Vitamin B12

What is Vitamin B12?

Vitamin B12, or cobalamin, is a water-soluble vitamin made by bacteria rather than by plants or animals. Animals take it in through their food and gut bacteria, which is why it is found naturally only in animal foods: meat, fish, eggs, milk and cheese. Plant foods contain none unless it has been added, as in some breakfast cereals, plant milks and nutritional yeast.

Supplements use cyanocobalamin, the most common form, or methylcobalamin, adenosylcobalamin or hydroxocobalamin. The NIH says research has not shown any one form to be better than the others.

The body stores 1,000 to 2,000 times the amount you would normally eat in a day, mostly in the liver. That is why a deficiency takes years to develop — and why it is easy to miss.

How it works

B12 is needed to make red blood cells, to maintain the protective sheath around nerves, and — with folate — to make DNA and recycle homocysteine. Too little causes megaloblastic anaemia and nerve damage, and the nerve damage can occur even without anaemia.

Absorbing B12 from food takes two steps. Stomach acid first frees it from the protein it is bound to; it then combines with intrinsic factor, a protein made by the stomach, and the pair is absorbed in the small intestine. Anything that disrupts either step causes deficiency even on a good diet: low stomach acid with age or medicines, pernicious anaemia (an autoimmune condition in which the body makes no intrinsic factor), stomach or bowel surgery, and coeliac or Crohn's disease.

B12 in supplements and fortified foods is not bound to protein, so it skips the first step — which is why people over 50 are advised to rely on those sources. A small share of any large dose is absorbed without intrinsic factor at all, which is why very high doses by mouth can work even for some people who absorb poorly.

What the research actually shows

Evidence strength varies by what is being claimed, so each use is graded separately against the published criteria rather than averaged into one reassuring letter.

Who should take it

Vegans, and vegetarians who eat few eggs or dairy foods, need a reliable source every day: a supplement, or foods fortified with B12. The Vegan Society advises either at least 10 micrograms a day or at least 2,000 micrograms once a week. This matters most in pregnancy and breastfeeding, when a vegan mother's baby can also become deficient.

People over 50 should get most of their B12 from fortified foods or supplements, according to the NIH, because many absorb less from food as stomach acid falls with age.

If you take metformin or a stomach-acid medicine long term, ask your doctor whether your B12 should be checked. After weight-loss surgery, or with pernicious anaemia, coeliac or Crohn's disease, B12 needs specialist management, often with injections.

Symptoms of deficiency include tiredness, weakness, pale skin, palpitations, pins and needles in the hands and feet, balance problems, memory problems and a sore mouth or tongue. They can take years to appear. If you have them, see your GP for a blood test rather than guessing.

Effective dosage

Adults need about 1.5 micrograms a day in the UK; the US recommended amount is 2.4 micrograms, rising to 2.6 in pregnancy and 2.8 while breastfeeding. Most people who eat animal foods get this from their diet.

Supplements commonly contain 10 to 1,000 micrograms. The body absorbs a smaller share as the dose rises, so larger, less frequent doses make up for it — which is why the Vegan Society's advice is either 10 micrograms daily or 2,000 micrograms weekly. Treating a diagnosed deficiency is a job for a doctor, with injections or high-dose tablets.

Where products diverge from the research. Most B12 products contain far more than anyone needs: 500 or 1,000 micrograms is hundreds of times the daily requirement. That is not dangerous, because only a small share is absorbed, but it undercuts "high-strength" premiums. For a vegan, a cheap 10 to 50 microgram daily tablet covers it.

The form is marketed more than it matters. Methylcobalamin is sold as "active" and better absorbed, and under-the-tongue sprays and lozenges as faster; the NIH says research has not shown any form of B12 to be better than the others.

Safety, side effects and interactions

No upper limit has been set for B12, and the NIH says it has not been shown to cause harm even at high doses. Some studies link high blood levels of B12 with a higher risk of cancer, while others find the opposite or no link; more evidence is needed.

The important interactions run the other way: metformin, proton pump inhibitors and H2 blockers can lower your B12. And folic acid above 1 mg a day can hide a B12 deficiency by correcting the anaemia but not the nerve damage — a particular concern for older people.

None of this is medical advice. See the medical disclaimer for what that means before you act on anything here.