Vitamin B12: where deficiency lives and who should supplement

Vitamin B12 (cobalamin) is one of the few water-soluble vitamins with a known deficiency risk profile, and deficiency that can progress to irreversible neurological damage if missed. The populations at risk are well-defined.

Where B12 comes from#

B12 is synthesised exclusively by bacteria. Humans get it from animal foods:

• Liver and kidney: densest source
• Meat including poultry
• Fish and seafood
• Eggs and dairy — lower density but reliable for ovo-lacto vegetarians
• Fortified foods: many plant milks, breakfast cereals, and nutritional yeasts

Plant foods reliably contain no B12 unless fortified. The claim that nori, spirulina, or fermented foods provide adequate B12 is not supported — these contain analogues that don't fulfil B12 function in humans.

Who's at deficiency risk#

Vegans. Without supplementation or fortified foods, deficiency is essentially inevitable on a long enough timeline. Some vegans go years before symptoms (the liver stores 2–5 years of B12), but progression is certain.

Vegetarians with limited dairy/eggs. Lower risk than vegans but still elevated, particularly long-term.

Older adults. Roughly 10–15% of adults over 65 have low B12, primarily due to atrophic gastritis (reduced stomach acid, reduced intrinsic factor production). This means even dietary B12 is poorly absorbed.

People on long-term metformin. Metformin reduces B12 absorption; UK GP guidance increasingly includes periodic B12 testing in long-term metformin users.

People on long-term proton-pump inhibitors (PPIs) like omeprazole. Reduced stomach acid impairs B12 release from protein-bound dietary sources.

Pernicious anaemia. Autoimmune destruction of intrinsic factor; requires lifelong supplementation, usually injection.

After gastric surgery or with malabsorption (coeliac, IBD).

Symptoms#

B12 deficiency progresses through stages:

Early:

• Fatigue
• Mild memory issues
• Mood symptoms

Intermediate:

• Macrocytic anaemia (large red blood cells)
• Glossitis (sore inflamed tongue)
• Paraesthesia (pins and needles)
• Numbness in hands and feet

Advanced (potentially irreversible):

• Severe peripheral neuropathy
• Cognitive impairment that can mimic dementia
• Subacute combined degeneration of the spinal cord
• Optic neuropathy

The neurological symptoms can become permanent if deficiency persists long enough. This is the main reason B12 deficiency warrants prompt action.

Testing#

Serum B12 is the standard test. NHS reference ranges typically:

• <150 pmol/L: deficient
• 150–250 pmol/L: borderline / suggestive

• 250 pmol/L: normal

Caveats:

• Serum B12 is imperfect — some people are symptomatic at "normal" levels
• Active B12 (holoTC) is a more sensitive test, available privately
• Methylmalonic acid (MMA) rises in functional B12 deficiency; useful confirmatory test
• Homocysteine can also be elevated

For vegans, periodic testing (annually) is reasonable.

Supplementation#

Cyanocobalamin is the cheapest and most-studied form. Standard doses:

• 10–50 µg/day for prevention in at-risk populations
• 1,000 µg/day for active deficiency repletion (oral)
• IM injection for pernicious anaemia or severe absorption issues

Methylcobalamin is widely marketed as superior; trial evidence supporting this is thin. Cyanocobalamin is converted to methylcobalamin in the body and works fine for most.

Sublingual vs oral absorption is similar in most studies; the sublingual marketing is overstated.

Daily oral B12 at 1,000 µg is sufficient even for many cases of poor absorption (passive diffusion contributes a small but reliable fraction at high doses).

What the wellness market overstates#

• "B12 boosts energy." Only if you were deficient. Vitamin shots for "energy" in B12-replete people don't work.
• "Everyone is B12 deficient." Not true — most omnivores eating adequate meat/dairy/eggs are fine.
• "Need fancy methylated B12." Cyanocobalamin works for the vast majority.
• "Liver detox" claims for B12. Not a clinical use.

What we'd actually say#

For vegans, supplementation is non-negotiable. The default should be daily oral B12 (10–50 µg) or weekly higher dose (1,000–2,000 µg), depending on preference.

For long-term vegetarians with limited dairy/egg intake, similar logic applies.

For adults over 65, particularly those on metformin or PPIs, periodic B12 testing is reasonable and supplementation if low.

For omnivores under 65 eating standard UK diets, routine supplementation isn't necessary.

If you have unexplained neurological symptoms (paraesthesia, numbness, balance issues, cognitive changes), B12 testing is part of basic work-up regardless of your demographic profile.

When to talk to a clinician#

• Neurological symptoms, particularly numbness, paraesthesia, or balance issues
• Macrocytic anaemia on a blood test
• Long-term metformin user without recent B12 check
• Long-term PPI user without recent B12 check
• Vegan or strict vegetarian without supplementation history
• Pernicious anaemia or family history of it
• Cognitive changes in older adults — B12 is part of dementia work-up