Vitamin B12 is one of the few nutrients where deficiency can cause permanent damage — specifically to the nervous system — before symptoms become obvious enough to prompt investigation. It's also one of the most common deficiencies in the UK, particularly among older adults, vegans, and people taking certain medications. Getting enough B12 is not optional: it's a genuine health priority.
What Vitamin B12 Does
B12 is essential for three fundamental biological processes. First, it's required for DNA synthesis — every cell that divides in your body needs B12 to replicate its genetic material correctly. Second, it's necessary for the formation of red blood cells; without adequate B12, red blood cells grow abnormally large and function poorly (macrocytic anaemia). Third — and most critically — B12 is required for maintaining the myelin sheath, the protective coating around nerve fibres that allows electrical signals to travel efficiently. Myelin damage causes neurological symptoms that, if allowed to progress, can be irreversible.
Who Is at Risk of Deficiency
B12 is found almost exclusively in animal products, making vegans and strict vegetarians at significant risk. However, dietary intake is only part of the story. B12 absorption is a complex process requiring a protein called intrinsic factor, produced by cells in the stomach lining. Several factors impair this system.
Pernicious anaemia — an autoimmune condition where the body attacks the cells that produce intrinsic factor — is the most common cause of B12 deficiency in the general population and is particularly prevalent in older adults of Northern European ancestry. It requires B12 injections rather than dietary change, because the absorption mechanism itself is broken.
Age is an independent risk factor. Gastric acid production decreases with age, and stomach acid is needed to release B12 from food proteins. Studies suggest that up to 20% of adults over 60 have low or marginal B12 status — a finding that is dramatically underappreciated in mainstream medicine.
Metformin, the most widely prescribed type 2 diabetes medication, is well-established to reduce B12 absorption. Anyone on long-term metformin should have B12 monitored regularly. Proton pump inhibitors (omeprazole, lansoprazole, and similar acid-suppressing medications) also impair B12 absorption — a problem given how commonly and long-term these are now prescribed.
Symptoms: What B12 Deficiency Actually Looks and Feels Like
The frustrating reality of B12 deficiency is that symptoms develop slowly and are easily attributed to other causes. Fatigue — often profound, disproportionate to circumstances — is usually the first complaint. This is followed by weakness, pallor, and breathlessness from anaemia. These symptoms are common enough that they rarely prompt immediate investigation for B12.
Neurological symptoms are more specific but often appear later and are sometimes dismissed as anxiety or depression: tingling and numbness in the hands and feet (peripheral neuropathy), problems with balance and coordination, memory difficulties, mood changes including irritability and low mood, and in severe cases, cognitive impairment that can be mistaken for early dementia. A sore, red tongue and mouth ulcers are also classic but underrecognised signs.
The critical point about neurological symptoms is that they can persist or become permanent if deficiency is not treated promptly. This is not a nutrient where "we'll wait and see" is appropriate once symptoms suggest deficiency.
Testing and Diagnosing B12 Deficiency
Standard serum B12 tests are imperfect. The normal reference range is usually set at 180–900 pmol/L, but symptoms of deficiency can occur at levels many laboratories would classify as "normal" — particularly in the 180–300 range. More sensitive markers include methylmalonic acid (MMA) and homocysteine, which rise early in B12 deficiency even before serum B12 falls below normal. If you have symptoms consistent with B12 deficiency and a borderline serum result, these secondary markers are worth requesting.
Food Sources of Vitamin B12
The richest animal sources of B12 include: liver and offal (extraordinarily high, but consumed infrequently); sardines, mackerel, salmon, and other oily fish (excellent sources); beef, lamb, and pork; eggs (useful but not high-potency); milk, yoghurt, and cheese. For people eating a varied omnivore diet with regular fish and dairy, B12 deficiency from dietary inadequacy alone is relatively unlikely, though absorption disorders remain a risk.
For vegans, the only reliable sources are fortified foods (nutritional yeast, plant milks, fortified cereals and spreads) and supplements. B12 supplementation is non-negotiable for vegans — it is not present in meaningful amounts in any unfortified plant food.
Supplements: Getting the Right Form and Dose
B12 supplements are widely available and inexpensive. The most common forms are cyanocobalamin (the most stable) and methylcobalamin (the active form, preferred by some practitioners for neurological conditions). For dietary supplementation, both are effective. For people with absorption problems, high-dose oral supplements (1,000–2,000mcg daily) can bypass the intrinsic factor system through passive diffusion and are effective even in pernicious anaemia in many cases. However, confirmed pernicious anaemia typically warrants intramuscular injections — usually hydroxocobalamin — administered by a GP.
If you're supplementing, don't underestimate the dose. The physiological requirement is tiny (around 2.4mcg daily), but absorption from supplements is inefficient — only about 1% of a 1,000mcg tablet is absorbed through passive diffusion. This is why supplement doses seem enormous relative to the dietary requirement.