Vitamin B12 may be the most essential vitamin your body cannot produce on its own. It is indispensable for the nervous system, the production of red blood cells, and DNA synthesis. Yet a significant portion of the population — particularly older adults and those who eat few animal products — has a deficiency without realising it.
Key takeaways
Vitamin B12 is essential for your nervous system, blood formation, and DNA synthesis. The body stores B12 in the liver — meaning a deficiency may only become apparent after months or years. The main sources are meat, fish, eggs and dairy. Vegans and older adults face the greatest risk and should supplement.
What does vitamin B12 do in your body?
B12 serves three critical functions:
1. Nervous system: B12 is required for the formation and maintenance of the myelin sheath — the protective layer around nerves that enables signal transmission. Prolonged deficiency can lead to irreversible nerve damage.
2. Blood formation: Together with folate (vitamin B9), B12 is involved in the production of red blood cells. Deficiency leads to megaloblastic anaemia — large, abnormal blood cells that transport oxygen less efficiently.
3. DNA synthesis: B12 acts as a cofactor in the conversion of homocysteine to methionine, a process essential for cell and DNA production. Elevated homocysteine levels are associated with increased cardiovascular risk.
Liver storage: why deficiency shows up so late
A unique feature of B12 is that the body can store it — primarily in the liver. Total liver storage in an adult is 2–5 mg, while the daily requirement is only 2.4 mcg. This means liver reserves are sufficient for 3 to 5 years. Deficiency therefore manifests late: it takes a long time to deplete the stores.
This makes B12 deficiency deceptive. Someone who stops eating animal products may not develop symptoms for years.
Symptoms of B12 deficiency
- Fatigue and weakness — due to reduced red blood cell production
- Tingling or numbness in hands and feet — nerve damage
- Forgetfulness and difficulty concentrating — cognitive decline
- Pale or yellowish skin — from anaemia and red cell breakdown
- Smooth, swollen tongue — a classic clinical sign
- Low mood or mood swings
- Balance problems and gait disturbances — in advanced deficiency
With prolonged deficiency, nerve damage may be irreversible. Early detection is therefore essential.
Who is most at risk?
Vegans and strict vegetarians
There is no reliable plant-based source of B12. Chlorella, spirulina, and seaweed contain so-called B12 analogues that appear as B12 in blood tests but have no B12 activity — and can actually mask a genuine deficiency. Supplementation is not optional for vegans; it is necessary.
Older adults (60+)
As you age, production of stomach acid and intrinsic factor — a protein required for B12 absorption in the small intestine — decreases. Even with sufficient dietary intake, absorption can be severely impaired. An estimated 10–15% of people over 65 have a subclinical deficiency.
People taking PPIs or metformin
Proton pump inhibitors reduce stomach acidity, impairing B12 absorption. Metformin (a widely used diabetes medication) actively inhibits B12 absorption through an independent mechanism. If you use either of these medications long-term, have your B12 checked periodically.
People with gastrointestinal conditions
Crohn's disease, coeliac disease, and other conditions that disrupt intestinal absorption can also lead to B12 deficiency.
Vitamin B12 in food
| Food | B12 per 100g |
|---|---|
| Beef liver | 70 mcg |
| Mackerel | 9 mcg |
| Beef | 2–3 mcg |
| Salmon | 3.2 mcg |
| Eggs (whole) | 0.9 mcg |
| Full-fat milk | 0.4 mcg |
| Greek yoghurt | 0.5 mcg |
The recommended daily intake is 2.4 mcg for adults. Pregnant and breastfeeding women need 2.6 and 2.8 mcg respectively.
Supplements: dosage and form
Supplements typically contain 500–1,000 mcg — which seems high compared to the daily need of 2.4 mcg, but is completely safe. B12 is water-soluble and excess amounts are excreted in urine. The higher dose compensates for passive absorption (only ~1% at high doses is absorbed passively without intrinsic factor).
The most common supplement forms are:
- Cyanocobalamin — synthetic, stable; the most affordable
- Methylcobalamin — active form; may be better absorbed by some people
- Adenosylcobalamin — active form that works specifically in mitochondria
For most people, the form makes little difference. Those with MTHFR gene variants may benefit from methylcobalamin.
For more context, see our articles on vitamin B12 deficiency and the complete B vitamins overview.
Want to track your daily B12 intake through food? With Moveno you photograph your meal and instantly see all the nutritional values.
Sources
- Dutch Health Council (2018). Dietary reference values for vitamins and minerals for adults. gezondheidsraad.nl
- Stabler SP. (2013). Vitamin B12 Deficiency. New England Journal of Medicine, 368:149–160. doi.org/10.1056/NEJMcp1113996
- RIVM (2023). Vitamin B12 in the Dutch diet. rivm.nl
- Pawlak R et al. (2013). How prevalent is vitamin B12 deficiency among vegetarians? Nutrition Reviews, 71(2):110–117. doi.org/10.1111/nure.12001
