Overview
Vitamin B12, also known as cobalamin, is an important cofactor in the homocysteine metabolism, haemoglobin synthesis and myelination.
Normal Range
- 150 - 750 pmol/L
Physiology
Absorption
- Vitamin B12 is isolated from ingested food by the action of stomach acid. If then binds to transcobalamin-1, which is secreted into salivary and gastric fluid. In the alkaline small bowel transcobalamin-1 is degraded by pancreatic enzymes. Free B12 then binds to intrinsic factor, a protein secreated by gastric parietal cells. The intrinsic factor-B12 complex is then absorbed within the terminal ileum.
Free B12 is not effectively absorbed by itself.
Roles
- Vitamin B12 is required for the action of methionine synthese, which converts homocysteine to the essential amino acid methionine. Methionine is converted to S-adenosyl-methionine, which is involved in methylation of DNA and RNA.
B12 is also a cofactor for the conversion of L-methylmalonyl-CoA into succinyl-CoA by L-methylmalonyl-CoA mutase. Succinyl CoA is an intermediate in the citric acid cycle and is also involved in the synthesis of haemoglobin.
Vitamin B12 Deficiency
Look For
- Reduced Vitamin B12 level
- Macrocytic, hyperchromic anaemia
- Hypersegmented neutrophils
Manifestations
- Manifestations
- Megaloblastic anaemia
- Atrophic glossitis
- Peripheral neuropathy
- Dementia
Causes of B12 Deficiency
- Atrophic gastritis - pernicious anaemia, H pylori infection
- Malabsorption - coeliac disease, tropical sprue, Crohn's disease, alcohol abuse
- Surgical resection - gastrectomy, gastric bypass, resection of terminal ileum
- Pancreatic insufficiency - chronic pancreatitis
- Inherited disorders - Imerslund-Gräsbeck syndrome
Dietary deficiency is rare as a cause of B12 deficiency.
Elevated Vitamin B12
Hypercobalaminaemia may be due to excess parenteral vitamin B12 administration, though have also been associated with certain other malignant and non-malignant conditions.
Causes of Hypercobalaminaemia
- Excessive vitamin B12 administration
- Solid organ malignancy
- Haematologic malignancy
- Autoimmune disease
- Acute or chronic liver disease
- Renal failure
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