Vitamin B12 (cobalamin) plays an essential role in enzymatic reactions responsible for red blood cell (RBC) formation and proper myelination of the nervous system. Thus, a deficiency of vitamin B12 may lead to megaloblastic anemia and a wide range of neurological disturbances. Deficiency may be caused by malabsorption, malnutrition, or increased demand. The most common underlying cause of vitamin B12 deficiency is pernicious anemia, an autoimmune disorder characterized by the absence of intrinsic factor (IF). IF is a protein that is crucial for vitamin B12 absorption. Patients present with signs of anemia (e.g., fatigue) and/or neurological manifestations such as paresthesia, spasticity, ataxia, and neuropsychiatric disorders. After detecting low serum vitamin B12 levels, the diagnostic approach consists of identifying the underlying cause by measuring autoantibodies and possibly conducting a Schilling test. When testing patients with suspected vitamin B12 deficiency, it is important to remember that folate deficiency also causes megaloblastic anemia. However, only vitamin B12 deficiency can be accompanied by neuropathy and exhibits elevated levels of methylmalonic acid (MMA). Treatment consists of parenteral supplementation; depending on the underlying cause, long-term supplementation may be needed.
↓ Intrinsic factor (IF)
Atrophic gastritis due to
- Autoimmune atrophic gastritis: most common cause of vitamin B12 deficiency
- H. pylori infection
- Gastrectomy 
- Atrophic gastritis due to
Reduced uptake of IF-vitamin B12 complex in terminal ileum due to:
- Alcohol use disorder
- Crohn disease, celiac disease
- Pancreatic insufficiency
- Surgical resection of the ileum
- Diphyllobothrium latum (tapeworm) infection
- Bacterial overgrowth
- ↓ Intrinsic factor (IF)
- Anorexia nervosa
- Strict vegan diets: occurs only after years of a strict diet that excludes all animal products (unlike folate deficiency, which occurs within a few months of insufficient intake)
- Increased demand: e.g., during pregnancy, breastfeeding, fish tapeworm (Diphyllobothrium latum) infection, and leukemia 
- Drugs: metformin, proton-pump inhibitors, colchicine
Dysfunctional biochemical reactions
- Physiological function: Vitamin B12 is a water-soluble cofactor for enzymatic reactions of DNA synthesis (via methionine synthase) and odd-chain fatty acid metabolism (via methylmalonyl CoA mutase). A deficiency of vitamin B12 leads to enzyme dysfunction.
Dysfunctional methionine synthase (normally converts homocysteine to methionine, thereby demethylating N5-methyl-THF to THF)
- ↓ Tetrahydrofolate (THF; cofactor in purine synthesis) → ↓ DNA synthesis → large, nucleated hematopoietic cells, including megaloblasts; → megaloblastic precursors undergo apoptosis or are phagocytosed by macrophages → pancytopenia (including megaloblastic anemia)
- ↓ Methionine → neuropathy 
- ↑ Homocysteine → endothelial damage → predisposes to cardiovascular disease
- Can also cause secondary folate deficiency
Dysfunctional methylmalonyl CoA mutase
- Methylmalonyl CoA cannot be converted 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 THF, resulting in megaloblastic anemia.
- A type of vitamin B12 deficiency caused by autoantibodies against intrinsic factor and/or gastric parietal cells (type II hypersensitivity reaction)
Antiparietal cell antibodies: target gastric parietal cells
- Causes ↓ acid production and atrophic gastritis
- ↓ Intrinsic factor production → ↓ vitamin B12 absorption in terminal ileum
- Anti-IF antibodies: bind intrinsic factor and block the vitamin B12 binding site
- Antiparietal cell antibodies: target gastric parietal cells
- Associated with other autoimmune diseases (e.g., hypothyroidism, vitiligo)
- Increases the risk of gastric cancer
- Signs of anemia (e.g., fatigue, pallor) 
- 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.
- It manifests with the following symptoms:
- Neuropsychiatric disease; (e.g., reversible dementia, depression, paranoia) 
- Worsening vision
- Autonomic dysfunction: impotence and incontinence
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.
Signs of megaloblastic anemia
- ↓ Hb
- ↑ MCV (macrocytic), ↑ MCH (hyperchromic)
- Hypersegmented neutrophils
- ↓ Reticulocytes
- Frequently thrombocytopenia and leukopenia (possibly pancytopenia)
If vitamin B12 serum levels are decreased, determine the underlying cause
- Test for autoantibodies
If autoantibodies are negative, perform:
Schilling test: a test to determine the cause of vitamin B12 deficiency 
- Consists of four stages that assess impaired vitamin B12 uptake (see table below)
- Patients ingest radiolabelled vitamin B12 and then its urinary excretion is measured.
- Urinary excretion of 8–40% (varies between laboratories) of radioactive vitamin B12 within 24 hours is considered normal. 
- No longer routinely used in clinical practice
- D-xylose absorption test
- Schilling test: a test to determine the cause of vitamin B12 deficiency 
|Stages of Schilling test|
|Description||Radiolabeled vitamin B12 measures in urine|
|Stage 1|| || |
|Stage 2|| || |
|Stage 3|| || |
|Stage 4|| || |
- If vitamin B12 serum levels are normal: 
- Measure homocysteine: elevated in both vitamin B12 deficiency and folate deficiency
- Measure methylmalonic acid (MMA); to help rule out folate deficiency (MMA is normal in folate deficiency and elevated in vitamin B12 deficiency )
Differential diagnoses of vitamin B12 deficiency
|Differential diagnosis of vitamin B12, B9, and B1 deficiencies|
|Vitamin B12 deficiency||Vitamin B9 deficiency||Vitamin B1 deficiency|
|Causes|| || || |
|Motor signs|| || || |
|Sensory signs|| || || || || |
|Other neurological features|| || |
|Neuropsychiatric signs|| || || || || |
|Diagnostics|| || |
| || || || |
|Management|| || |
|Prognosis|| || || || |
Starting folate treatment before excluding vitamin B12 deficiency may correct anemia, but it can worsen neuropathy!
In contrast to vitamin B12 deficiency, folic acid deficiency is generally not associated with neurological symptoms.
Other causes of macrocytic anemia
- 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.
- IM supplementation of vitamin B12 (e.g., cyanocobalamin, hydroxycobalamin)
- Prevent future vitamin B12 deficiency by treating the underlying disease if possible (causative therapy): e.g., in patients with fish tapeworm infestation, intestinal bacterial overgrowth, or chronic pancreatitis. 
- If causative therapy is not possible, lifelong IM supplementation may be necessary.
- Secondary folate deficiency: Folate supplementation can improve and mask hematologic symptoms but has no influence on neurologic symptoms of vitamin B12 deficiency.