Where are folate, vitamin B12, and thiamine absorbed in the gastrointestinal (GI) tract?

Absorption Sites of Folate, Vitamin B12, and Thiamine in the Gastrointestinal Tract

Folate is absorbed primarily in the duodenum and jejunum, vitamin B12 is absorbed exclusively in the terminal ileum, and thiamine is absorbed predominantly in the proximal small intestine (duodenum and jejunum).

Folate Absorption

• Absorption site: Duodenum and jejunum (proximal small intestine) 1

• Absorption mechanism:

  • Occurs through an active, carrier-mediated, saturable transport process 1
  • Dietary folates (polyglutamates) must first be hydrolyzed to monoglutamates before absorption 2
  • Vitamin C improves folate bioavailability by limiting degradation in the stomach 1

• Important considerations:

  • Folate is not stored in the body in large amounts, making constant dietary supply necessary 1
  • Folate works closely with vitamin B12; deficiency of one can affect the metabolism of the other 3
  • High doses of folate (>1 mg/day) may mask vitamin B12 deficiency by correcting hematological abnormalities while allowing neurological damage to progress 3

Vitamin B12 (Cobalamin) Absorption

• Absorption site: Terminal ileum exclusively 4, 5

• Absorption mechanism:

  • Complex process requiring multiple steps:
    1. In the stomach: B12 is released from food and binds to haptocorrin 6
    2. In the duodenum: Pancreatic proteases degrade haptocorrin, and B12 transfers to intrinsic factor (IF) 6
    3. In the terminal ileum: The B12-IF complex is recognized by a specific receptor complex composed of cubilin and amnionless proteins 5, 6
  • A small amount (~1% of ingested amount) can be absorbed by simple diffusion, but only with very large doses 4

• Important considerations:

  • Vitamin B12 is not present in foods of plant origin 4
  • Intrinsic factor deficiency causes pernicious anemia 4
  • Malabsorption is common in the elderly and in conditions affecting the terminal ileum 5, 6
  • After absorption, B12 is transported in blood by transcobalamin binding proteins 4

Thiamine Absorption

• Absorption site: Proximal small intestine (duodenum and jejunum) 1, 7

• Absorption mechanism:

  • Absorbed through a carrier-mediated, sodium-independent process 2, 7
  • Absorption appears to be controlled and limited compared to other B vitamins 7
  • Well absorbed orally except in cases of alcohol-related gastritis 1

• Important considerations:

  • Thiamine is not stored in large amounts in the body 1
  • After oral administration, thiamine shows modest increases in serum concentration followed by active renal clearance 7
  • Absorption can be upregulated during thiamine deficiency 2

Clinical Implications

• Malabsorptive disorders significantly affect vitamins requiring specialized digestive or transport mechanisms (B12, folate, and fat-soluble vitamins) 8

• Specific gastrointestinal conditions affecting absorption sites:

  • Terminal ileal disease or resection: B12 malabsorption 1, 6
  • Proximal small bowel disease: Folate and thiamine malabsorption 1, 8
  • Bariatric surgery: Risk for all three vitamins, requiring supplementation 1, 6

• Supplementation considerations:

  • Multivitamin supplementation is recommended after restrictive bariatric procedures 1
  • Vitamin B12 may require parenteral administration in cases of malabsorption 4
  • Thiamine supplementation (100-300 mg/day) should be considered in patients with reduced food intake or high alcohol consumption 1

Understanding these specific absorption sites is crucial for diagnosing and managing nutritional deficiencies, especially in patients with gastrointestinal disorders or after surgical procedures affecting specific segments of the GI tract.