How Vitamin B12 is Absorbed
Vitamin B12 (cobalamin) is absorbed through a complex, multi-step process that requires intrinsic factor binding in the stomach and selective uptake in the distal ileum via the cubilin-amnionless receptor complex 1.
The Absorption Pathway
Step 1: Release and Initial Binding in the Stomach
- Dietary B12 is released from food proteins by gastric acid and pepsin in the stomach
- Once freed, B12 initially binds to haptocorrin (HC), a salivary protein that protects B12 in the acidic gastric environment 2
- Simultaneously, gastric parietal cells secrete intrinsic factor (IF), which is essential for subsequent absorption 3
Step 2: Transfer in the Duodenum
- In the duodenum, pancreatic proteases degrade haptocorrin
- The pH change triggers transfer of B12 from haptocorrin to intrinsic factor 2
- This IF-B12 complex is resistant to further digestion and travels to the ileum
Step 3: Absorption in the Distal Ileum
- The distal ileum is the exclusive site of physiological B12 absorption 1
- The IF-B12 complex binds to a highly specific receptor composed of two proteins: cubilin (CUBN) and amnionless (AMN) 1, 4
- This receptor-mediated endocytosis allows B12 to enter ileal enterocytes 5
Step 4: Systemic Distribution
- After crossing the ileal cell, B12 is released into the bloodstream
- It binds to transcobalamin II (TC), which transports it to all body tissues 2, 4
- The liver stores significant B12, some of which undergoes enterohepatic circulation through bile 2
Clinical Implications of This Absorption Mechanism
Why Malabsorption Occurs
Gastric causes:
- Lack of intrinsic factor (pernicious anemia, gastrectomy) prevents IF-B12 complex formation 3
- Gastric achlorhydria and atrophic gastritis impair release of B12 from food proteins, affecting approximately 15% of adults over 60 3
Pancreatic causes:
- Pancreatic insufficiency prevents degradation of haptocorrin, blocking B12 transfer to intrinsic factor 5
Ileal causes:
- Resection of >20 cm of distal ileum eliminates the absorption site 1
- Inflammatory bowel disease (particularly Crohn's disease) affecting the ileum impairs receptor function 1
- Inherited disorders (Imerslund-Gräsbeck syndrome) involve defective cubilin-amnionless receptors 4, 6
Bariatric surgery:
- Roux-en-Y gastric bypass impairs both protein-bound B12 digestion and IF-B12 complex formation 5
Alternative Absorption Pathway
Passive diffusion allows approximately 1-2% of oral crystalline B12 to be absorbed independent of intrinsic factor across the entire intestinal mucosa 3. This explains why high-dose oral B12 (1000 μg daily) can effectively treat most B12 deficiency, even without intrinsic factor 3.
Key Clinical Pitfalls
- The ileum is the only physiological absorption site - colonic bacteria do not contribute significantly to serum B12 levels 7
- Protein-bound B12 from food requires gastric acid for release, while crystalline B12 supplements do not 3
- Metformin increases B12 deficiency risk (7-11% absolute risk over 4 years) through unclear mechanisms 3
- Folate supplementation does not mask B12 deficiency anemia in most cases, contrary to older teaching 3