Vitamin B12: Who's Deficient, Which Form Absorbs Best, and What Happens When You Run Out

Vitamin B12 occupies a unique place in nutrition: it is one of the few micronutrients where deficiency leads to irreversible damage. Prolonged B12 deficiency destroys the myelin sheath around nerve fibres — and once that damage progresses far enough, it cannot be fully reversed even with supplementation. This is not a hypothetical risk for a narrow population. B12 deficiency is remarkably common, systematically underdiagnosed, and in many cases entirely preventable.

What B12 Does

Vitamin B12 (cobalamin) is involved in several critical processes:

• Myelin synthesis: B12 is essential for producing myelin — the insulating sheath around nerve fibres that enables rapid signal transmission. Deficiency causes progressive demyelination, leading to neuropathy and neurological dysfunction
• Red blood cell formation: B12 (along with folate) is required for DNA synthesis in red blood cell precursors. Deficiency causes megaloblastic anaemia — abnormally large, dysfunctional red blood cells that cannot carry oxygen efficiently
• DNA synthesis: Every dividing cell in the body requires B12. Rapidly dividing tissues — blood, gut epithelium, immune cells — are most affected by deficiency
• Homocysteine metabolism: B12 (with folate and B6) converts homocysteine to methionine. Elevated homocysteine is an independent cardiovascular risk factor; B12 deficiency raises homocysteine levels
• Energy metabolism: B12 is a cofactor in the conversion of methylmalonyl-CoA to succinyl-CoA, a step in the citric acid cycle. Deficiency impairs energy extraction from fatty acids and amino acids — contributing to fatigue
• Cognitive function: B12 is required for synthesis of neurotransmitter precursors and for brain cell maintenance. Deficiency is associated with cognitive decline, depression, and — in older adults — dementia risk

Who Is Most at Risk?

B12 is found almost exclusively in animal products. Any diet that reduces or eliminates animal food intake creates B12 risk.

Vegans and Vegetarians

Vegans who do not supplement B12 will become deficient. This is not a debate — it is physiological certainty over a long enough timeline. The body has liver stores that can last 3–5 years, which is why deficiency symptoms may take years to appear after going vegan. This delay creates a false sense of security.

Vegetarians who consume dairy and eggs have some dietary B12, but studies consistently show lower serum B12 levels than omnivores, and many vegetarians are borderline insufficient.

The recommendation is unambiguous: all vegans must supplement B12. This is not optional; it is a non-negotiable aspect of a vegan diet done responsibly.

Older Adults

The most common cause of B12 deficiency in non-vegetarians is atrophic gastritis — a condition in which the stomach lining atrophies with age, reducing production of hydrochloric acid and intrinsic factor (the protein required to absorb food-bound B12). Atrophic gastritis affects an estimated 10–30% of people over 60.

The critical distinction: people with intrinsic factor deficiency cannot absorb B12 from food, but can absorb free B12 (the form in supplements) through passive diffusion in the gut, which doesn't require intrinsic factor. This is why high-dose oral B12 supplementation works for many older adults who can't absorb food-bound B12 — not because they're absorbing it efficiently, but because even 1–2% passive absorption of a large dose delivers adequate B12.

Metformin Users

Metformin — one of the world's most widely prescribed medications, used for type 2 diabetes — reduces B12 absorption by interfering with calcium-dependent B12 intrinsic factor receptor binding in the gut. Studies show B12 deficiency in 5–40% of long-term metformin users, and the risk increases with dose and duration.

This is clinically underrecognised. Many patients take metformin for years without anyone checking B12 status. The American Diabetes Association recommends periodic B12 monitoring in metformin users, but this recommendation is inconsistently followed.

PPI (Proton Pump Inhibitor) Users

PPIs like omeprazole and lansoprazole reduce stomach acid, which is required to cleave B12 from food proteins before absorption. Long-term PPI use is associated with reduced B12 status — a 2013 study in JAMA found PPI use for 2+ years associated with a 65% increased risk of B12 deficiency.

People with Pernicious Anaemia

Pernicious anaemia is an autoimmune condition where the immune system destroys intrinsic factor-producing cells in the stomach lining. Without intrinsic factor, food-bound B12 cannot be absorbed. This affects approximately 0.1% of the population but is more common in older adults and those with other autoimmune conditions. Treatment requires either high-dose oral B12 or intramuscular injections — because passive absorption still works even without intrinsic factor.

Diagnosing Deficiency: The Testing Problem

Standard serum B12 tests are imperfect. The NHS, European, and US reference ranges typically flag deficiency below 180–200 pmol/L, but research increasingly suggests that neurological and cognitive effects can appear at "normal" levels — particularly below 300–400 pmol/L in symptomatic individuals.

More sensitive markers:

• Methylmalonic acid (MMA): Elevated in B12 deficiency (MMA accumulates when the B12-dependent enzyme can't process it). Better functional marker than serum B12
• Holotranscobalamin (HoloTC / Active B12): Measures the biologically active fraction of serum B12; more sensitive early marker of deficiency than total B12
• Homocysteine: Elevated in B12 deficiency (and folate deficiency); non-specific but useful as part of a panel

If your serum B12 is in the 200–400 range and you have symptoms (fatigue, numbness, brain fog, poor memory), it's worth asking for MMA and holotranscobalamin testing rather than accepting "normal" serum B12 at face value.

B12 Forms Compared

B12 supplements come in several forms, and the form matters:

Form	Bioavailability	Stability	Best For	Notes
Methylcobalamin	High (sublingual best)	Less stable — light-sensitive	General supplementation; neurological support	Active form; doesn't need conversion; preferred for neurological symptoms
Adenosylcobalamin	High	Less stable	Energy metabolism; mitochondrial support	Active form used in mitochondria; rarely found alone, usually combined with methylcobalamin
Hydroxocobalamin	Good — stays in blood longer	More stable	Injection (clinical use); general supplementation	Not active — converted to methyl and adenosyl forms in body; longer half-life than cyanocobalamin
Cyanocobalamin	Good when converted	Most stable (shelf life)	Budget supplementation; fortified foods	Synthetic form requiring conversion; contains trace cyanide (harmless at normal doses); most studied, most common in research

The honest assessment: the evidence strongly supporting methylcobalamin over cyanocobalamin for general supplementation is more limited than supplement marketing suggests. Both forms raise serum B12 effectively in most people. Methylcobalamin is the preferred form for people with known neurological symptoms, those with MTHFR polymorphisms (affecting methylation pathways), and those who want the most directly active form. For general B12 maintenance in healthy individuals, cyanocobalamin works well and is significantly cheaper.

Sublingual delivery (tablets dissolved under the tongue) increases absorption compared to swallowed tablets for oral B12, because some B12 absorbs directly through the mucous membranes — bypassing the stomach acid and intrinsic factor pathway. This makes sublingual B12 particularly useful for older adults with absorption issues or pernicious anaemia patients using high-dose oral supplementation.

Dosing: More Is Needed Than the RDA Suggests

The EU RDA for B12 is 2.4 mcg/day. This reflects the minimum to prevent deficiency in healthy adults with normal absorption — not the optimal dose for supplementation in at-risk populations.

Food Sources and Why Diet Alone Often Falls Short

Food	B12 per Serving	Notes
Beef liver	70–80 mcg per 85g	Extraordinary source; once/week covers weekly needs
Clams	84 mcg per 85g (cooked)	Richest common shellfish source
Oysters	28 mcg per 85g	Also excellent zinc source (see zinc article)
Sardines	8.2 mcg per 85g	Practical everyday source; also rich in omega-3
Salmon	4.9 mcg per 85g	Good source alongside omega-3 benefits
Beef	2.4 mcg per 85g	Covers RDA in one serving
Milk	1.2 mcg per 240ml	Significant for lacto-vegetarians
Egg (1 large)	0.6 mcg	Modest contribution; eggs are B12 present but not rich
Nutritional yeast (fortified)	2–4 mcg per 2 tablespoons	Reliable vegan source; check label to confirm fortification
Tempeh, spirulina, seaweed	Variable / often zero	Despite marketing, these are NOT reliable B12 sources — contain mostly inactive B12 analogues that may actually block absorption

The last row is important: spirulina and some fermented plant foods are frequently promoted as vegan B12 sources. The evidence does not support this. Most plant-derived "B12" is in the form of inactive analogues that don't function as B12 in the body and may even compete with real B12 for absorption. Vegans must supplement; plant-food "sources" are not a substitute.

Symptoms to Watch For

Early deficiency (often months to years before neurological damage):

• Persistent fatigue and weakness
• Pale or slightly jaundiced skin
• Mouth ulcers and sore tongue (glossitis)
• Shortness of breath with mild exertion
• Mood changes: depression, irritability

Later deficiency (neurological — indicates more serious depletion):

• Numbness or tingling in hands and feet (peripheral neuropathy)
• Difficulty walking, balance problems (subacute combined degeneration)
• Vision changes or visual disturbances
• Cognitive decline, memory problems, brain fog
• Dementia-like symptoms in older adults

Critical point: the anaemia caused by B12 deficiency (megaloblastic anaemia) can be masked by folate supplementation — folate allows red blood cells to form normally even in B12 deficiency, preventing the anaemia that would otherwise alert doctors. But the neurological damage continues in the background. This is why high-dose folate supplementation in vegans or vegetarians without B12 is potentially dangerous — it masks the early warning sign while neurological damage progresses.

The Bottom Line

B12 is the nutritional deficiency with the most severe irreversible consequences and the easiest prevention. The stakes are unusually high: caught early, B12 deficiency is correctable. Left until neurological symptoms are established, some damage may be permanent.

If you're vegan: supplement. Non-negotiably, consistently, adequately. 250 mcg/day of methylcobalamin or 1,000 mcg several times per week. Not occasional nutritional yeast, not spirulina — actual B12 supplementation.

If you're over 60: get your B12 tested. If it's below 400 pmol/L and you have any symptoms, supplement with sublingual B12 and retest in 3 months.

If you take metformin or PPIs long-term: ask your doctor to check B12 status annually. It's a simple blood test that catches a preventable problem before it becomes an irreversible one.

The irony of B12 deficiency is that it is simultaneously one of the most serious nutritional problems in the developed world and one of the most preventable. A few pence worth of supplement per day prevents irreversible neurological damage. Get your levels checked.