Vitamin B12 deficiency deserves more urgency than most nutrient gaps, because the neurological damage it causes can become permanent if left untreated for long enough. Haematological symptoms are reversible; some nerve damage is not.
⚠️ Medical disclaimer: Neurological symptoms (numbness, tingling, balance problems, memory change) with suspected B12 deficiency need prompt medical assessment, not a shopping list.
Early and general
Neurological (take seriously)
Laboratory
Neurological symptoms in the absence of anaemia occur in a meaningful proportion of cases — roughly a quarter in some case series — which is exactly why a normal full blood count does not exclude deficiency.
| Group | Mechanism |
|---|---|
| Vegans | B12 occurs naturally only in animal foods and fortified products |
| Vegetarians | Dairy and egg intake is often insufficient |
| Adults over 60 | Atrophic gastritis reduces acid needed to free food-bound B12 |
| Pernicious anaemia | Autoimmune loss of intrinsic factor; affects ~1–2% of older adults |
| Metformin users | Long-term use reduces ileal B12 absorption |
| PPI and H2 blocker users | Reduced gastric acid impairs release from food |
| Coeliac, Crohn's, ileal resection | Terminal ileum is the absorption site |
| Bariatric surgery | Both acid and absorptive surface reduced |
| Nitrous oxide use | Irreversibly oxidises B12; can precipitate acute neurological disease |
| Pregnancy and breastfeeding | Increased requirement; infant stores depend on maternal status |
Serum total B12 is the first-line test, but 20–30% of results in the low-normal range are misleading, because most circulating B12 is bound to haptocorrin and metabolically unavailable.
Interpretation guide (labs vary):
| Serum B12 | Interpretation |
|---|---|
| <148 pmol/L (200 pg/mL) | Deficiency likely |
| 148–258 pmol/L (200–350 pg/mL) | Grey zone — check MMA/homocysteine |
| >258 pmol/L (350 pg/mL) | Deficiency unlikely unless symptoms strongly suggest it |
Confirmatory markers
Dietary deficiency without neurological symptoms: oral cyanocobalamin 1,000–2,000 µg/day is effective. At these doses roughly 1% is absorbed by passive diffusion, bypassing intrinsic factor entirely — which is why high-dose oral works even in some malabsorption.
Pernicious anaemia or neurological involvement: intramuscular hydroxocobalamin, typically a loading series (for example 1 mg on alternate days for around 2 weeks, or until improvement plateaus in neurological cases) followed by maintenance every 2–3 months, lifelong.
Monitoring
| Food | Serving | Approx. B12 |
|---|---|---|
| Lamb's liver | 100 g | ~80 µg |
| Clams | 100 g | ~50 µg |
| Mackerel | 100 g | ~8 µg |
| Salmon | 100 g | ~3 µg |
| Beef | 150 g | ~3 µg |
| Eggs | 2 large | ~1.1 µg |
| Milk | 250 ml | ~1.2 µg |
| Cheddar | 40 g | ~0.5 µg |
| Fortified nutritional yeast | 5 g | 2–5 µg |
| Fortified plant milk | 250 ml | ~1 µg |
Adult requirement is around 2.4 µg/day, but absorption from a single meal is capped at roughly 1.5–2 µg by intrinsic factor saturation — spreading intake across meals matters more than the daily total suggests.
Vegan strategy: rely on a supplement, not chance fortification. Either 50 µg daily or 2,000 µg weekly of cyanocobalamin is a well-established regimen. Spirulina, nori, tempeh and unfortified nutritional yeast contain B12 analogues that show up on assays but are not biologically active — treating them as sources is a common and consequential mistake.
Upload your blood work to Nutrtion.io — B12, folate, ferritin and MCV are read together, so the app can tell you whether a normal blood count is genuinely reassuring or the result of two deficiencies cancelling out.
Can I fix B12 deficiency with diet alone? Only if the cause is purely dietary and absorption is intact. Malabsorption requires high-dose oral or injections.
How fast do symptoms improve? Fatigue and anaemia typically improve within weeks. Nerve symptoms improve over months and may not fully resolve.
Are B12 injections better than tablets? For pernicious anaemia and neurological disease, yes. For dietary deficiency, high-dose oral is equally effective in trials.
Can you take too much B12? B12 is water-soluble with no established upper limit; excess is excreted. Very high serum B12 without supplementation, however, should be investigated.
Does a normal B12 result rule out deficiency? Not in the 148–258 pmol/L range. Ask for MMA or active B12 if symptoms fit.