Vitamin B12, or cobalamin, is an essential nutrient that originates exclusively from microbial synthesis by certain bacteria and archaea. Whilst these microorganisms produce B12 in nature, humans obtain it primarily through animal-based foods—such as meat, fish, dairy, and eggs—where it accumulates via the food chain. Plant foods do not naturally contain reliable amounts of bioavailable B12, making fortified foods and supplements crucial for those following vegan or vegetarian diets. Understanding where B12 comes from, how it is absorbed, and who may need additional intake is vital for preventing deficiency and maintaining neurological and haematological health.

Natural Sources of Vitamin B12 in Food

Vitamin B12, also known as cobalamin, is a water-soluble vitamin found naturally in animal-based foods. This essential nutrient is produced exclusively by certain bacteria and archaea, which means it enters the food chain primarily through animal products that have accumulated B12 from microbial synthesis in their digestive systems or from consuming B12-containing feed.

The richest natural dietary sources include:

• Meat and poultry – beef, lamb, pork, and chicken, with liver and kidney being particularly concentrated sources

• Fish and seafood – salmon, trout, tuna, sardines, mackerel, and shellfish such as clams and mussels

• Dairy products – milk, cheese, and yoghurt provide B12 that can be well absorbed when gastric function is normal

• Eggs – particularly the yolk, which contains moderate amounts of the vitamin

The bioavailability of B12 varies between food sources. Animal products generally provide B12 in forms that can be absorbed by the human digestive system when gastric and ileal function are intact. According to UK food composition data, a 100g serving of cooked beef liver can provide over 60 micrograms of B12—more than 20 times the recommended daily intake for adults. A glass of milk provides approximately 1.2 micrograms, whilst a medium egg contains around 0.6 micrograms.

It is important to note that plant-based foods do not naturally contain vitamin B12 in significant, reliable amounts. Whilst some fermented foods, certain seaweeds (such as nori), and mushrooms may contain B12-like compounds, these are generally not bioavailable to humans or present in inconsistent amounts, and cannot be relied upon to meet nutritional requirements. This is why individuals following vegan or strict vegetarian diets require particular attention to B12 intake through fortified foods or supplements.

How Vitamin B12 Is Produced and Absorbed

Vitamin B12 is synthesised exclusively by certain microorganisms, including specific bacteria and archaea. These microbes possess the complex enzymatic machinery required to produce cobalamin—a process that involves approximately 30 different enzymatic steps. In nature, these B12-producing bacteria are found in soil, water, and the gastrointestinal tracts of animals, where they contribute to the animal's B12 stores.

The absorption of vitamin B12 in humans is a sophisticated, multi-step process that requires several components to function correctly. When B12-containing food is consumed, the vitamin is initially bound to proteins in the food. Gastric acid and pepsin in the stomach work to release B12 from these food proteins. The freed B12 then binds to a protein called R-binder (haptocorrin), which is secreted in saliva.

As this complex moves into the small intestine, pancreatic enzymes break down the R-binder, releasing B12 once again. The vitamin then binds to intrinsic factor, a glycoprotein produced by parietal cells in the stomach lining. This B12-intrinsic factor complex is essential for absorption—without intrinsic factor, only about 1–2% of dietary B12 can be absorbed through passive diffusion, as noted in NICE Clinical Knowledge Summaries.

The B12-intrinsic factor complex travels to the terminal ileum (the final section of the small intestine), where specialised receptors recognise and absorb it. Once absorbed into the intestinal cells, B12 is released into the bloodstream, where it binds to transport proteins called transcobalamins. The active form of B12 (holotranscobalamin or active B12) is bound to transcobalamin II, which delivers B12 to tissues throughout the body, including the liver (which stores several years' supply), bone marrow, and nervous system. Some UK laboratories now measure holotranscobalamin as a more sensitive marker of B12 status.

This complex absorption mechanism explains why B12 deficiency can arise not only from inadequate dietary intake but also from conditions affecting any stage of this process, including reduced stomach acid production, pancreatic insufficiency, or damage to the terminal ileum.

Fortified Foods and B12 Supplements

Given that vitamin B12 occurs naturally only in animal products, food fortification plays a crucial role in preventing deficiency, particularly for those following plant-based diets. In the UK, numerous foods are fortified with synthetic B12 (cyanocobalamin), which is produced through bacterial fermentation in controlled laboratory conditions—the same microbial process that occurs in nature, but optimised for commercial production.

Common fortified foods available in the UK include:

• Plant-based milk alternatives – soya, almond, oat, and coconut milk are frequently fortified, with levels typically varying by brand (approximately 0.38–1.0 micrograms per 100ml)

• Breakfast cereals – many brands fortify their products, with some providing 25–100% of the recommended daily intake per serving

• Nutritional yeast – often fortified and popular among vegan communities

• Meat substitutes – plant-based burgers, sausages, and other alternatives are increasingly fortified

• Yeast extracts – certain spreads contain added B12

When fortified foods are insufficient or impractical, B12 supplements are available in several forms. Cyanocobalamin is the most common and stable form used in supplements and fortified foods. Other forms include methylcobalamin and adenosylcobalamin, which are the active forms found in human metabolism. Supplements are available as tablets (including sublingual forms that dissolve under the tongue), capsules, liquids, and sprays.

For individuals with absorption difficulties, such as those with pernicious anaemia or following gastric surgery, intramuscular B12 injections (hydroxocobalamin) may be prescribed. These bypass the digestive system entirely, delivering B12 directly into muscle tissue for gradual release into the bloodstream. According to NHS guidance, these injections are typically administered at intervals ranging from every other day initially to every two to three months for maintenance (or every two months if neurological symptoms are present), depending on the underlying cause and severity of deficiency.

It is worth noting that B12 from fortified foods and supplements is often more readily absorbed than B12 from natural food sources, as it is already in free form and does not require separation from food proteins by stomach acid. However, intrinsic factor is still required for absorption unless very high oral doses are taken.

Who May Need Additional B12 and Why

Whilst most people consuming a varied diet including animal products obtain sufficient vitamin B12, several groups are at increased risk of deficiency and may require additional B12 through fortified foods, supplements, or medical treatment.

Dietary factors represent a primary risk category. Individuals following vegan or strict vegetarian diets cannot obtain adequate B12 from unfortified plant foods alone and require reliable supplementation or fortified food consumption. Those with poor overall nutrition or restrictive eating patterns may also be at risk.

Age-related changes can affect B12 status. Some older adults experience reduced stomach acid production (hypochlorhydria or atrophic gastritis), which may impair the release of B12 from food proteins. Those with diagnosed low stomach acid or atrophic gastritis may benefit from B12 in crystalline form (supplements or fortified foods), as this form does not require stomach acid for absorption.

Medical conditions affecting the digestive system can severely compromise B12 absorption:

• Pernicious anaemia – an autoimmune condition destroying parietal cells that produce intrinsic factor, preventing B12 absorption

• Crohn's disease or coeliac disease – particularly when affecting the terminal ileum

• Gastric or ileal surgery – including gastric bypass procedures or removal of sections of the small intestine

• Pancreatic insufficiency – reducing enzyme availability needed for B12 processing

Certain medications can interfere with B12 absorption or metabolism. Metformin, commonly prescribed for type 2 diabetes, may reduce B12 levels with long-term use. The MHRA Drug Safety Update (June 2022) advises testing B12 levels in patients with symptoms of deficiency and considering periodic monitoring in those with risk factors. Proton pump inhibitors (PPIs) and H2-receptor antagonists, used for acid reflux and peptic ulcers, reduce stomach acid production and may affect B12 absorption when used long-term. Individuals taking these medications who develop symptoms of B12 deficiency should discuss testing with their GP.

Nitrous oxide exposure (from recreational use or during surgery) can inactivate B12 and cause neurological symptoms in some individuals.

Pregnant and breastfeeding women need to ensure adequate B12 intake to support foetal development and provide sufficient amounts in breast milk. This is particularly important for women following plant-based diets, who should discuss appropriate supplementation with their midwife or GP.

NHS Guidance on B12 Intake and Deficiency

According to NHS guidance, adults aged 19 and over require approximately 1.5 micrograms of vitamin B12 daily. For adolescents, the UK dietary reference values recommend 1.5 micrograms for those aged 15-18 years and 1.2 micrograms for those aged 11-14 years. Most individuals consuming animal products can easily meet this requirement through diet alone—for example, a small portion of salmon or a glass of milk provides the daily requirement.

The signs and symptoms of B12 deficiency develop gradually and can be non-specific, making early recognition challenging. Common manifestations include:

• Extreme tiredness and lack of energy (lethargy)

• Breathlessness and feeling faint

• Headaches and pale skin

• Pins and needles (paraesthesia)

• Sore, red tongue (glossitis)

• Mouth ulcers

• Disturbed vision

• Irritability, depression, or cognitive changes

• Problems with memory, understanding, and judgement

If left untreated, B12 deficiency can lead to megaloblastic anaemia (where red blood cells are larger than normal and reduced in number) and potentially irreversible neurological damage, including peripheral neuropathy and subacute combined degeneration of the spinal cord.

When to contact your GP: You should arrange an appointment if you experience persistent tiredness, unexplained neurological symptoms such as pins and needles, memory problems, or any combination of the symptoms listed above. Seek prompt medical review if you develop neurological symptoms such as numbness, paraesthesia, gait disturbance, or visual changes. This is particularly important if you have risk factors for B12 deficiency, such as following a vegan diet, having a digestive condition, or taking medications that may affect B12 absorption.

Investigation and diagnosis typically involve a blood test measuring serum B12 levels. Some UK laboratories also measure active B12 (holotranscobalamin). The NHS may also check full blood count, folate levels, and, in some cases, antibodies associated with pernicious anaemia (intrinsic factor antibodies and gastric parietal cell antibodies). For borderline results, additional tests such as methylmalonic acid (MMA) or homocysteine may be used to confirm deficiency.

Treatment approaches depend on the cause and severity of deficiency. For dietary insufficiency without absorption problems, oral supplements (typically 50–150 micrograms daily) are usually effective. For deficiency caused by absorption problems, such as pernicious anaemia, the standard NHS treatment involves hydroxocobalamin injections—initially given on alternate days until symptoms improve, then typically every two to three months for life (or every two months if neurological symptoms are present). Some patients may require more frequent injections if neurological symptoms persist.

The NHS emphasises that individuals at risk of deficiency, particularly those following plant-based diets, should proactively ensure adequate B12 intake through fortified foods or supplements rather than waiting for deficiency symptoms to develop, as prevention is considerably more straightforward than treating established deficiency with potential neurological complications.

Frequently Asked Questions