Vitamin B12 | B12 and Folate - MedSchool
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B12 and Folate
 
 
B12 and Folate
Vitamin B12, also known as cobalamin, is an important cofactor in the homocysteine metabolism, haemoglobin synthesis and myelination.
 

Vitamin B12

 
 

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
Last updated on January 13th, 2020
 
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