Vitamin B12, also called cobalamin, is a water-soluble vitamin the body needs to make red blood cells, build the myelin sheath around nerves, and recycle homocysteine back into methionine. No plant or animal makes it: bacteria do, and it reaches us through animal foods, fortified foods or supplements. Absorption is unusually fussy, which is why deficiency turns up even where diets are rich. The fuller clinical reference is the Vitamin B12 (cobalamin) entry in the Smart Longevity glossary.
What it measures
The standard test is serum total B12 from a routine blood draw. It counts all the circulating vitamin, most of it bound to a carrier that cannot deliver it into cells, so a normal total can sit on top of a real shortage. It is a screen, not a verdict.
Three markers sharpen it. Holotranscobalamin (active B12) measures only the fraction tissues can actually take up, and it falls earlier than the total. Methylmalonic acid, or MMA, rises when cells run short, because the enzyme that clears it stalls. Homocysteine rises too, but it also rises in folate and B6 deficiency, so MMA is the more specific of the two. A blood count often adds a clue first: red cells too few and too large.
Typical values
Reference intervals differ by assay and laboratory, so read the interval printed on your own report first. The commonly published landmarks sit roughly here:
| Result | Commonly cited range | Read as |
|---|---|---|
| Low | Below about 200 pg/mL (roughly 148 pmol/L) | Deficiency likely; labs set their own cut-off |
| Borderline | Roughly 200-300 pg/mL (about 148-221 pmol/L) | Grey zone; MMA or active B12 decides |
| Normal | Above roughly 300 pg/mL (about 221 pmol/L) | Usually adequate, unless symptoms persist |
No tolerable upper intake level has been set, because dietary and supplemental B12 is not toxic. A high result without supplements is therefore rarely about intake and can flag liver or blood disease.
Why it matters for longevity
Untreated deficiency does two kinds of damage. It causes megaloblastic anemia, which is reversible. It also degrades the spinal cord and peripheral nerves, producing numbness, unsteady walking, memory trouble and mood change, and that damage can become permanent. Neurological signs sometimes appear before the blood count moves, so normal hemoglobin does not rule deficiency out.
The longer-range evidence is observational. Low B12 status is associated with raised homocysteine, faster cognitive decline and greater brain atrophy in older adults. But trials that lowered homocysteine with B vitamins did not reduce heart attacks or strokes, and cognitive results were mixed, with any benefit concentrated in people already deficient. Correcting a genuine deficiency protects nerves and blood; extra B12 on top of adequate status has not been shown to extend healthspan.
What changes it
Intake sets the floor. Meat, fish, shellfish, eggs and dairy carry it; plants do not, so a vegan diet needs fortified foods or a supplement. The adult recommended intake in the United States is 2.4 micrograms a day, slightly higher in pregnancy and lactation.
Absorption matters more for most people who run low. Stomach acid frees the vitamin from food protein, intrinsic factor escorts it, and the last stretch of the small intestine takes it in. Break any link and diet stops helping: atrophic gastritis, pernicious anemia, gastric bypass, ileal resection and Crohn’s disease all do that. Long-term metformin and acid-suppressing drugs lower levels, and repeated nitrous oxide exposure inactivates the vitamin outright.
Repletion is straightforward once the cause is known. High-dose oral B12 works for most people, including many with pernicious anemia, because a small fraction crosses the gut wall without intrinsic factor; injections are used when absorption is severely impaired or symptoms are neurological. The liver holds several years’ worth, so a new vegan diet can look fine for a long time and then not. One caution: high-dose folic acid can mask the anemia of a B12 deficiency while nerve damage continues, so test before supplementing either.
Related reading
- Which Blood Tests Reflect Aging and How to Read Them
- How to Build a Longevity Supplement Plan
- How to Eat for Brain Health: A Cognitive Longevity Guide
- Folate
Disclaimer. This article is for information only and does not replace medical advice. Talk to a qualified clinician before changing anything about your health.