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B12 and Folate
 
 

Vitamin B12

 
 
 
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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.
  • Physiology

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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