and contains a tiny, harmless amount of cyanide, which the body removes as it converts cyanocobalamin into the active forms. Very well studied, and the form used in most supplements and fortified foods.
• Methylcobalamin: one of the two active forms, used by methionine synthase, and found naturally in food. Often marketed as superior, but there’s no good evidence that it works better than cyanocobalamin for most people — and it’s less stable. It’s widely used for nerve conditions in some countries, though the evidence is limited.
• Adenosylcobalamin: the other active form, used in the mitochondria. Less commonly sold on its own; often combined with methylcobalamin in “complete B12” products. Any B12 form can supply both active forms, since the body interconverts them.
• Hydroxocobalamin: a natural form that stays in the body longer than cyanocobalamin after injection. It’s the standard B12 injection in the UK and much of Europe, while cyanocobalamin injections are common in the US. It’s also the antidote for cyanide poisoning — it binds cyanide in the blood.

Which form for your goal:

How to Take Vitamin B12

The intrinsic factor route can only handle about 1.5–2µg at a time, so small doses spread across the day are absorbed more efficiently than a single dose. For moderate doses (10–25µg a day), splitting into two doses helps; at high doses, where passive absorption does most of the work, splitting matters less.

For vegans: common recommendations include 25–100µg daily or 1,000µg two to three times a week. The UK’s Vegan Society recommends at least 10µg a day, or 2,000µg once a week, or fortified foods providing at least 3µg a day spread across two or three meals. Cyanocobalamin and methylcobalamin both work.

For pernicious anemia or low intrinsic factor, without injections: 1,000–2,000µg a day of oral cyanocobalamin or methylcobalamin, under medical supervision. At these doses, passive absorption provides enough B12 for maintenance. Sublingual tablets are an option but don’t work better than swallowed ones.

For confirmed pernicious anemia with injections: in the UK, treatment usually starts with a series of hydroxocobalamin injections, followed by 1mg every 2–3 months for life. In the US, cyanocobalamin injections, typically monthly, are common.

For older adults and long-term PPI or metformin users: the crystalline B12 in supplements and fortified foods doesn’t need stomach acid to be released from food protein, so it’s absorbed much better than B12 from food when acid is low. Doses of 25–100µg a day are commonly used, and higher doses if absorption is significantly impaired.

Always make sure vitamin B12 is adequate when taking folate supplements — the two work together, and neither works fully without the other.

Vitamin B12 – Interactions

Metformin reduces B12 absorption — periodic B12 checks are recommended for long-term users. PPIs and H2 blockers reduce the stomach acid needed to release B12 from food. Nitrous oxide inactivates vitamin B12 — a serious risk for people with borderline B12 status. Colchicine and some antibiotics may reduce absorption. High-dose folic acid can hide B12 deficiency by correcting the anemia while nerve damage continues.

Vitamin B12 – Synergy

• Works with folate (B9) in the methylation cycle — B12 frees folate from the 5-MTHF “trap,” and folate supplies the methyl group that B12 passes on.
• Works with B6 in clearing homocysteine — all three B vitamins need to be adequate.
• Supports red blood cell production alongside iron — B12 deficiency and iron deficiency can occur together, and both need correcting for full recovery.
• Works with biotin (B7) in the same pathway — biotin’s enzyme produces methylmalonyl-CoA, which B12’s enzyme then converts.
• Like B12, magnesium is needed for normal nerve function and energy metabolism, and low levels of either can cause overlapping symptoms such as fatigue and tingling — though they don’t directly interact.
• Linked with choline: SAM from the B12-supported methylation cycle is used in one of the body’s routes for making phosphatidylcholine.

Vitamin B12 – Antagonistic Interactions

Colchicine and some antibiotics can reduce B12 absorption.

Nitrous oxide inactivates vitamin B12 — a serious risk for people with borderline status.

Metformin, PPIs, and H2 blockers reduce B12 absorption.

Lack of intrinsic factor (pernicious anemia, stomach surgery) removes the main absorption route.

High-dose folic acid can hide B12 deficiency by correcting the anemia but not the nerve damage.

Heavy alcohol use can impair B12 absorption.

Food values: USDA FoodData Central — clams, cooked (SR Legacy, FDC ID 171975); lamb kidneys, braised (SR Legacy, 174355); lamb liver, braised (SR Legacy, 172532); Pacific oysters, cooked (SR Legacy, 174250); blue mussels, cooked (SR Legacy, 174217); Atlantic mackerel, cooked (SR Legacy, 175120); anchovies, canned in olive oil (Foundation Foods, 2747652); sardines, canned in oil (SR Legacy, 175139); Atlantic salmon, farmed, cooked (SR Legacy, 175168); ground beef, cooked (SR Legacy, 171797); light tuna, canned in water (Foundation Foods, 334194); egg, hard-boiled (SR Legacy, 173424); cheddar cheese (Foundation Foods, 328637); plain nonfat Greek yogurt (Foundation Foods, 330137); chicken breast, braised (Foundation Foods, 331960). RDA values from the US National Academies, via the NIH Office of Dietary Supplements. % Daily Values use the FDA’s current Daily Value of 2.4µg.