Vitamin B12: Roles, Food Sources, Forms, Absorption and Safety

Overview

Vitamin B12, also called cobalamin, helps the body make red blood cells and keep the nervous system healthy. It also helps release energy from food and helps the body use folate. [1] It is unusual among vitamins because it is not found naturally in fruit, vegetables or grains, so people who eat no animal foods may not get enough. [1]

If you have numbness, tingling, balance problems, memory changes or unusual tiredness, please see a doctor or other clinician rather than treating yourself. The NHS (UK) says some problems caused by low B12 can be irreversible if treatment is delayed. [2]

Evidence labels used on this page: established nutritional role / deficiency treatment, evidence in adequately nourished people, insufficient evidence, marketing claim.

What it is and its forms

MedlinePlus lists "cobalamin" and "cyanocobalamin" as alternative names for vitamin B12. [3] Supplements come as tablets, under-the-tongue (sublingual) forms, a nasal gel and prescription injections. [3] In UK practice, injections for deficiency use hydroxocobalamin, which the NHS prefers to cyanocobalamin because it stays in the body longer. [4]

Is methylcobalamin better? Proper evidence not found. We searched PubMed for head-to-head trials of methylcobalamin and cyanocobalamin and found none that answered the question. The documented differences are covered at B12 forms compared.

Role in the body and food sources

MedlinePlus says B12 is important for protein metabolism, forming red blood cells and maintaining the nervous system. [3] It is found naturally in fish, meat, poultry, eggs, milk and milk products. Organ meats such as beef liver and shellfish such as clams are especially rich. [3] It is not present in plant foods unless they are fortified. Fortified breakfast cereals and some nutritional yeast products contain it. [3] MedlinePlus adds that the B12 used to fortify foods is very easy to absorb, and that the nutrition label shows whether it has been added. [3] See foods high in vitamin B12.

Absorption: why low B12 is often an absorption problem

In the stomach, B12 combines with a protein called intrinsic factor, and the mix is absorbed in a part of the gut called the distal ileum. [5] Pernicious anemia, an autoimmune condition that damages the cells making intrinsic factor, is the most common cause of B12 deficiency in the UK. [5] Stomach surgery such as gastrectomy and Crohn's disease can also reduce absorption. [5]

The body stores B12 in the liver. The NHS says stores can last around 2 to 5 years, so problems can take a long time to appear after a diet change. [5]

Who may need assessment

MedlinePlus (US) lists people over 50, people on vegetarian or vegan diets, people who have had stomach or intestinal surgery including weight-loss surgery, and people with celiac disease or Crohn disease. [3] The NHS says B12 deficiency affects around 1 in 10 people aged 75 or over and 1 in 20 aged 65 to 74. [2]

A 2013 review of 18 studies that used sensitive blood markers found B12 depletion or deficiency among vegetarians and vegans in every population studied, with higher rates in vegans. [6] See B12 for vegetarians and vegans.

A 2017 clinical review says screening average-risk adults is not recommended, but may be warranted with risk factors such as gut surgery, inflammatory bowel disease, long-term metformin or acid-reducing medicines, a vegan diet, or age over 75. [7]

Deficiency, tests and treatment

Symptoms the NHS lists include extreme tiredness, pins and needles, a sore red tongue, muscle weakness, vision problems and changes in mood or memory. [2] These are not specific to B12, so symptoms alone cannot show a deficiency.

Tests: a 2017 review says initial assessment includes a blood count and a serum B12 level, and that methylmalonic acid (MMA) can help confirm deficiency when a person at high risk has low-normal B12. [7] The NHS notes that some people have neurological problems despite normal blood B12 levels, called functional deficiency. [5] Ranges differ by lab and country. See the B12 test explained and low B12: what to do next.

Treatment is led by a clinician. In the UK it usually starts with hydroxocobalamin injections, then continues as tablets or injections depending on the cause. [4] A Cochrane review of three small trials (153 people) found low-quality evidence of no clinically relevant difference in blood B12 between 1,000 mcg a day by mouth and injections. [8] A 2017 review says injections act faster and should be considered in severe deficiency or severe neurological symptoms. [7] See B12 injections.

Official intake levels

Upper limits. EFSA (EU, 2006) found no clearly defined adverse effects that could be used to set an upper limit. [11] Health Canada also lists no upper limit, because suitable data were lacking. [9] The NHS says 2 mg or less a day of B12 supplements is unlikely to cause harm, and that there is not enough evidence on higher doses. [1] No limit is not proof of safety at any amount. See RDA, AI, DV and UL.

Evidence by use

Safety, interactions and precautions

• Medicines: the NHS lists proton pump inhibitors, metformin and nitrous oxide as medicines that can lower B12. [5] The NHS says your GP will know which medicines affect B12 and will monitor you if necessary. [5] Please do not stop a prescribed medicine yourself. See B12 and medicines and medication interactions.
• Folic acid can hide B12 deficiency: the NHS says folic acid above 1 mg a day can mask B12 deficiency symptoms, which matters most for older people. [1] Before treating folate deficiency, a GP checks B12 first. [4]
• Pregnancy and breastfeeding: needs are higher (see table). Discuss supplements with your midwife or doctor: prenatal vitamins, pregnancy safety.
• Children and older adults: vitamins for children and teens, vitamins for older adults.
• Kidney and liver conditions, and before surgery: Proper evidence not found in the sources read for B12-specific advice. Tell your care team about every supplement; see supplements and surgery.
• After weight-loss surgery: the 2017 review says ongoing B12 supplementation is needed; the care team sets the amount. [7] See vitamins after bariatric surgery.
• Product quality: check the label and see quality testing and certifications.

Related

All vitamins · The eight B vitamins · Folate (B9) · Choline