B12 Deficiency

Vitamin B12 (cobalamin) is an essential water-soluble vitamin required for DNA synthesis, erythropoiesis, and maintenance of myelin integrity in the central and peripheral nervous systems. Deficiency results in megaloblastic anaemia and, when prolonged, subacute combined degeneration of the spinal cord — a potentially irreversible demyelinating condition affecting the posterior and lateral columns.

B12 deficiency is common in Australian primary care. Prevalence estimates suggest 5–6% of the general population aged ≥50 years have low serum B12, with significantly higher rates among older adults in residential aged care (up to 20%), vegans and vegetarians (up to 50–80%), and individuals with gastrointestinal disease or prior surgery. The Australian Institute of Health and Welfare (AIHW) data indicate that B12 deficiency-related presentations account for a significant proportion of nutritional anaemia diagnoses nationally.

In Australia, the most frequent aetiology is pernicious anaemia — an autoimmune condition characterised by destruction of gastric parietal cells and loss of intrinsic factor production — followed by malabsorptive disorders including coeliac disease, inflammatory bowel disease affecting the terminal ileum, and iatrogenic causes such as post-gastrectomy and post-bariatric surgery states. Dietary insufficiency is increasingly relevant given rising rates of plant-based diets, particularly in younger Australians.

The clinical spectrum ranges from asymptomatic biochemical deficiency with subtle haematological changes to severe megaloblastic anaemia with pancytopenia and profound neurological disability. Early recognition and treatment are critical, as neurological damage — particularly subacute combined degeneration — may become irreversible within months if untreated.

This guideline provides a comprehensive framework for the diagnosis and management of B12 deficiency in Australian clinical practice, aligned with current evidence and Australian therapeutic standards.

Vitamin B12 is obtained exclusively from animal-derived dietary sources (meat, dairy, eggs, fish) and requires a complex absorption pathway:

• Dietary B12 is released from food proteins by gastric acid and pepsin.
• Free B12 binds to R-proteins (haptocorrin) in the stomach.
• In the duodenum, pancreatic proteases degrade R-proteins, releasing B12 which then binds to intrinsic factor (IF) — a glycoprotein secreted by gastric parietal cells.
• The B12–IF complex is absorbed in the terminal ileum via the cubilin–amnionless receptor complex.
• Absorbed B12 binds to transcobalamin II (TC II) in the portal circulation and is delivered to tissues.

Metabolic roles: B12 serves as a cofactor for two critical enzymatic reactions:

Haematological effects: Impaired DNA synthesis leads to megaloblastic erythropoiesis — large erythroblasts with immature nuclei relative to cytoplasm. This extends to all rapidly dividing cells, explaining the pancytopenia and glossitis seen in severe deficiency.

Neurological effects: Demyelination of the posterior columns (dorsal columns) causes loss of proprioception and vibration sense; lateral corticospinal tract involvement causes upper motor neuron signs (spasticity, hyperreflexia, extensor plantar responses). Peripheral neuropathy with axonal degeneration also occurs. The mechanism is thought to relate to impaired methylation of myelin basic protein and accumulation of abnormal fatty acids in myelin sheaths.

Pernicious Anaemia

Pernicious anaemia (PA) is the most common cause of B12 deficiency in Australia, accounting for 20–50% of cases. It is an autoimmune condition characterised by:

• Autoimmune destruction of gastric parietal cells → atrophic gastritis, achlorhydria, and loss of intrinsic factor production.
• Presence of anti-parietal cell antibodies (sensitivity ~80%, specificity low) and/or anti-intrinsic factor antibodies (sensitivity ~50%, specificity ~95%).
• Increased incidence in patients with other autoimmune conditions: autoimmune thyroid disease (Hashimoto's, Graves'), type 1 diabetes mellitus, vitiligo, and Addison's disease.
• Typically presents in patients aged ≥60 years, but may occur at any age.
• Slight female predominance (F:M = 1.5:1).

Malabsorptive Causes

Dietary Insufficiency

• Strict vegans (no animal products): 5–10 year hepatic B12 stores become depleted; deficiency develops in 2–5 years without supplementation.
• Lacto-ovo vegetarians: lower risk but still at increased risk, especially elderly vegetarians with reduced absorptive capacity.
• Restrictive eating disorders (anorexia nervosa, ARFID): may present with B12 and other micronutrient deficiencies.

Drug-Induced Causes

Other Causes

• Pregnancy and lactation: Increased demand; may unmask marginal deficiency.
• Transcobalamin II deficiency: Rare autosomal recessive condition presenting in infancy.
• Functional B12 deficiency: B12 bound to haptocorrin (inactive) is elevated while TC II-bound (active) B12 is low; serum B12 may be normal — check holotranscobalamin (active B12) if available.

The clinical presentation of B12 deficiency spans a wide spectrum, from asymptomatic biochemical abnormality to severe haematological and neurological disease. Haematological and neurological manifestations may occur independently — a patient can develop subacute combined degeneration without significant anaemia, and vice versa.

Haematological Manifestations

Neurological Manifestations

Other Manifestations

• Fatigue, weakness, dyspnoea on exertion (from anaemia).
• Anorexia, weight loss, mild diarrhoea.
• Recurrent oral ulcers.
• Hyperpigmentation of skin creases (more common in darker-skinned individuals).
• Infertility and recurrent miscarriage (rare; associated with severe deficiency).

Laboratory Investigations

Investigations to Identify the Underlying Cause

• Coeliac serology: Anti-tissue transglutaminase IgA (anti-tTG IgA) with total IgA — MBS Item 66595. Screen all patients with unexplained B12 deficiency.
• Gastroscopy with biopsies: Indicated if PA suspected (look for atrophic gastritis, intestinal metaplasia), or if coeliac serology positive. Consider in patients with upper GI symptoms or iron deficiency coexisting with B12 deficiency.
• Schilling test: Historical gold standard — now obsolete in Australia. Replaced by anti-IF antibody testing and clinical assessment.
• Colonoscopy: If Crohn's disease suspected or if ileal disease on imaging.
• CT/MRI abdomen: If ileal Crohn's disease, SIBO, or small bowel pathology suspected.

Neurological Investigations

• MRI spine: In patients with suspected subacute combined degeneration — look for T2 hyperintensity in posterior and lateral columns (inverted V sign on axial images).
• Nerve conduction studies / EMG: May confirm sensorimotor axonal or demyelinating peripheral neuropathy.
• Neuropsychological testing: In patients presenting with cognitive impairment or psychiatric features.

Principles of Treatment

• Treat the underlying cause where possible (e.g., gluten-free diet for coeliac disease, treat H. pylori, discontinue/limit causative drugs).
• Replace B12 — intramuscular hydroxocobalamin is first-line.
• Always check and correct concurrent folate deficiency.
• Do NOT give folate alone without B12 — this may mask the haematological picture while allowing irreversible neurological progression.
• Monitor for hypokalaemia during treatment (rapid erythropoiesis consumes potassium).

Intramuscular Hydroxocobalamin — First-Line Therapy

Oral Cyanocobalamin — Alternative for Selected Patients

Management of Specific Scenarios

Folic Acid Replacement

When to Refer

• Neurologist: Suspected subacute combined degeneration, significant peripheral neuropathy, or diagnostic uncertainty.
• Gastroenterologist: Suspected coeliac disease, Crohn's disease, suspected pernicious anaemia for gastroscopy/biopsy, or unexplained malabsorption.
• Haematologist: Pancytopenia, diagnostic uncertainty (e.g., concern for myelodysplastic syndrome), or refractory B12 deficiency.
• Dietitian: All vegan/vegetarian patients with deficiency; post-bariatric surgery patients.

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