Expansion
Bound by R protein, transferred to intrinsic factor, absorbed in the terminal ileum
Expansion
- Stomach: acid and pepsin release B12 from dietary protein; parietal cells secrete intrinsic factor
- B12 binds R protein (haptocorrin) from saliva, which protects it from acid
- Duodenum: pancreatic proteases degrade R protein, freeing B12 to bind intrinsic factor
- Terminal ileum: the B12-intrinsic factor complex binds cubilin receptors and is absorbed
- Transported in blood on transcobalamin II
Causes of deficiency map onto the steps:
- Diet: strict vegan diet
- Stomach: pernicious anaemia (autoimmune loss of parietal cells), gastrectomy, atrophic gastritis, long-term proton pump inhibitors
- Pancreas: chronic pancreatitis
- Small bowel: bacterial overgrowth (bacteria consume B12), fish tapeworm
- Terminal ileum: Crohn disease, ileal resection
Liver stores last 3 to 5 years, which is why deficiency develops slowly. Folate by contrast has stores of only a few months, and giving folate alone can correct the anaemia while allowing subacute combined degeneration of the cord to progress.