Vitamin B₁₂ (cobalamin) is a complex cobalt-containing, water-soluble vitamin essential for DNA synthesis, hematopoiesis, and normal neurological function. Unlike most other water-soluble vitamins, vitamin B₁₂ is stored in significant amounts in the liver, and deficiency typically develops slowly over several years.

Deficiency of vitamin B₁₂ results in a spectrum of hematological, neurological, and metabolic abnormalities. The most characteristic manifestation is megaloblastic anemia, often accompanied by irreversible neurological damage if untreated.

Chemical Forms and Biologically Active Derivatives

Vitamin B₁₂ exists in several forms:

• Cyanocobalamin: A synthetic form used in supplements
• Hydroxocobalamin: A naturally occurring form used therapeutically
• Methylcobalamin: Active coenzyme form in cytosol
• Adenosylcobalamin: Active coenzyme form in mitochondria

The latter two forms are biologically active and participate directly in enzymatic reactions.

   Dietary Sources, Requirement, and Storage

Vitamin B₁₂ is synthesized exclusively by microorganisms and is found primarily in foods of animal origin such as liver, meat, fish, eggs, and dairy products. Plant foods generally lack vitamin B₁₂ unless fortified.

• Recommended daily intake: ~2–3 µg/day for adults
• Body stores: Approximately 2–5 mg, mainly in the liver
• Daily loss: Very small (~0.1% of total stores)

Due to substantial hepatic reserves, deficiency may take several years to manifest after cessation of intake.

   Absorption, Transport, and Enterohepatic Circulation

Gastrointestinal Handling

The absorption of vitamin B₁₂ is a complex, multistep process:

1. Release from food: Gastric acid and pepsin liberate B₁₂ from dietary proteins.
2. Binding to haptocorrin: In the stomach, B₁₂ binds to salivary R-proteins (haptocorrins).
3. Transfer to intrinsic factor: In the duodenum, pancreatic proteases degrade haptocorrin, allowing B₁₂ to bind intrinsic factor (IF), a glycoprotein secreted by gastric parietal cells.
4. Ileal absorption: The B₁₂–IF complex binds to specific receptors (cubilin) in the terminal ileum and is internalized by receptor-mediated endocytosis.
5. Intracellular processing: Intrinsic factor is degraded, and B₁₂ is released into circulation.

Transport

In plasma, vitamin B₁₂ is transported primarily by transcobalamin-II (an α₁-globulin), which delivers it to tissues.

Enterohepatic Circulation

Vitamin B₁₂ is secreted into bile and reabsorbed in the ileum, contributing to conservation of body stores.

   Biochemical Functions of Vitamin B₁₂

Vitamin B₁₂ acts as a cofactor in two critical enzymatic reactions:

1. Methionine Synthase Reaction

• Converts homocysteine to methionine
• Requires methylcobalamin
• Links vitamin B₁₂ with folate metabolism

This reaction is central to DNA synthesis via regeneration of tetrahydrofolate.

2. Methylmalonyl-CoA Mutase Reaction

• Converts methylmalonyl-CoA to succinyl-CoA
• Requires adenosylcobalamin

This reaction is important in fatty acid and amino acid metabolism.

   Pathophysiology of Vitamin B₁₂ Deficiency

Impaired DNA Synthesis

Deficiency of vitamin B₁₂ leads to trapping of folate as methyl-tetrahydrofolate, a phenomenon explained by the Methyl trap hypothesis. This results in reduced availability of tetrahydrofolate for thymidine synthesis, causing defective DNA replication.

Consequently, rapidly dividing cells, particularly in the bone marrow, are affected, leading to megaloblastic anemia.

Neurological Dysfunction

Accumulation of methylmalonic acid disrupts myelin synthesis, leading to demyelination of the central and peripheral nervous systems. This results in characteristic neurological manifestations.

   Etiology of Vitamin B₁₂ Deficiency

1. Pernicious Anemia
   An autoimmune disorder characterized by destruction of gastric parietal cells and deficiency of intrinsic factor, leading to impaired absorption of vitamin B₁₂.
2. Gastric Surgery
   Total gastrectomy results in loss of intrinsic factor. Partial gastrectomy may reduce absorption depending on the extent of resection.
3. Dietary Deficiency
   Occurs in strict vegans or malnourished individuals lacking animal-derived foods.
4. Malabsorption Syndromes
   Includes conditions such as celiac disease and tropical sprue affecting the terminal ileum.
5. Bacterial Overgrowth (Blind Loop Syndrome)
   Intestinal bacteria compete for vitamin B₁₂.
6. Parasitic Infestation
   Infection with Diphyllobothrium latum leads to depletion of vitamin B₁₂.
7. Hereditary Disorders
   Rare genetic defects affecting intrinsic factor, cubilin receptor, or transcobalamin-II.
8. Drug-Induced Deficiency
   Long-term use of proton pump inhibitors or metformin may impair absorption.

   Clinical Manifestations

Hematological Features

• Megaloblastic anemia
• Macrocytosis (increased MCV)
• Hypersegmented neutrophils
• Fatigue, pallor, and weakness

Neurological Features

• Peripheral neuropathy (paresthesia, numbness)
• Subacute combined degeneration of the spinal cord
• Ataxia and impaired coordination
• Cognitive impairment and memory loss

Neurological symptoms may precede or occur independently of anemia.

Other Features

• Glossitis (inflamed tongue)
• Mild jaundice due to ineffective erythropoiesis

   Diagnosis of Vitamin B₁₂ Deficiency

Laboratory Investigations

• Serum vitamin B₁₂ levels: Decreased
• Methylmalonic acid: Increased (specific for B₁₂ deficiency)
• Homocysteine: Increased
• Complete blood count: Macrocytic anemia
• Peripheral smear: Megaloblastic changes

Immunological Tests

• Anti–intrinsic factor antibodies
• Anti–parietal cell antibodies

Schilling Test (Historical)

The Schilling test was previously used to assess vitamin B₁₂ absorption using radioactive cobalt-labeled vitamin B₁₂. Urinary excretion of the vitamin was measured to distinguish between malabsorption and intrinsic factor deficiency.

Although of historical importance, this test is now obsolete and has been replaced by modern diagnostic methods.

   Differentiation from Folate Deficiency

Both vitamin B₁₂ and folate deficiencies cause megaloblastic anemia; however:

• Neurological symptoms are present only in vitamin B₁₂ deficiency
• Methylmalonic acid levels are elevated only in vitamin B₁₂ deficiency

   Conclusion

Vitamin B₁₂ is essential for DNA synthesis and neurological integrity. Its absorption requires a complex interaction between gastric, pancreatic, and intestinal factors. Deficiency may arise from nutritional, autoimmune, gastrointestinal, or genetic causes. Early diagnosis using modern biochemical markers is critical to prevent irreversible neurological damage.