Place · Level 3
Vitamin B12
只有细菌会造 · 需要胃里出门票 · 让神经有外套，让 DNA 能复印
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Chapter 1
Food
Food
B12 (cobalamin) is the largest and most complex of all the vitamins — at its core is a cobalt (Co) ion wrapped by a corrin ring, structurally homologous to heme (which holds Fe) and chlorophyll (which holds Mg).
One key fact: only prokaryotes (bacteria and archaea) have the enzymes to synthesize B12. Plants, fungi, and animals cannot make it — animals accumulate B12 by eating microbe-laden forage or via gut microbial symbionts, and then concentrate it in liver, muscle, eggs, and milk. So dietary B12 comes almost entirely from animal sources:
Clams: ~99 µg / 100 gBeef liver: ~70 µg / 100 g (off the charts)Salmon, tuna: ~4-7 µg / 100 gEggs: ~0.6 µg per eggMilk: ~1.3 µg per cup
The RDA is only 2.4 µg/day — a small number. But plant sources are almost all unreliable:
Nori / seaweed contain pseudo-B12 analogs — structurally similar but with zero biological activity, and they actually interfere with real B12 absorptionUnfermented legumes and nuts: noneFortified foods (fortified breakfast cereal, plant milks) contain real B12 — for vegans, this is the only reliable plant-based dietary source of B12
So strict or near-vegan eaters typically need fortified foods plus a supplement.
One key fact: only prokaryotes (bacteria and archaea) have the enzymes to synthesize B12. Plants, fungi, and animals cannot make it — animals accumulate B12 by eating microbe-laden forage or via gut microbial symbionts, and then concentrate it in liver, muscle, eggs, and milk. So dietary B12 comes almost entirely from animal sources:
Clams: ~99 µg / 100 gBeef liver: ~70 µg / 100 g (off the charts)Salmon, tuna: ~4-7 µg / 100 gEggs: ~0.6 µg per eggMilk: ~1.3 µg per cup
The RDA is only 2.4 µg/day — a small number. But plant sources are almost all unreliable:
Nori / seaweed contain pseudo-B12 analogs — structurally similar but with zero biological activity, and they actually interfere with real B12 absorptionUnfermented legumes and nuts: noneFortified foods (fortified breakfast cereal, plant milks) contain real B12 — for vegans, this is the only reliable plant-based dietary source of B12
So strict or near-vegan eaters typically need fortified foods plus a supplement.
The 'plant B12' truth
"This plant contains B12, so vegans don't have to worry" is a common claim online. Let's check the evidence one item at a time:Nori / seaweed: contains cobalamin analogs — structurally similar but biologically inactive. Worse, these pseudo-B12 molecules occupy IF receptors and interfere with real B12 absorption (*Watanabe 2014* and others have confirmed this).Spirulina / chlorella: 99% pseudo-B12 — the "high B12" claim on the label is misleading, and EFSA does not recognize them as B12 sources.Fermented soy (miso, natto, tempeh): contain real B12, but in trace amounts (< 0.1 µg/serving) — far below the 2.4 µg RDA, so cannot be a primary source.Mushrooms (especially shiitake): contain a small amount of real B12 — about 5 µg per 100 g of dried mushrooms, but a realistic meal portion contributes < 0.5 µg.Bacterial contamination on unwashed organic vegetables: exists in theory, unreliable in practice — modern hygiene standards mostly wash it away.
The only reliable non-animal sources of B12 are:
1. Fortified foods: fortified breakfast cereal (~6 µg/serving), fortified plant milks (~1-3 µg/cup), fortified nutritional yeast (~8 µg/tbsp)
2. B12 supplements: cyanocobalamin is the most common; methylcobalamin also works
A practical plan for strict vegans: every day, take at least 6 µg from fortified foods plus an oral supplement of 2.4-10 µg/day — or, alternatively, take a single high-dose 2000 µg tablet once a week as a storage strategy. Test serum B12, MMA, and homocysteine once a year.
This is not a "limitation of veganism" — once you understand the mechanism, you know exactly how to fill the gap.
Chapter 2
Stomach · Intrinsic factor
Stomach · Intrinsic factor
B12 absorption is one of the most complex pathways in all of nutrition — a three-stage handshake.
Stage 1, the stomach:
Gastric acid (HCl) and pepsin strip B12 off dietary proteinParietal cells in the stomach wall secrete a carrier called R-binder (haptocorrin), which binds B12 first to protect it from acid damageThe same parietal cells also secrete intrinsic factor (IF) — but IF does not bind B12 yet in the stomach
Stage 2, the duodenum: pancreatic protease cleaves the R-binder → B12 is released; in the mildly alkaline environment, B12 binds IF to form the IF-B12 complex.
Stage 3: IF-B12 travels along the small intestine to the ileum (covered below).
The step most often overlooked in this pathway is inadequate stomach acid — once this stage fails, the entire handshake collapses:
Atrophic gastritis in the elderly has a prevalence of about 15-20% over age 60, with naturally declining acid outputLong-term PPI / H2 blockers (omeprazole, etc.) directly suppress acidLong-term metformin interferes with B12 absorption (mechanism not fully understood)Gastric bypass / gastrectomy directly removes IF-secreting cells, usually requiring lifelong B12 injections
Stage 1, the stomach:
Gastric acid (HCl) and pepsin strip B12 off dietary proteinParietal cells in the stomach wall secrete a carrier called R-binder (haptocorrin), which binds B12 first to protect it from acid damageThe same parietal cells also secrete intrinsic factor (IF) — but IF does not bind B12 yet in the stomach
Stage 2, the duodenum: pancreatic protease cleaves the R-binder → B12 is released; in the mildly alkaline environment, B12 binds IF to form the IF-B12 complex.
Stage 3: IF-B12 travels along the small intestine to the ileum (covered below).
The step most often overlooked in this pathway is inadequate stomach acid — once this stage fails, the entire handshake collapses:
Atrophic gastritis in the elderly has a prevalence of about 15-20% over age 60, with naturally declining acid outputLong-term PPI / H2 blockers (omeprazole, etc.) directly suppress acidLong-term metformin interferes with B12 absorption (mechanism not fully understood)Gastric bypass / gastrectomy directly removes IF-secreting cells, usually requiring lifelong B12 injections
Metformin & B12
Metformin is first-line therapy for type 2 diabetes and one of the most widely used prescription drugs in the world. Long-term use (over 4 years) is clearly associated with B12 deficiency.Epidemiology: long-term follow-up from DPPOS (*NEJM 2017*) showed that people on metformin for ≥ 5 years had a B12 deficiency rate of about 9.5%, vs 3.6% in the placebo arm. Higher daily doses (> 1500 mg) increased risk linearly. After an average of 5 years, B12 levels were about 20% below baseline.
The mechanism is not fully settled. The leading hypothesis is interference with calcium-dependent uptake of the B12-IF complex by the cubilin receptor in the ileum. Some studies suggest indirect effects through changes in the gut microbiome. Direct oxidative damage to B12 is weakly supported.
A clinically important pitfall: "diabetic peripheral neuropathy (DPN)" is often attributed to diabetes itself, but a fraction of cases are actually B12-deficiency neuropathy, which presents almost identically (foot numbness, tingling, poor proprioception). Misdiagnosis leads to glucose control without B12 repletion, and the neurological damage continues; meanwhile B12 deficiency raises homocysteine, worsening cardiovascular risk.
Guidelines: ADA + EASD (2019 update) recommend monitoring B12 every 1-2 years in long-term metformin users (serum B12 + MMA + homocysteine); AACE makes the same recommendation. Real-world execution is low (< 30% in surveys).
Practical guidance: if you are on long-term metformin, ask your physician to test B12 (ideally with MMA) at least annually; any unexplained numbness, balance issues, fatigue, or memory decline warrants early testing. Prophylactic B12 is not required, but deficiency must be treated promptly. Standard oral 1000 µg/day cyanocobalamin is usually sufficient (even with poor IF function, passive diffusion at ~1% gives ~10 µg, still 4× the RDA); severe deficiency typically warrants IM 1000 µg weekly × 8 weeks followed by monthly maintenance.
Chapter 3
Ileum · the lock
Ileum · the lock
The apical surface of the epithelial cells in the distal small intestine (the ileum) expresses a receptor called cubilin, which recognizes only the IF-B12 complex — not free B12. This is one of the most specific lock-and-key pairings in all of nutrient absorption.
The sequence: cubilin pulls IF-B12 into the epithelial cell → inside the cell, B12 dissociates from IF → it swaps onto transcobalamin II (TCII) → TCII-B12 enters portal blood → systemic circulation.
This pathway maxes out at about 1.5-2 µg per day. Even if you take a 1000 µg oral B12 tablet, less than 2% actually goes through the IF channel. The good news is that at high concentrations there is a second, passive diffusion pathway (about 1% absorption), so a large oral dose (1000 µg) still delivers about 10 µg — 4× the RDA. This is why:
A 1000 µg oral B12 tablet often works even for people with impaired IF channel function (the elderly, PPI users, mild atrophic gastritis)Complete gastrectomy or severe pernicious anemia usually still requires injections (bypassing the entire absorption pathway)
The sequence: cubilin pulls IF-B12 into the epithelial cell → inside the cell, B12 dissociates from IF → it swaps onto transcobalamin II (TCII) → TCII-B12 enters portal blood → systemic circulation.
This pathway maxes out at about 1.5-2 µg per day. Even if you take a 1000 µg oral B12 tablet, less than 2% actually goes through the IF channel. The good news is that at high concentrations there is a second, passive diffusion pathway (about 1% absorption), so a large oral dose (1000 µg) still delivers about 10 µg — 4× the RDA. This is why:
A 1000 µg oral B12 tablet often works even for people with impaired IF channel function (the elderly, PPI users, mild atrophic gastritis)Complete gastrectomy or severe pernicious anemia usually still requires injections (bypassing the entire absorption pathway)
Oral vs sublingual vs IM
B12 supplementation comes in three forms, with clearly tiered evidence.Regular oral tablets: completely sufficient for healthy people with an intact IF pathway. Dose 250-1000 µg/day (cyanocobalamin or methylcobalamin). Even when the active channel saturates (~1.5-2 µg), passive diffusion at about 1% adds another 10-25 µg. Cheapest option, with the highest long-term adherence.
Sublingual lozenges: in theory, absorbed through oral mucosa, bypassing the IF channel. But Sharabi 2003 RCT (n=30) compared oral vs sublingual 1000 µg/day for 4 weeks and found blood B12 rose identically; *Castelli 2011* and *Bensky 2019* reached similar conclusions. There is no evidence that sublingual is meaningfully better than ordinary oral — the truth is that oral mucosal absorption of B12 is minimal, and the "effect" of sublingual dosing mostly comes from swallowing and then absorbing it orally. Unless swallowing is impaired, it is not worth the extra cost.
Intramuscular (IM) injection has well-defined indications: severe pernicious anemia (autoimmune destruction of IF-secreting cells), total gastrectomy, severe atrophic gastritis with multiple GI conditions, intolerance of oral intake (persistent vomiting / severe malabsorption), or acute neurological symptoms requiring rapid repletion. The classic regimen: 1000 µg IM weekly × 8 weeks for acute correction, then 1000 µg IM monthly for maintenance. High-dose oral 1000-2000 µg/day is an alternative: multiple RCTs (*Kuzminski 1998*, *Bolaman 2003*) show it is equally effective for pernicious anemia, provided the patient takes it daily without fail.
As for "energy shots / B12 injections for vitality" — the influencer-clinic B12 booster shot ($30-100) gives no energy boost in people without documented deficiency. This conflates "reversing the symptoms of a real deficiency" with "polishing things up when you are already sufficient". Placebo effect plus the clinic experience is the actual product.
To summarize: the vast majority of deficiencies are managed with 1000 µg/day orally plus identifying the underlying cause; only a small set of strict indications calls for IM; sublingual is marketing and unnecessary; and supplementing when you are not deficient does not raise your energy.
Chapter 4
Liver · long reserve
Liver · long reserve
The liver stores about 2-5 mg of B12 — roughly 3-5 years of daily requirement (RDA 2.4 µg/day). This is the largest reserve among the water-soluble vitamins; most of the others are barely stored at all.
Mechanistically, there is also an enterohepatic recirculation loop: the liver secretes about 1.4 µg of B12 per day into bile → bile returns to the small intestine → most of it is re-absorbed through the IF channel. So even if you completely stop B12 intake, the reservoir leaks slowly rather than emptying quickly.
The implications:
Newly vegan eaters will not immediately develop a B12 deficiency — most people can coast for 2-3 yearsBut waiting until the reservoir runs dry is too late: neurological damage is gradual, and by the time symptoms appear they are often irreversibleThe safer move is to set the habit at the start of a vegan diet, not to wait until problems appear
Clinically, strict vegans should test serum homocysteine and methylmalonic acid (MMA) annually — these are more sensitive than measuring B12 directly and can catch subclinical deficiency before clinical symptoms arise.
Mechanistically, there is also an enterohepatic recirculation loop: the liver secretes about 1.4 µg of B12 per day into bile → bile returns to the small intestine → most of it is re-absorbed through the IF channel. So even if you completely stop B12 intake, the reservoir leaks slowly rather than emptying quickly.
The implications:
Newly vegan eaters will not immediately develop a B12 deficiency — most people can coast for 2-3 yearsBut waiting until the reservoir runs dry is too late: neurological damage is gradual, and by the time symptoms appear they are often irreversibleThe safer move is to set the habit at the start of a vegan diet, not to wait until problems appear
Clinically, strict vegans should test serum homocysteine and methylmalonic acid (MMA) annually — these are more sensitive than measuring B12 directly and can catch subclinical deficiency before clinical symptoms arise.
Vegan timing myth
The "3-5 year reserve" number is widely misunderstood, and new vegans often use it to delay starting B12. Here is why you should not rely on the estimate.The 3-5 year reserve is calculated for someone who has eaten animal foods for decades and has a healthy liver. Several situations shorten the reserve meaningfully:
First, high reliance on enterohepatic recirculation. The liver secretes about 1.4 µg of B12 into bile per day, most of which is reabsorbed in the ileum — and that recycling depends on a working IF pathway. If you also have atrophic gastritis, are on long-term PPI, or are on metformin, the recycling is leaking faster even if the reservoir looks full.
Second, the starting reserve may already be much lower than you assume. Most people are far below the theoretical ceiling — actual stores may be only 1-2 mg, not the textbook 5 mg. People with chronically low BMI, monotonous diets, or chronic disease before going vegan may have < 1 mg.
Third, early subclinical deficiency is not the same as clinical symptoms. Serum B12 can fall within 6-12 months of going vegan; MMA and homocysteine rise at 12-24 months; numbness and anemia (the textbook clinical symptoms) are what the 3-5 year figure refers to. But the subclinical period is already accumulating neurological damage — by the time symptoms appear, some of it may be permanent.
So in practice, strict vegans should not wait for the reservoir to empty — establish a B12 habit from the very first day of a vegan diet:
6+ µg/day from fortified foods (fortified breakfast cereal, plant milk, nutritional yeast), or25-100 µg/day oral supplement (cyanocobalamin), or2000 µg weekly tablet (a one-shot storage strategy, fine for forgetful people)Test serum B12 + MMA + homocysteine in the first year and annually thereafter
One situation that deserves special attention: strictly vegan breastfeeding mothers can have infants who develop severe B12 deficiency within 6 months, with permanent neurological damage. The mother must supplement, and the infant should receive 0.4-5 µg/day from birth.
The "I get enough from nutritional yeast / nori" claim is addressed in the "Plant B12 truth" section on the food page — it is not reliable.
Chapter 5
Methylation cycle
Methylation cycle
B12 is a cofactor for two enzymes. The first is methionine synthase (MTR), in the cytoplasm, which catalyzes:
Homocysteine (Hcy) + 5-methyl-THF → Methionine (Met) + THF (with B12 as cofactor and folate as the methyl donor)
Methionine then becomes S-adenosylmethionine: The body's main methyl-group donor — it tags DNA, neurotransmitters, and more with methyl groups. (S-adenosylmethionine) — the body's universal methyl donor, used in:
DNA and histone methylation (gene expression regulation)Phospholipid synthesis (phosphatidylcholine)Neurotransmitter synthesis (dopamine, serotonin, and epinephrine are derived from methionine)Myelin phospholipid synthesisCreatine synthesis
Low B12 → MTR stalls → SAM drops → every pathway above runs at a discount. At the same time, homocysteine accumulates in blood, producing hyperhomocysteinemia — a risk marker linked with cardiovascular disease, stroke, and dementia.
Folate is B12's partner here, but folate on its own can keep supplying nucleotide synthesis (masking anemia) while neurological damage continues to progress — this is the biochemical root of "folate masking B12 deficiency". This is why clinicians measure B12 first, then folate, never the other way around.
Homocysteine (Hcy) + 5-methyl-THF → Methionine (Met) + THF (with B12 as cofactor and folate as the methyl donor)
Methionine then becomes S-adenosylmethionine: The body's main methyl-group donor — it tags DNA, neurotransmitters, and more with methyl groups. (S-adenosylmethionine) — the body's universal methyl donor, used in:
DNA and histone methylation (gene expression regulation)Phospholipid synthesis (phosphatidylcholine)Neurotransmitter synthesis (dopamine, serotonin, and epinephrine are derived from methionine)Myelin phospholipid synthesisCreatine synthesis
Low B12 → MTR stalls → SAM drops → every pathway above runs at a discount. At the same time, homocysteine accumulates in blood, producing hyperhomocysteinemia — a risk marker linked with cardiovascular disease, stroke, and dementia.
Folate is B12's partner here, but folate on its own can keep supplying nucleotide synthesis (masking anemia) while neurological damage continues to progress — this is the biochemical root of "folate masking B12 deficiency". This is why clinicians measure B12 first, then folate, never the other way around.
Methylation testing hype
"MTHFR positive — you must use methylcobalamin, not cyanocobalamin" has been one of the biggest marketing narratives in direct sales and functional medicine circles for the past decade. Let's check the evidence point by point.MTHFR C677T: the T allele is common globally, with TT homozygosity in 30-50% of Han Chinese. TT homozygotes have 30-70% lower MTHFR enzyme activity; the main consequence is reduced 5-MTHF production, leading to slightly elevated homocysteine and somewhat higher folate requirements in some people.
The major B12 supplement forms:
Cyanocobalamin: the most common industrial form, stabilized with a -CN group; everyone removes the -CN in the body and converts it to the active formsMethylcobalamin: already in the methylated form, ready to enter the methionine synthase pathway directlyAdenosylcobalamin: enters the MUT pathway directlyHydroxocobalamin: the preferred injectable form, with a long half-life
*Thakkar 2015* (a review of 10+ RCTs) showed that, in both healthy and deficient populations, the various forms raise plasma B12 and lower MMA and Hcy almost identically — and there is no significant difference between cyanocobalamin and methylcobalamin. The exceptions are very rare inherited disorders such as CblC disease, which require combined hydroxocobalamin plus methylcobalamin.
So "MTHFR positive means you must use methylcobalamin" is a marketing claim: MTHFR affects the folate cycle, not B12 conversion. Even TT homozygotes still uncap cyanocobalamin normally; "skipping the conversion step" has no meaningful benefit for 99% of people.
When might methylcobalamin be a reasonable choice? Severe liver impairment may slightly reduce conversion efficiency. Heavy smokers have somewhat higher cyanide burden, so methylcobalamin theoretically reduces CN exposure — but the absolute amounts are negligible; a single cigarette delivers far more cyanide than a cyanocobalamin tablet.
In practice: the vast majority of people can simply use a 25-1000 µg/day cyanocobalamin tablet — cheap, stable, effective. Methylation testing (~¥1000) is fine to do for curiosity, but it cannot be read as "you must use methylcobalamin or methylfolate". What really determines your B12 status is intake, absorption, and medications — not the chemical form.
Chapter 6
Nerve · myelin
Nerve · myelin
B12's second enzyme is methylmalonyl-CoA mutase (MUT), located in the mitochondria. It converts the metabolic intermediates of odd-chain fatty acids and certain amino acids — methylmalonyl-CoA → succinyl-CoA — so they can enter the tricarboxylic acid (Krebs) cycle: The mitochondrial hub cycle that fully oxidizes fuel and harvests electrons for energy. cycle for energy production.
Low B12 → MUT stalls → methylmalonic acid (MMA) accumulates in blood — this is the most sensitive biochemical marker of B12 deficiency. At the same time, odd-chain fatty acids are incorrectly built into myelin phospholipids, the structure of myelin (the insulating sheath around nerve fibers — the "insulation" on the wire) becomes abnormal, and demyelination sets in.
The clinical correlate is subacute combined degeneration: symmetric demyelination of the lateral and posterior columns of the spinal cord, presenting as foot numbness → unsteady gait → inability to stand with eyes closed (loss of proprioception) → dementia. The time window is exquisitely sensitive: reversible if caught within 6 months; often permanent deficits if it has been > 12 months.
This is the highest-risk part of the B12 story — folate alone can mask the anemia, but it cannot mask the neurological damage. Anyone with unexplained numbness, balance problems, or cognitive decline should have B12 measured early.
Low B12 → MUT stalls → methylmalonic acid (MMA) accumulates in blood — this is the most sensitive biochemical marker of B12 deficiency. At the same time, odd-chain fatty acids are incorrectly built into myelin phospholipids, the structure of myelin (the insulating sheath around nerve fibers — the "insulation" on the wire) becomes abnormal, and demyelination sets in.
The clinical correlate is subacute combined degeneration: symmetric demyelination of the lateral and posterior columns of the spinal cord, presenting as foot numbness → unsteady gait → inability to stand with eyes closed (loss of proprioception) → dementia. The time window is exquisitely sensitive: reversible if caught within 6 months; often permanent deficits if it has been > 12 months.
This is the highest-risk part of the B12 story — folate alone can mask the anemia, but it cannot mask the neurological damage. Anyone with unexplained numbness, balance problems, or cognitive decline should have B12 measured early.
B12 testing: more than one number
Looking at serum B12 alone misses about 30% of true deficiencies — the assay method, recent supplementation, and hepatic stores all distort the result. A complete panel usually has three components.Serum total B12 (standard):
< 200 pg/mL: definite deficiency200-350 pg/mL: gray zone — about 30-50% of people in this range are already functionally deficient> 350 pg/mL: usually sufficient (but false reassurance is possible)
Methylmalonic acid (MMA) is the most specific marker: MUT stalls → MMA accumulates, and in people with normal kidney function it is essentially undisturbed by other factors. > 0.4 µmol/L strongly suggests B12 deficiency. When B12 is in the gray zone, this is the most useful next test.
Homocysteine (Hcy) is sensitive but not specific: deficiencies of B12, folate, or B6 will all raise it. > 15 µmol/L suggests deficiency of one of these B vitamins — you need MMA to disambiguate.
HoloTC (holotranscobalamin, "active B12") reflects the fraction actually available to cells: < 35 pmol/L suggests functional deficiency. Common in European clinical practice; less common in US labs.
Two common patterns to distinguish:
MMA ↑ + Hcy ↑: B12 deficiencyMMA normal + Hcy ↑: folate (or B6) deficiency, not B12
This is why testing B12 alone is not enough — gray-zone cases may be missed, and people taking folate supplements can have B12 deficiency masked.
High-risk groups for whom annual screening is recommended: strict or near-vegan eaters, age > 65, long-term PPI / H2 / metformin users, post-gastric-bypass / gastrectomy patients, autoimmune pernicious anemia (anti-IF antibodies), and anyone with chronic unexplained fatigue, numbness, balance problems, or cognitive decline.
The B12 test is cheaper than the supplement — measure first, then decide what to take and how.