Vitamin B12 is the only vitamin whose absorption requires an entire biological pathway: it is released from the protein in food with the help of stomach acid, binds to a protein called intrinsic factor that is secreted in the stomach, and only then is absorbed at the end of the small intestine (the ileum).
A disruption at any one of these stations causes deficiency — which is why B12 deficiency is far more common than it seems, even in people who eat meat.
It is also the deficiency with the highest price: if it persists, the nerve damage it causes may be irreversible. It develops slowly, sometimes over years, and in some cases it appears before anemia has even shown up in blood tests.
Dosage and sources
| Item | Value |
|---|---|
| Recommended Dietary Allowance (RDA) — adults | 2.4 mcg |
| Pregnancy / breastfeeding | 2.6 / 2.8 mcg |
| Tolerable Upper Intake Level (UL) | None — no toxicity has been found |
| Common supplement dose for vegans | 250-1,000 mcg per day, or 2,000 mcg weekly |
| Source | B12 per 100 grams |
|---|---|
| Oysters | 16 mcg (667%) |
| Beef liver | 70 mcg |
| Mackerel | 8.7 mcg |
| Sardines | 8.9 mcg |
| Beef | 2.6 mcg |
| One egg | 0.5 mcg |
| Milk (a cup) | 1.2 mcg |
| Fortified nutritional yeast | Depends on the product — read the label |
B12 is made only by bacteria — not by plants and not by animals. It ends up in animal foods because animals accumulate it from their microbiome or from fortified feed.
Benefits (Pros)
1. The nervous system and myelin
B12 is needed to maintain the myelin sheath that wraps nerve fibers. Deficiency causes degeneration of the spinal cord columns — numbness, loss of vibration and position sense, unsteady gait and, in advanced cases, cognitive disturbances. Some of the damage is irreversible.
2. Red blood cell production
Together with folic acid, B12 is needed for DNA synthesis in bone marrow cells. Deficiency causes megaloblastic anemia — large, immature cells.
3. Metabolism and homocysteine
B12 is a cofactor for only two enzymes, but both are central: methionine synthase (which breaks down homocysteine and produces methionine) and methylmalonyl-CoA mutase.
4. A precise diagnostic marker
Methylmalonic acid (MMA) in the blood is a specific and sensitive marker of B12 deficiency — often better than the B12 level itself, which can look normal even when there is cellular deficiency.
Downsides and side effects (Cons)
1. Drugs that deplete B12 — the common cause nobody talks about
This is the most important practical part of this page.
- Metformin — the most widely used drug for type 2 diabetes — reduces B12 absorption through a calcium-dependent mechanism in the ileum. Rates of deficiency in long-term metformin users are high, and the common recommendation is periodic monitoring.
- Proton pump inhibitors (omeprazole, pantoprazole) and H2 blockers — suppress stomach acid, which is needed to release B12 from the protein in food. Prolonged use over years raises the risk of deficiency.
Millions of people take these two groups of drugs, and many are unaware of the link.
2. Age — atrophic gastritis
About 10-30% of people aged 50 and over develop thinning of the stomach lining and reduced acid secretion. They have trouble releasing B12 from food — but absorb B12 from supplements or fortified foods well, since there the vitamin is not bound to protein.
3. Veganism and vegetarianism
B12 does not exist in unfortified plant foods. Spirulina, seaweed and mushrooms are not a source — they contain pseudo-B12, analogs that are counted in tests but are inactive in the body and may even interfere. Vegans must take a supplement or eat fortified foods, without exception.
4. Pernicious anemia
An autoimmune disease in which the body attacks the cells that make intrinsic factor. Active absorption is completely blocked, and treatment is lifelong.
5. Folic acid masks the deficiency
High-dose folic acid corrects the anemia but not the nerve damage. The result: the blood test looks normal, and the spinal cord damage continues quietly. This is why you don’t take high-dose folic acid without checking your B12 level.
6. No toxicity — but no magic either
No Tolerable Upper Intake Level has been set for B12, and it is considered safe even at high doses. On the other hand, there is no evidence that it adds energy or focus for people who are not deficient — “B12 shots for energy” in someone with normal levels are not supported by research.
Summary
B12 is the vitamin with the most complicated absorption, so deficiency is more common than it seems — even among meat eaters. Three groups should get tested: people taking metformin, people taking omeprazole and similar drugs for years, and those aged 50 and over. Vegans must take a supplement, and spirulina is no substitute — it contains inactive pseudo-B12. The good news: most cases are treated with a pill rather than an injection — a 1,000 mcg dose is partly absorbed even without intrinsic factor, and that is enough.
Frequently asked questions
Question: How much B12 do you need per day? Answer: 2.4 mcg for adults, 2.6 in pregnancy and 2.8 while breastfeeding. No Tolerable Upper Intake Level has been set, because no toxicity has been found. Supplements for vegans are usually given at 250-1,000 mcg per day or 2,000 mcg weekly — the high doses are needed because passive absorption is only about one percent efficient.
Question: Does metformin cause B12 deficiency? Answer: Yes, and it is one of the most common causes of deficiency that doesn’t get talked about enough. Metformin reduces B12 absorption in the ileum through a calcium-dependent mechanism, and deficiency rates among long-term users are high. The common recommendation is periodic monitoring of B12 levels in anyone taking the drug long term.
Question: Is spirulina a source of B12? Answer: No, and this mistake can cause harm. Spirulina, seaweed and mushrooms contain pseudo-B12 — analogs that are counted in lab tests but are inactive in the body, and there is evidence they even compete for binding sites and interfere with the absorption of real B12. Vegans must take a dedicated supplement or eat fortified foods, without exception.
Question: Pills or injections? Answer: In most cases pills are enough. Besides active absorption via intrinsic factor, there is also passive absorption of about one percent of the dose — so a 1,000 mcg pill delivers about 10 mcg absorbed, four times the daily requirement, even in people without intrinsic factor. Injections are reserved for severe deficiency, neurological symptoms or adherence problems.
Question: Why is folic acid dangerous when B12 is deficient? Answer: Because it corrects the anemia without correcting the nerve damage. High-dose folic acid “normalizes” the blood count so the blood test looks normal — while the damage to the spinal cord keeps progressing quietly. That is why you don’t take high-dose folic acid without first checking your B12 level.
Question: Does omeprazole affect B12? Answer: Yes, with prolonged use. Proton pump inhibitors and H2 blockers suppress stomach acid secretion, which is needed to release B12 from the protein it is bound to in food. Years of use raise the risk of deficiency. It is worth noting that fortified foods and supplements are not affected — in them the vitamin is not bound to protein to begin with.
Question: How do I know if I’m low in B12? Answer: A blood test, but not just the B12 level itself — it can look normal even when there is cellular deficiency. Methylmalonic acid (MMA) in the blood is a more specific and sensitive marker, and homocysteine is sometimes tested too. Suspicious symptoms include numbness in the hands and feet, fatigue, unsteady gait and trouble concentrating.
Question: Do B12 shots give you energy? Answer: Only if you are deficient. In someone with normal levels there is no evidence that injections or high doses add energy, focus or performance. B12 is very safe and has no known toxicity, but “safe” is not “beneficial” — its place is in correcting a deficiency, not in improving a normal state.