Vitamin B12 deficiency

Vitamin B₁₂ (cobalamin) plays an essential role in enzymatic reactions responsible for red blood cell (RBC) formation and myelination of the nervous system. Vitamin B₁₂ deficiency is caused by insufficient dietary intake, malabsorption, or increased demand and can result in megaloblastic anemia and various neurological disturbances. Patients typically present with signs of anemia (e.g., fatigue) and/or neurological manifestations (e.g., paresthesia, ataxia, and neuropsychiatric disorders). Pernicious anemia is the most common type of vitamin B₁₂ deficiency and is caused by autoimmune gastritis (AIG). Vitamin B₁₂ deficiency in AIG is due to mucosal atrophy, achlorhydria, and loss of intrinsic factor, which is crucial for vitamin B₁₂ absorption. Laboratory studies often show macrocytic anemia, with or without pancytopenia, and circulating megaloblasts. Serum vitamin B₁₂ level may support the diagnosis but has low sensitivity and specificity. If levels are normal, diagnosis is confirmed based on elevated methylmalonic acid (MMA) and/or homocysteine levels. Additional studies (e.g., autoantibodies for pernicious anemia, EGD with biopsy) may be required to establish the underlying cause. Treatment of vitamin B₁₂ deficiency consists of oral or parenteral supplementation. Depending on the underlying cause, long-term vitamin B₁₂ replacement may be needed.

• Insufficient dietary intake ^[1]
  • Vegan diet: Vitamin B₁₂ deficiency typically only develops after years of strictly avoiding all animal products (unlike folate deficiency, which occurs within a few months of insufficient intake).
  • Malnutrition
  • Chronic heavy alcohol use ^[2]
• Malabsorption
  • Gastric: ↓ intrinsic factor and achlorhydria ^[3]
    • Atrophic gastritis: autoimmune; , resulting in pernicious anemia (most common type of vitamin B₁₂ deficiency; ), or nonautoimmune, e.g., Helicobacter pylori infection ^[1][4]
    • Gastrectomy (partial or total), gastric bypass, bariatric surgery
    • Proton pump inhibitors (PPIs)
    • H₂ receptor blockers ^[1][2]
  • Pancreas: chronic pancreatitis
  • Jejunum: ↑ vitamin B₁₂ demand ^[1]
    • Bacterial overgrowth; or parasites, including fish tapeworm (Dibothriocephalus latus)
  • Terminal ileum: ↓ uptake of intrinsic factor-vitamin B₁₂ complex ^[3]
    • Inflammation; : celiac disease; , tropical sprue, Crohn disease ^[4]
    • Ileal resection and/or reconstructive surgery ^[1]
• Other ^[3]
  • Pregnancy or breastfeeding due to increased vitamin B₁₂ demand ^[2][4]
  • Inherited disorders of vitamin B₁₂ absorption (e.g., transcobalamin deficiency) or inherited metabolic disorders ^[4]
  • Drug-induced deficiency, e.g., due to metformin, nitrous oxide misuse, PPIs

Malabsorption, autoimmune gastritis (pernicious anemia), gastrectomy, gastric bypass, nitric oxide misuse, and inherited disorders can cause severe vitamin B₁₂ deficiency. ^[3]

For information on sources, absorption, transport, and storage of vitamin B₁₂, see “Vitamins.”

Vitamin B₁₂ deficiency

Vitamin B₁₂ is a water-soluble cofactor for enzymes involved in DNA synthesis (i.e., methionine synthase) and odd-chain fatty acid metabolism (i.e., methylmalonyl-CoA mutase), and deficiency leads to enzymatic dysfunction. Enzymatic dysfunction has the following consequences: ^[1]

• Methionine synthase demethylates 5-methyltetrahydrofolate, producing tetrahydrofolic acid, a cofactor in purine synthesis, and also converts homocysteine to methionine
  • ↓ Tetrahydrofolic acid → ↓ DNA synthesis → impaired hematopoiesis
    • Large, nucleated hematopoietic cells, including megaloblasts, are produced, which undergo apoptosis or are phagocytosed by macrophages
    • Pancytopenia (including megaloblastic anemia)
  • ↓ Methionine → neuropathy ^[2][5]
  • ↑ Homocysteine → endothelial damage (increases risk for cardiovascular disease)
  • Can cause secondary folate deficiency
• Methylmalonyl-CoA mutase (converts methylmalonyl-CoA to succinyl CoA)
  • Accumulation of methylmalonyl-CoA and its precursor propionyl-CoA, as well as their associated odd-chain fatty acids, which cannot be completely metabolized
  • Propionyl CoA replaces acetyl CoA in neuronal membranes → demyelination → neurological manifestations

Folate deficiency also leads to low levels of tetrahydrofolic acid, causing megaloblastic anemia.

Pernicious anemia

The pathophysiology of AIG has not been clearly defined but is associated with: ^[2][6][7]

• Antiparietal cell antibodies; : target gastric parietal cells; → ↓ acid production and atrophic gastritis and ↓ intrinsic factor production → ↓ vitamin B₁₂ absorption in terminal ileum ^[2][7]
• Autoreactive T cells target gastric proton pump (H+/K+ ATPase) and are likely the main mediators of inflammation in AIG. ^[7]
• Anti-intrinsic factor antibodies bind intrinsic factor, blocking the vitamin B₁₂ binding site and most vitamin B₁₂ absorption; a small amount of passive diffusion occurs throughout the intestinal tract. ^[2][7]
• See also “Pathophysiology” in “Atrophic gastritis.”

AIG causes chronic destruction of gastric parietal cells, leading to mucosal atrophy, achlorhydria, and loss of intrinsic factor. Vitamin B₁₂ deficiency is a late manifestation of AIG, termed pernicious anemia.

AIG is associated with other autoimmune diseases (e.g., hypothyroidism, vitiligo) and an increased risk of gastric cancer.

• Signs of anemia (e.g., fatigue, pallor) ^[8]
• Mild scleral icterus and/or jaundice
• Neurological disturbances are generally symmetrical
  • Peripheral neuropathy: tingling, numbness, pins-and-needles sensation, coldness (especially in the lower extremities)
  • Subacute combined degeneration of spinal cord: symmetrical demyelination of the spinal cord tracts occurs in vitamin B₁₂ deficiency due to insufficient vitamin B₁₂-dependent fatty acid synthesis and production/maintenance of myelin
    • It manifests with the following symptoms:
      • Paresthesia, impaired proprioception, loss of vibratory sensation, tactile sensation, and position discrimination due to demyelination of the dorsal columns
      • Spastic paresis due to demyelination of the lateral corticospinal tracts (axons of upper motor neurons)
      • Gait abnormalities (spinal ataxia) resulting from the damage of spinocerebellar tracts and dorsal columns
    • Long-term deficiency can lead to irreversible neurological damage.
  • Neuropsychiatric disease; (e.g., reversible dementia, depression, paranoia) ^[8]
  • Worsening vision
  • Autonomic dysfunction: impotence and incontinence
• Glossitis

The Spinocerebellar tracts, lateral Corticospinal tracts, and Dorsal columns are affected in Subacute Combined Degeneration.

Always consider vitamin B₁₂ deficiency when evaluating patients with dementia.

Approach ^[3]

Consider vitamin B₁₂ deficiency in patients with risk factors, clinical features, and/or supportive laboratory findings (e.g., macrocytic anemia).

• Obtain initial studies and vitamin B₁₂ levels.
• Consider additional confirmatory studies (e.g., in patients with normal vitamin B₁₂ levels).
• Obtain additional studies if the cause is not immediately evident (e.g., low vitamin B₁₂ intake or malabsorption).

Consider screening for vitamin B₁₂ deficiency in individuals taking metformin or with a vegan diet. ^[4]

Do not wait for diagnostic confirmation to initiate treatment if there is a strong clinical suspicion for vitamin B₁₂ deficiency. ^[4]

Initial workup ^[2]

• CBC
  • Macrocytic anemia: ↓ Hb, ↑ MCV, ↑ MCH, ↓ reticulocyte index ^[7]
  • Pancytopenia may occur. ^[3]
• Blood smear: : oval macrocytes, circulating megaloblasts, > 5% hypersegmented neutrophils
• Folate level: Folate deficiency causes a similar hematologic presentation to vitamin B₁₂ deficiency. ^[4][7]
• Hemolysis markers: Bilirubin and LDH levels may be elevated. ^[3]
• Patients with peripheral neuropathy: See “Diagnostics for polyneuropathy.” ^[2]

Diagnostic confirmation ^[2][4]

Diagnosis is confirmed based on circulating vitamin B₁₂ and elevated MMA and/or homocysteine levels. There is a lack of consensus on cutoff values and many laboratories use a combination of biomarkers for diagnosis.

• Serum vitamin B₁₂ level (first-line test) : deficiency confirmed if levels are low ^[2][3][4]
• Serum MMA and/or homocysteine levels
  • Indication: normal serum vitamin B₁₂ levels (serum MMA and homocysteine testing have higher sensitivity than serum vitamin B₁₂ testing)
  • Findings: Elevation confirms the diagnosis.
    • MMA is elevated in vitamin B₁₂ deficiency and normal in folate deficiency. ^[2][3][4]
    • Homocysteine is elevated in both vitamin B₁₂ and folate deficiency. ^[3][4]

Schilling test

• Description
  • A test historically used to assess vitamin B₁₂ uptake and determine the cause of deficiency. ^[9]
  • This test is now obsolete but the principles are useful for understanding the causes of vitamin B₁₂ deficiency.
• Stages
  • Consists of four stages that assess impaired vitamin B₁₂ uptake
  • Patients ingest radiolabeled vitamin B₁₂ and urinary excretion is measured.
  • Urinary excretion of 8–40% (varies between laboratories) of radioactive vitamin B₁₂ within 24 hours is considered normal. ^[10]

Additional studies ^[3]

Consider the following additional studies based on clinical suspicion of malabsorption as the cause of vitamin B₁₂ deficiency. ^[2]

• Autoantibodies for pernicious anemia (against intrinsic factor or parietal cells ) ^[2]
  • Presence of serum autoantibodies supports the diagnosis.
  • Consider testing even if vitamin B₁₂ deficiency is not confirmed if there is strong clinical suspicion. ^[2]
• EGD with biopsy ^[6][7]
  • Recommended in patients with autoantibodies indicating pernicious anemia
  • Consider if the cause is unclear, e.g., in patients without autoantibodies if there is strong clinical suspicion.
  • See also “Diagnosis of atrophic gastritis.”
• Testing for malabsorption: See “Diagnosis of malabsorption.” ^[2]

Autoantibodies may not be present in all patients with pernicious anemia.

Differential diagnoses of vitamin B₁₂ deficiency

Starting folate treatment before excluding vitamin B₁₂ deficiency may correct anemia, but it can worsen neuropathy!

In contrast to vitamin B₁₂ deficiency, folate deficiency is generally not associated with neurological symptoms.

Other causes of macrocytic anemia

• Myelodysplastic syndrome
  • CBC may show macrocytic (but not megaloblastic) anemia
  • Most patients have other concurrent hematological alterations (e.g., neutropenia, thrombocytopenia).

Other causes of neuropathy

• Multiple sclerosis
• Friedreich ataxia
• Charcot-Marie-Tooth disease
• Infectious diseases (e.g., syphilis/tabes dorsalis, HIV myelopathy, herpes virus myelitis)
• Other deficiencies (e.g., copper deficiency, vitamin E deficiency)
• Neoplasms (e.g., astrocytoma, ependymoma)

The differential diagnoses listed here are not exhaustive.

Vitamin B₁₂ replacement ^[3]

Do not wait for diagnostic confirmation to initiate treatment in patients with severe neurological deficits, if there is a strong clinical suspicion of vitamin B₁₂ deficiency, and/or in pregnant individuals. ^[2]

• Cyanocobalamin IM (off-label) ^[2][4]
  • Preferred route for patients with any of the following: ^[1][3][11]
    • Neurological deficit
    • Severely low cell counts on CBC
    • Very low serum vitamin B₁₂ levels
  • Patients sometimes develop itch, flu-like symptoms, and/or anaphylaxis.
• Cyanocobalamin PO
  • High dose (off-label) ^[2]
    • Effective alternative to IM route even in patients with pernicious anemia ^[1][3]
    • Suitable for patients with good therapy adherence ^[1]
  • Low dose (off-label) : for subclinical deficiency or preemptively for individuals with a low-vitamin B₁₂ diet (e.g., vegan diet) ^[2]
• Sublingual or nasal routes: not recommended ^[12]

Follow-up ^[2][3]

• Response to therapy ^[2]
  • Monitor clinical response. ^[2]
  • Repeat CBC. ^[2]
• Duration of vitamin B₁₂ replacement therapy ^[3]
  • Reversible causes: Treat the underlying cause and stop replacement once clinical features stabilize or resolve.
  • Irreversible causes (e.g., in pernicious anemia): Continue indefinitely and counsel the patient on the importance of ongoing vitamin B₁₂ replacement therapy. ^[12]

Consider alternative diagnoses if there is an insufficient response to treatment. ^[2]

Additional management ^[1][12]

• Management of the underlying condition: See “SIBO,” “Pancreatic insufficiency,” and “Crohn disease.”
• Supportive care, e.g.:
  • Avoidance of PPIs in patients with pernicious anemia ^[7]
  • Neurological rehabilitation: See “Treatment of polyneuropathy.”
  • RBC transfusion for anemia-related cardiac symptoms, e.g., angina pectoris, dyspnea ^[1][12]

In combined vitamin B₁₂ and folate deficiency, folate supplementation without vitamin B₁₂ replacement may exacerbate neurological symptoms, likely due to ongoing damage from untreated vitamin B₁₂ deficiency. ^[2][7]