Vitamin B12 deficiency: symptoms, who's at risk and how to check

Diet low in animal products

Vitamin B12 is found almost exclusively in animal-derived foods: meat, fish, eggs and dairy. Plant-based foods do not contain meaningful amounts of bioavailable B12, with the exception of some fortified products. Research using the EPIC-Oxford cohort found that 52% of vegans were classified as vitamin B12 deficient by serum B12 measurement, compared to 7% of vegetarians and very few omnivores. The prevalence of B12 deficiency is increasing in the UK alongside the growing adoption of vegan and vegetarian diets. People following plant-based diets who are not supplementing consistently should regard B12 testing as a routine annual priority, not an occasional check.

Pernicious anaemia and intrinsic factor deficiency

Pernicious anaemia is an autoimmune condition in which the stomach fails to produce intrinsic factor, a protein required for B12 absorption in the small intestine. Without intrinsic factor, dietary B12 cannot be absorbed regardless of intake. Pernicious anaemia is more common in older adults, in people with a family history of the condition, and in those with other autoimmune conditions. It is one of the most frequently missed diagnoses in people presenting with neurological symptoms, fatigue and anaemia, and requires lifelong B12 supplementation (usually by injection) rather than dietary changes.

Age-related changes in absorption

The absorption of vitamin B12 requires stomach acid for liberation from food proteins, intrinsic factor for uptake in the terminal ileum, and functional transport proteins in the blood. All three of these mechanisms can become less efficient with age. Gastric acid production declines after the age of 50, which reduces the release of B12 from food, and conditions including atrophic gastritis (chronic stomach inflammation) can progress to more significant absorption failure. Older adults often have normal dietary B12 intake but inadequate absorption, making B12 testing a relevant consideration in anyone over 50 presenting with fatigue or cognitive changes.

Medications affecting B12 status

Metformin, used widely for type 2 diabetes, consistently reduces B12 absorption over time. Long-term use of proton pump inhibitors (PPIs) and H2 blockers, prescribed for acid reflux and stomach ulcers, reduces gastric acid and impairs B12 liberation from food. Nitrous oxide (used recreationally and in dentistry) irreversibly inactivates B12 with a single exposure in susceptible individuals, and can precipitate acute B12 deficiency in people who already have borderline levels. Anyone on long-term metformin or PPIs should include B12 testing as part of routine monitoring.

MTHFR gene variants and B12 utilisation

Up to half of the UK population carry variants in the MTHFR gene that reduce the efficiency of the methylation pathway. This pathway depends on both B12 and folate to function correctly. People with MTHFR variants may have serum B12 levels that appear normal while functional B12 activity at the cellular level is compromised, because the gene variant impairs conversion of B12 to its active methylated form. Elevated homocysteine, a downstream marker of this inefficiency, provides evidence of functional B12 insufficiency even when the B12 number itself looks acceptable. This is the specific connection between B12 deficiency and methylation that makes Stride's DNA testing particularly relevant for anyone with persistent neurological or cognitive symptoms despite apparently normal B12 levels.

Gut conditions affecting absorption

Coeliac disease, Crohn's disease and other conditions affecting the small intestine can reduce B12 absorption, particularly when the terminal ileum (the section of small bowel where intrinsic factor-bound B12 is taken up) is involved. Gastric bypass and other weight loss surgeries that reduce the surface area of the stomach or small intestine carry a significant risk of B12 deficiency that requires monitoring and, typically, supplementation.

Standard B12 testing measures total serum B12, which includes both active and inactive forms of the vitamin. The active form (holotranscobalamin) is only around 20% of total B12 but is the fraction actually available to cells. This means that serum B12 can be within the reference range while functional B12 at the tissue level is inadequate. A B12 blood test UK that includes homocysteine alongside B12 provides significantly more information about whether the methylation system is actually working efficiently.

Serum vitamin B12 provides the primary screening measure. Levels below 150 pmol/L are generally considered deficient; levels below 200-250 pmol/L are considered borderline and can be associated with symptoms, particularly neurological ones.

Homocysteine is the most clinically useful functional marker for B12 status. Homocysteine is an amino acid that builds up in the blood when either B12 or folate is insufficient for the methylation reactions that clear it. Elevated homocysteine above 12-15 micromol/L is a reliable indicator of functional B12 or folate insufficiency, even when serum B12 appears within the normal range. It is also independently associated with increased risk of cardiovascular disease, cognitive decline and stroke.

Folate is assessed alongside B12 because both nutrients work in concert in the methylation pathway. B12 deficiency and folate deficiency can produce overlapping symptoms, and both elevate homocysteine. Testing them together allows accurate attribution.

Dietary sources and supplementation for vegans and vegetarians

For people who eat animal products, dietary B12 from meat, fish, eggs and dairy is generally sufficient to maintain adequate levels unless an absorption problem is present. For vegans and vegetarians, reliable supplementation is essential. A daily supplement containing at least 10-25 micrograms of cyanocobalamin is the standard recommendation, since the absorption of B12 from supplements is dose-dependent and only a small fraction of each dose is absorbed. Fortified foods including certain plant milks, breakfast cereals and nutritional yeast contribute some B12 but are generally insufficient as the sole source. Retesting B12 and homocysteine annually confirms whether your supplementation approach is maintaining functional adequacy.

Choosing the right form of B12

Cyanocobalamin is the most widely available and studied form. Methylcobalamin is the active, pre-converted form that bypasses the methylation steps that MTHFR variants can impair. For people with confirmed MTHFR variants, methylcobalamin may be the more effective choice. Hydroxocobalamin, used in NHS injections for pernicious anaemia, has a longer half-life in the body than cyanocobalamin. If standard B12 supplementation is not raising levels or resolving symptoms, discussing the form of B12 with a practitioner and testing homocysteine to assess functional response is the appropriate next step.

Monitoring through blood testing

B12 deficiency is reversible when caught early, but neurological damage from prolonged severe deficiency can be irreversible. The most common missed opportunity is the diagnostic delay that occurs when borderline B12 levels are observed but no action is taken, sometimes for years. Testing B12 alongside homocysteine gives a functional picture that is more clinically actionable than serum B12 alone. If homocysteine is elevated, B12 and folate intervention is warranted regardless of where serum B12 sits within the reference range.

The B12 and methylation connection

B12 is a cofactor in the methylation cycle, the biochemical process that adds methyl groups to DNA, neurotransmitters, hormones and other molecules. This cycle is essential for nerve function, cognitive health, mood regulation and gene expression. Homocysteine accumulates when the cycle is running inefficiently, providing a measurable marker of downstream impact. People with elevated homocysteine alongside low-normal B12 are experiencing functional methylation impairment that a standard normal result would miss entirely. This is why Stride's approach of testing B12 and homocysteine together provides materially more useful information than either marker alone.