Vitamin B12

Vitamin B12 — chemically known as cobalamin — is the single nutrient that deserves dedicated attention from anyone eating a fully plant-based diet. It is not produced by plants. It is not, technically, produced by animals either. It is synthesized by a specific class of bacteria and archaea, and every gram of it in the modern food system originated, directly or indirectly, in a microbial fermenter. Vegans take it from a bottle; omnivores take it from livestock who were either fed B12-fortified feed or given injections of the same substance. The destination is identical. The route is shorter for vegans.

This article is the definitive veganism.wiki reference on B12: what it does, what you need, how to supplement, how to test for deficiency, and the common misconceptions worth dispatching.

What vitamin B12 actually does

B12 is a cofactor for just two enzymes in the human body, but the two reactions they run are profoundly important:

• Methionine synthase — which converts homocysteine to methionine, the feeder reaction for nearly every methylation event in the body, including DNA synthesis, neurotransmitter production, and myelin sheath maintenance.
• Methylmalonyl-CoA mutase — which converts methylmalonyl-CoA to succinyl-CoA, a step inside the mitochondrial energy-production pathway.

When B12 runs low, the first reaction fails and homocysteine rises; the second reaction fails and methylmalonic acid (MMA) rises. These are the two biomarkers clinicians use to detect functional deficiency well before symptoms appear.

Where B12 comes from

Bacteria. That’s it. Neither plants nor animals synthesize B12 — animals merely store it in their tissues after consuming it from soil, feed, or their own gut flora (in ruminants). In the modern factory-farming system, livestock are routinely supplemented with B12 themselves because feedlot feed is sterile enough that natural microbial sources are insufficient. The B12 in the meat aisle is, in a very real sense, laundered from the same industrial bacterial fermenters that make vegan supplements.

Trace amounts of B12-like compounds appear in some plants grown in B12-rich soil, in unwashed organic produce, and in certain algae and fermented foods — but most of these are B12 analogues (corrinoids that the body cannot use, and which may even block true B12 uptake at the receptor). Relying on them is not reliable.

How much you need

Adult daily requirements, compared across authorities:

• U.S. / NIH: 2.4 µg per day for adults. 2.6 µg during pregnancy, 2.8 µg while lactating.
• European Food Safety Authority: 4.0 µg per day — reflects more recent evidence that the NIH figure is on the low side.
• The Vegan Society (UK): recommends either ~10 µg/day from fortified foods + supplement, or ≥2,000 µg once weekly.

There is no known upper limit. B12 is water-soluble; the body excretes excess. Overdose at supplemental levels is not a concern for healthy people.

How to supplement (the practical part)

Two equally good regimens, pick whichever you’ll actually stick with:

1. Daily: 25–100 µg of cyanocobalamin or methylcobalamin.
2. Weekly: 1,000–2,000 µg once or twice per week.

Why the big dose gap between “daily” and “weekly”? Absorption is saturable. The intrinsic-factor-mediated uptake pathway in the terminal ileum caps at around 1.5–2 µg per meal. Above that, the body switches to passive diffusion, which absorbs roughly 1% of whatever’s present. A single 1,000 µg pill passively delivers ~10 µg — plenty for a week of needs.

Which form?

• Cyanocobalamin — the cheapest, most stable, most studied. Converts in the body to the active coenzyme forms. Recommended as the default unless you have kidney disease (in which case talk to your doctor about hydroxocobalamin or methylcobalamin, since the “cyano” group, though trivial in healthy people, is metabolized via cyanide-detox pathways).
• Methylcobalamin — active form. Popular in “natural” supplements. Unnecessary for most healthy adults but harmless.
• Hydroxocobalamin — the injectable form clinicians use for severe deficiency. Highest bioavailability.
• Adenosylcobalamin — the other active form, works in mitochondria.

For healthy vegans eating well, cyanocobalamin is the honest answer. If supplement marketing has made you nervous about it, methylcobalamin is the price of peace of mind; both work.

Fortified foods are not a replacement

Plant milks, nutritional yeast (“nooch”), breakfast cereals, and some meat analogues are often fortified with B12. A heavy, consistent consumer can meet their needs from these alone. But “consistent” is doing a lot of work in that sentence. If you switch brands, go on vacation, pick the wrong carton, or simply have a low-appetite week, you fall short. A dedicated supplement eliminates the variability for pennies per day. Use fortified foods as a floor, not a ceiling.

How to test for deficiency

If you’ve been vegan for more than a year and have never supplemented — get tested. Ask your doctor for:

• Serum B12 — the standard test. Imperfect (measures both active and inactive B12), but widely available.
• Homocysteine — rises when B12 is functionally low. Elevated homocysteine independently raises cardiovascular risk.
• Methylmalonic acid (MMA) — the most sensitive marker of true intracellular B12 deficiency.
• Holotranscobalamin (holoTC) — the “active B12” test. Increasingly available and arguably the best early indicator.

Serum B12 below 200 pg/mL is deficient; 200–300 pg/mL is borderline and worth confirming with MMA or holoTC; above 400 pg/mL is comfortable. Many vegans aiming for optimal (rather than merely adequate) status target over 500 pg/mL.

Symptoms of deficiency, in rough order of appearance

1. Fatigue, low mood, pallor
2. Tingling, numbness, or “pins and needles” in hands and feet
3. Glossitis (sore, smooth tongue)
4. Memory lapses, cognitive fog
5. Gait disturbances, balance problems
6. Subacute combined degeneration of the spinal cord — irreversible if untreated

The last stage is rare and takes years to develop, but by the time it appears the damage may be permanent. Neurological symptoms can progress before hematological (anemia) signs show up — especially in people taking high-folate diets, which can mask the blood signs while the nerves quietly deteriorate. This is a specific reason vegans should not rely on “I feel fine” as a substitute for supplementation.

Pregnancy, infants, and the elderly

• Pregnancy and breastfeeding. Maternal B12 is crucial for fetal and infant neurological development. Exclusively breastfed infants of B12-deficient mothers can develop severe, sometimes permanent neurological damage. Pregnant and breastfeeding vegans should be meticulous about supplementation and consider having their levels checked.
• Infants and children. Never rely on fortified foods alone for infants. Pediatric vegan multivitamins or an age-appropriate liquid supplement is standard practice.
• Adults over ~50. Intrinsic-factor production drops with age, and many omnivores over 50 are also deficient. Ironically, aging vegans who already supplement may have better status than their omnivorous peers.

Common misconceptions

• “Nutritional yeast has plenty of B12.” Only if it is explicitly fortified. Many brands are. Many aren’t. Read the label.
• “Spirulina / chlorella / tempeh / nori provides B12.” Most of the “B12” in these is analogue (pseudovitamin B12) that does not satisfy human requirements and may actively compete with true B12 at the gut receptor.
• “Soil-dwelling bacteria on unwashed vegetables covered our ancestors’ needs.” Possibly, in some regions and eras. Not a safe plan for the modern, heavily sanitized food supply.
• “Supplementing B12 is unnatural.” Synthetic B12 is molecularly identical to B12 made in a bacterium’s gut. Factory-farmed animals receive the same molecule before it gets served to omnivores. The directness is arguably more natural, not less.
• “Methylcobalamin is meaningfully better than cyanocobalamin for healthy people.” The evidence is thin. Use whichever is available and affordable; both work.

What the evidence does not say

• It does not say vegans are uniquely vulnerable — B12 deficiency is common in the general population, especially older adults and people on acid- suppressing medications.
• It does not say one form of B12 is dramatically superior for cognition, energy, or longevity. Marketing around “active” forms has outrun the data.
• It does not say megadoses are harmful in healthy people, but they are also not particularly helpful.

The punchline

B12 supplementation is not a failure of veganism. It is a feature of the modern food system, which cannot be escaped whether you eat plants or animals — only the route changes. Taking a 250 µg tablet with your toothbrush each morning is cheaper, kinder, and arguably more honest than routing the same molecule through an animal first.

If you do one thing after reading this page, make it this: order a B12 supplement today. Everything else is detail.