Vitamin B12, or cobalamin, is an essential water-soluble vitamin that the body cannot produce independently, making dietary intake or supplementation vital for health. This crucial nutrient supports red blood cell formation, neurological function, DNA synthesis, and numerous metabolic processes throughout the body. B12 deficiency can lead to serious complications including anaemia and irreversible nerve damage if left untreated. Understanding what B12 does for the body helps identify deficiency risks and ensures adequate intake through diet or supplementation, particularly for at-risk groups such as vegans, older adults, and those with absorption disorders.

What Does Vitamin B12 Do for the Body?

Vitamin B12, also known as cobalamin, is an essential water-soluble vitamin that plays a fundamental role in maintaining human health. Unlike many other vitamins, B12 contains the trace element cobalt, which gives it its alternative name. The body cannot produce vitamin B12 independently, making dietary intake or supplementation absolutely necessary for optimal physiological function.

This vital nutrient is involved in numerous critical biological processes throughout the body. B12 acts as a cofactor for key enzymes, particularly methionine synthase and methylmalonyl-CoA mutase, which facilitate essential biochemical reactions involved in DNA synthesis, red blood cell formation, and neurological function. Without adequate B12, these processes cannot proceed efficiently, potentially leading to significant health consequences.

The body stores vitamin B12 primarily in the liver, with reserves typically sufficient to last several years in healthy individuals. However, this storage capacity means that deficiency symptoms may not appear immediately when dietary intake becomes inadequate. Once consumed, B12 is absorbed in the terminal ileum of the small intestine through a complex mechanism involving intrinsic factor—a protein produced by the stomach—before circulating in the bloodstream bound to transport proteins.

Understanding what B12 does for the body is particularly important in the UK, where the NHS recognises B12 deficiency as a relatively common condition, especially among certain population groups. NICE Clinical Knowledge Summaries emphasise the importance of identifying and treating B12 deficiency promptly to prevent irreversible neurological damage and other serious complications.

Key Functions of B12 in Your Body

Red Blood Cell Formation and Oxygen Transport

Vitamin B12 is essential for the production of healthy red blood cells through a process called erythropoiesis. It works alongside folate to enable DNA synthesis during cell division, which is particularly important for rapidly dividing cells such as those in bone marrow. Without sufficient B12, red blood cells become abnormally large and dysfunctional—a condition known as megaloblastic anaemia. These malformed cells cannot effectively transport oxygen throughout the body, leading to fatigue and weakness.

Neurological Function and Myelin Synthesis

One of B12's most critical roles involves maintaining the nervous system. The vitamin is essential for synthesising myelin, the protective fatty sheath that surrounds nerve fibres and enables rapid electrical signal transmission. B12 deficiency can lead to demyelination, causing neurological symptoms ranging from tingling and numbness (paraesthesia) to more severe complications including cognitive impairment, memory problems, and difficulty with balance and coordination. This neurological involvement occurs partly through the methylmalonyl-CoA mutase pathway, where B12 deficiency leads to accumulation of methylmalonic acid that can damage nerve cells. Importantly, neurological damage from prolonged B12 deficiency may become irreversible if left untreated.

DNA Synthesis and Cell Division

Vitamin B12 serves as a cofactor for methionine synthase, an enzyme crucial for DNA synthesis and cellular replication. This function is vital for all rapidly dividing cells throughout the body, including those in the gastrointestinal tract, immune system, and bone marrow.

Homocysteine Metabolism

B12 helps convert homocysteine—an amino acid that, at elevated levels, is associated with cardiovascular risk—into methionine. This metabolic pathway helps maintain healthy homocysteine levels. However, while B12 supplementation can lower homocysteine levels, there is no established evidence that this reduces cardiovascular events in otherwise healthy individuals.

Signs Your Body May Need More B12

Haematological Symptoms

The most common manifestation of B12 deficiency is anaemia, which develops gradually as the body's stores become depleted. Symptoms include persistent tiredness and lethargy that doesn't improve with rest, unusual pallor of the skin and mucous membranes, shortness of breath during normal activities, and heart palpitations. Some individuals may also experience a sore, red tongue (glossitis) or mouth ulcers that are slow to heal. It's important to note that B12 deficiency can exist without anaemia, and macrocytosis (enlarged red blood cells) is a common finding.

Neurological and Cognitive Symptoms

Neurological symptoms can occur even before anaemia develops and may include pins and needles sensations (paraesthesia), typically in the hands and feet; numbness or altered sensation; difficulty with balance and coordination (ataxia); muscle weakness; and vision disturbances. Cognitive symptoms may manifest as memory problems, difficulty concentrating, confusion, or mood changes including depression and irritability. In severe cases, particularly in elderly patients, B12 deficiency can mimic dementia.

Other Clinical Features

Additional signs that may indicate B12 deficiency include unexplained weight loss, loss of appetite, constipation, and general weakness. Some patients report a yellow tinge to the skin (mild jaundice) due to increased breakdown of abnormal red blood cells.

When to Contact Your GP

You should arrange to see your GP if you experience persistent tiredness alongside other symptoms mentioned above, particularly if you have risk factors for B12 deficiency. Your GP can arrange diagnostic tests including a full blood count, serum B12 and folate levels. If pernicious anaemia is suspected, testing for anti-intrinsic factor antibodies may be performed.

Seek urgent medical attention if you develop severe neurological symptoms such as significant balance problems, vision changes, or marked cognitive impairment. Early diagnosis and treatment are crucial, as prolonged deficiency can lead to irreversible neurological damage. Importantly, if B12 deficiency is suspected, folic acid should not be started alone without excluding B12 deficiency, as this could precipitate or worsen neurological symptoms.

How to Get Enough Vitamin B12

Dietary Sources

Vitamin B12 is naturally found almost exclusively in animal-based foods, making dietary intake straightforward for most omnivores. Excellent sources include:

• Meat, particularly liver and kidney (organ meats contain the highest concentrations)

• Fish, especially salmon, trout, tuna, and sardines

• Shellfish, particularly clams and mussels

• Eggs, with B12 concentrated in the yolk

• Dairy products including milk, cheese, and yoghurt

• Fortified foods such as certain breakfast cereals, plant-based milk alternatives, and nutritional yeast

The NHS recommends that adults need approximately 1.5 micrograms of vitamin B12 daily. Most people consuming a varied diet that includes animal products will easily meet this requirement. However, cooking methods can affect B12 content, with some loss occurring during high-heat preparation.

Supplementation

For individuals unable to obtain sufficient B12 from diet alone—particularly vegans, vegetarians, and those with absorption problems—supplementation becomes necessary. Oral B12 supplements (usually cyanocobalamin) are available over the counter in various forms including tablets, sublingual preparations, and sprays. Doses typically range from 25 to 2,000 micrograms. Standard oral tablets are generally effective for dietary deficiency where absorption is intact; there is no clear evidence that sublingual or spray forms offer superior absorption.

For those with confirmed deficiency due to malabsorption or with significant neurological symptoms, intramuscular B12 injections (hydroxocobalamin) are recommended in the UK. The standard NHS protocol for patients without neurological involvement is hydroxocobalamin 1 mg intramuscularly three times a week for 2 weeks, then 1 mg every 2-3 months. For patients with neurological involvement, the regimen is 1 mg on alternate days until no further improvement, followed by 1 mg every 2 months. Treatment is lifelong for conditions like pernicious anaemia.

Hydrocobalamin injections may occasionally cause side effects including injection site reactions, itching, or rashes. Very rarely, anaphylactic reactions can occur. Suspected adverse reactions should be reported via the MHRA Yellow Card scheme.

Optimising Absorption

B12 absorption requires adequate stomach acid and intrinsic factor production. Certain medications, particularly proton pump inhibitors (PPIs) and metformin, can interfere with B12 absorption when used long-term. If you take metformin regularly, your doctor may test your B12 levels if you develop symptoms of anaemia or neuropathy, and may consider periodic monitoring if you have other risk factors for deficiency.

Who Is at Risk of B12 Deficiency?

Dietary Factors

Vegans and strict vegetarians face the highest dietary risk of B12 deficiency, as plant-based foods contain virtually no naturally occurring B12 unless fortified. The Vegan Society and NHS both recommend that individuals following plant-based diets take regular B12 supplements or consume adequate amounts of fortified foods. Vegetarians who consume some dairy products and eggs have lower risk but should still monitor their intake carefully.

Gastrointestinal Conditions

Several digestive disorders impair B12 absorption. Pernicious anaemia, an autoimmune condition where the body attacks the cells producing intrinsic factor, is a common cause of B12 deficiency in the UK. Other conditions affecting absorption include Crohn's disease, coeliac disease, and atrophic gastritis. Individuals who have undergone gastrointestinal surgery, particularly gastric bypass or removal of parts of the stomach or small intestine (especially the terminal ileum), are at significantly increased risk due to reduced intrinsic factor production or loss of absorption sites.

Age-Related Risk

Older adults, particularly those over 60, are at increased risk due to reduced stomach acid production (achlorhydria), which impairs the release of B12 from food proteins. The prevalence of B12 deficiency increases with age, affecting a significant proportion of elderly individuals.

Medications

Long-term use of certain medications increases deficiency risk. Proton pump inhibitors (PPIs) and H2-receptor antagonists reduce stomach acid production, whilst metformin (used for type 2 diabetes) can interfere with B12 absorption. The MHRA advises that patients taking metformin should be tested for B12 deficiency if they develop symptoms of anaemia or peripheral neuropathy.

Other Risk Groups

Pregnant and breastfeeding women who follow vegan or vegetarian diets need to ensure adequate B12 intake through supplementation or fortified foods. Breastfed infants of B12-deficient mothers are at risk of developing deficiency themselves. Individuals with chronic alcohol misuse may have both poor dietary intake and impaired absorption. Exposure to nitrous oxide (including recreational use) can inactivate B12 and cause acute neurological symptoms. If you belong to any of these risk groups, discuss B12 screening with your GP, particularly if you experience any symptoms suggestive of deficiency.

Frequently Asked Questions