Vitamin B12: Needs, Deficiency Risks for Vegans and Older Adults

Nutrition · Published 2026-10-24

A cracked egg with its yolk in the shell on a wooden board — A cracked egg on a wooden board.

Vitamin B12 is needed in tiny amounts, measured in micrograms, but a lack of it can damage nerves in ways that are not always reversible. It is found naturally almost only in animal foods, and absorbing it depends on a chain of steps in the stomach and gut that often weakens with age.

What vitamin B12 does

Vitamin B12 (cobalamin) is required to make red blood cells, to synthesise DNA and to maintain the myelin sheath that insulates nerve fibres. It works closely with folate: both are needed for cells to divide normally. A shortage of either causes megaloblastic anaemia, in which the bone marrow produces large, immature red blood cells that carry oxygen poorly.

How much is needed

Body	Adults	Pregnancy	Breastfeeding
US National Academies (NIH ODS)	RDA 2.4 mcg	2.6 mcg	2.8 mcg
EFSA (EU, 2015)	Adequate intake 4.0 mcg	4.5 mcg	5.0 mcg
UK reference nutrient intake	1.5 mcg	1.5 mcg	2.0 mcg

The spread between 1.5 and 4.0 mcg reflects different methods: EFSA based its value on intakes associated with healthy biomarker levels in European populations, while older US and UK values were derived from the amount needed to maintain normal blood counts. No tolerable upper intake level has been set by the US National Academies or EFSA, because no adverse effects have been firmly linked to high intakes from food or supplements in healthy people. That is not the same as saying unlimited doses are useful: the body absorbs only a small fraction of a large oral dose.

Where it comes from

Vitamin B12 is made by microorganisms, not by plants or animals. It accumulates in animal tissue, so the main dietary sources are meat, fish and shellfish (clams and liver are especially rich), eggs, milk and dairy products. Fortified foods, such as some breakfast cereals, plant drinks, yeast extracts and nutritional yeast products, provide synthetic B12 in a form that is absorbed well. Seaweed, spirulina, mushrooms and fermented foods are not reliable sources: some contain B12 analogues that the body cannot use.

How it is absorbed

In food, B12 is bound to protein. Stomach acid and enzymes release it; it then binds to intrinsic factor, a protein made by the stomach lining, and the complex is absorbed at the end of the small intestine (the ileum). Anything that interferes with these steps can cause deficiency even when intake looks adequate. A small proportion of a large oral dose (roughly 1%) is also absorbed by passive diffusion, without intrinsic factor, which is why high-dose oral B12 can work for some people with absorption problems under medical supervision.

Who is at risk of deficiency

Vegans, and to a lesser extent vegetarians, because plant foods contain no reliable B12. Vegan diets need fortified foods or a supplement; this is the position of the NHS and of dietetic associations, and it applies especially in pregnancy and breastfeeding, since infants of B12-deficient vegan mothers can develop serious deficiency.
Older adults. Atrophic gastritis, which reduces stomach acid, affects an estimated 10% to 30% of older adults according to the NIH ODS, making food-bound B12 harder to absorb. For this reason the US National Academies advise people over 50 to obtain most of their B12 from fortified foods or supplements, whose free B12 does not need stomach acid to be released.
People with pernicious anaemia, an autoimmune condition that destroys the cells making intrinsic factor. It is a common cause of B12 deficiency in the UK, according to the NHS. Oral B12 from food does little here; treatment usually involves injections of hydroxocobalamin.
People who have had stomach or bowel surgery (including weight-loss surgery), or who have conditions affecting the ileum, such as Crohn's disease.
People taking certain medicines long term: metformin (used for type 2 diabetes) and acid-reducing drugs such as proton pump inhibitors and H2 blockers are associated with lower B12 levels. UK medicines regulators advise that B12 levels be checked in people on metformin who develop symptoms of deficiency.

Symptoms

Deficiency develops slowly because the liver stores several years' worth of B12. Symptoms include tiredness, breathlessness, pale or yellowish skin, a sore red tongue, mouth ulcers, pins and needles or numbness in the hands and feet, problems with balance and walking, memory problems, confusion and changes in mood. Neurological symptoms can appear before, or without, anaemia, and nerve damage can become permanent if deficiency is left untreated. Anyone with these symptoms should see a doctor rather than self-treat, because the cause matters for the treatment.

How deficiency is diagnosed

Doctors usually start with a full blood count and a serum B12 test. Serum B12 is an imperfect marker: results in a borderline range are common, and some people with normal levels still have functional deficiency. Additional tests may include methylmalonic acid (MMA) and homocysteine, which rise when cells lack B12, and tests for intrinsic factor antibodies to look for pernicious anaemia. Folate is normally checked at the same time.

The folic acid caution

High intakes of folic acid can correct the anaemia caused by B12 deficiency without correcting the nerve damage. In the past this "masking" could delay diagnosis while neurological harm progressed. It is one reason the folic acid upper limit exists, and why doctors check B12 before or alongside treating megaloblastic anaemia with folic acid. See our page on folate and folic acid.

Do B12 supplements help people who are not deficient?

B12 is often marketed for energy. There is no good evidence that extra B12 boosts energy or mental performance in people with normal levels. Trials of B vitamins (B12 with folic acid and B6) to lower homocysteine and prevent heart disease or stroke have generally not shown meaningful benefit on major cardiovascular outcomes. The value of B12 lies in preventing and correcting deficiency in the groups above.

In short

Adult reference intakes range from 1.5 mcg (UK) to 2.4 mcg (US) and 4.0 mcg (EFSA) a day; no upper limit has been set.
Vegans need fortified foods or a supplement; people over 50 are advised by US authorities to rely mainly on those forms.
Pernicious anaemia, gut surgery, metformin and acid-reducing drugs raise the risk of deficiency.
Neurological symptoms need prompt medical assessment; the cause decides whether tablets or injections are appropriate.

This page is general information, not medical advice. If you have symptoms of B12 deficiency, are pregnant, follow a vegan diet or take metformin or acid-reducing medicine, ask your doctor or pharmacist about testing and treatment.

Sources

NIH Office of Dietary Supplements, Vitamin B12 fact sheet for health professionals.
NHS, Vitamin B12 or folate deficiency anaemia.
NHS, B vitamins and folic acid.
EFSA Panel on Dietetic Products, Nutrition and Allergies. Scientific opinion on dietary reference values for cobalamin (vitamin B12). EFSA Journal 2015;13(7):4150.
Medicines and Healthcare products Regulatory Agency, Drug Safety Update: metformin and reduced vitamin B12 levels (2022).