Vitamin B12

How much you need

The official Recommended Dietary Allowance for adults, per day:

People aged 18 fall in the official 14–18 group, which has the same values — 2.4 mcg a day, 2.6 mcg in pregnancy and 2.8 mcg when breastfeeding. Pregnancy and breastfeeding change what you need — confirm yours with your midwife, obstetrician or doctor.

Upper limit

No upper limit has been set for vitamin B12, because its potential for toxicity is low. That does not make large doses useful — most of a large dose is not absorbed.

What it does

Vitamin B12 is needed for the development, myelination and function of the central nervous system, for healthy red blood cells and for making DNA ^[1]. Compounds with vitamin B12 activity are called cobalamins, because the vitamin contains the mineral cobalt ^[1]. The body stores about 1 to 5 mg — 1,000 to 2,000 times as much as a person typically eats in a day — which is why deficiency can take several years to show ^[1].

Forms

Methylcobalamin and 5-deoxyadenosylcobalamin are the forms the body uses; cyanocobalamin and hydroxycobalamin become active once converted to them ^[1]. Most supplements contain cyanocobalamin, and others use adenosylcobalamin, methylcobalamin or hydroxycobalamin; no evidence indicates that absorption differs by form ^[1]. Tablets or lozenges dissolved under the tongue appear no more effective than those swallowed ^[1]. Injections and a nasal gel are prescription medicines, used mainly to treat deficiency ^[1].

Where it comes from

1. Beef liver, cooked, pan fried3 ounces70.7 mcg
2. Nutritional yeast, fortified, from several brands (check label)fortifiedabout ¼ cup8.3–24 mcg
3. Clams (without shells), cooked3 ounces17 mcg
4. Oysters, eastern, wild, cooked3 ounces14.9 mcg
5. Salmon, Atlantic, cooked3 ounces2.6 mcg

How these five were chosen: The five foods with the most vitamin B12 per serving in the NIH fact sheet's food table. Fortified foods are marked.

Getting the most from it

In food, vitamin B12 is bound to protein. Stomach acid and enzymes release it, and it is then absorbed together with intrinsic factor, a protein made by the stomach ^[1]. The vitamin added to fortified foods and supplements is already free, so it does not need that release step ^[1]. Absorption drops steeply once a dose exceeds what intrinsic factor can carry, about 1 to 2 mcg: around half of a small dose is absorbed, but only about 2% of 500 mcg and 1.3% of 1,000 mcg ^[1]. Vitamin B12 from dairy appears to be about three times as available as from meat, fish and poultry, and from supplements about 50% more available than from food ^[1].

What about folic acid?

Vitamin B12 is often sold in B-complex supplements alongside other B vitamins, usually in larger amounts than in multivitamins ^[1], and several of the trials described below gave it with folic acid. The idea is that together they lower homocysteine, a compound associated with heart disease and dementia. They do lower it — but in those trials, that did not translate into fewer heart attacks or better cognitive function ^[2][3]. The combination may also carry risks. In U.S. survey data on adults aged 60 and older, low vitamin B12 status together with high folic acid or folate was associated with about two to three times the risk of cognitive impairment — a cross-sectional finding that cannot show cause ^[6]. In a trial of 400 mcg of folic acid and 500 mcg of vitamin B12 a day for 2 to 3 years, colorectal cancer was more common in the supplement group (3.4% against 2%), although this might have been due to the folic acid rather than the vitamin B12, and overall cancer risk did not differ ^[5].

In the United States

Most people in the United States get enough vitamin B12 ^[1]. In national survey data from 2017 to March 2020, men aged 20 and older averaged 5.84 mcg a day from food and drink and women 3.69 mcg; only 5% of men and 11% of women fell below the average requirement of 2 mcg ^[1]. About 24% of men and 29% of women take a supplement containing it ^[1]. Even so, in 2007–2018 data about 3.6% of adults had vitamin B12 deficiency (blood levels below 200 pg/mL) and about 12.5% had insufficiency (below 300 pg/mL) ^[1]. People of low socioeconomic status, women and non-Hispanic Black Americans are the most likely to have low intakes ^[1].

Who is most likely to run low

• Older adults — between 3% and 43% of older adults living in the community are deficient, depending on the definition used; atrophic gastritis, which reduces stomach acid and intrinsic factor, affects 8% to 9% of adults 65 and older ^[1]
• People with pernicious anemia, an autoimmune disease that stops the stomach making intrinsic factor and the most common cause of clinically evident deficiency worldwide ^[1]
• People with celiac disease, who have higher rates of deficiency; for Crohn's disease the evidence is mixed ^[1]
• People who have had stomach or intestinal surgery, including weight-loss surgery, which removes cells that make stomach acid and intrinsic factor ^[1]
• Vegans and vegetarians, because natural sources are limited to animal foods; fortified foods and supplements substantially reduce the risk ^[1]

What deficiency looks like

Causes include difficulty absorbing the vitamin from food, lack of intrinsic factor (as in pernicious anemia), stomach or intestinal surgery, long-term use of some medicines, and a diet without it ^[1]. The hallmark is megaloblastic anemia — large, abnormal red blood cells — along with low blood counts, an inflamed tongue, fatigue, palpitations, pale skin, dementia, weight loss and infertility ^[1]. Numbness and tingling in the hands and feet can occur without anemia, so early diagnosis matters to avoid irreversible damage ^[1]. In pregnant and breastfeeding women, deficiency might cause neural tube defects, developmental delays, failure to thrive and anemia in the baby ^[1]. Blood levels below 200 or 250 pg/mL are generally considered low, and methylmalonic acid is the most sensitive marker ^[1]. Deficiency is usually treated with injections; in three small trials, very high oral doses (1,000 to 2,000 mcg) normalised blood levels about as well, though the evidence was of low quality ^[7].

What the evidence supports, and what it doesn't

Established. Healthy blood and nerves. For adults, the official values were set mainly at the amount needed to maintain healthy blood cell status and serum vitamin B12 levels, and enough vitamin B12 prevents the anemia and nerve damage of deficiency ^[1].

Not supported by trials. Heart disease: supplements lower homocysteine, but a Cochrane review of 15 studies in 71,422 people concluded that vitamin B12, alone or with other B vitamins, does not prevent heart attacks or reduce deaths in people with or at risk of cardiovascular disease ^[2]. Cognition: in a trial of 2,919 adults aged 65 and older given 500 mcg of vitamin B12 and 400 mcg of folic acid a day or a placebo for 2 years, homocysteine fell but cognitive test scores did not differ ^[3], and a Cochrane review of 14 studies in 27,882 mostly older people found little to no effect on overall cognitive function ^[1]. Cancer: a meta-analysis of 18 trials in 74,498 people found that B-vitamin supplements including 20 to 2,000 mcg of vitamin B12 a day had little or no effect on cancer incidence, cancer deaths or overall mortality ^[4]. Energy: although it is often promoted as an energy and endurance booster, vitamin B12 appears to do nothing for athletic performance in people who are not deficient ^[1].

Mixed. Whether vitamin B12 status affects cancer risk is unclear. Some observational studies link high blood levels, or long-term use of at least 55 mcg a day from supplements, with more cancer — in one study, lung cancer in men; others link low levels with more gastric, colorectal or prostate cancer; and others find no link at all ^[1]. Some studies have found associations between low vitamin B12 and depression ^[1].

Too much

No upper limit has been set: the Food and Nutrition Board judged vitamin B12's potential for toxicity to be low, and even large doses are generally considered safe because the body does not store the excess ^[1]. That is a statement about safety, not benefit — see the evidence above, including the unresolved observational links between high levels and some cancers ^[1].

Medicines that interact

Proton pump inhibitors such as omeprazole and lansoprazole, and H2 blockers such as cimetidine, reduce stomach acid and can interfere with absorbing vitamin B12 from food, leading to deficiency. Metformin, used for prediabetes and diabetes, might reduce absorption and significantly lower blood levels ^[1]. If you take any of these regularly, talk to your doctor or pharmacist about your vitamin B12 status.