Intrinsic factor and vitamin B12 absorption
Parietal cells (the stomach cells that secrete acid) make intrinsic factor, a protein that binds vitamin B12 for absorption in the last part of the small intestine.
Remember
Without the intrinsic factor that parietal cells make, vitamin B12 cannot be absorbed, so the body runs out of B12 (B12 deficiency).
Memory aid
no intrinsic factor, no passport into the ileum
Think of intrinsic factor as the passport that vitamin B12 needs to get into the ileum.
Parietal cells in the stomach lining secrete acid and intrinsic factor, a protein that binds vitamin B12. Acid and pepsin (a protein-digesting enzyme) help to release B12 from food. In the duodenum, the freed B12 is then handed to intrinsic factor.
The B12 and intrinsic factor are absorbed together in the terminal ileum, the last part of the small intestine. With no intrinsic factor, B12 is not absorbed even when the diet contains enough, and this causes B12 deficiency.
In pernicious anaemia the immune system attacks the parietal cells and intrinsic factor, so the body runs out of B12. The deficiency causes a macrocytic anaemia (an anaemia with large red cells), tiredness, glossitis (a sore, smooth tongue) and nerve damage with tingling (paraesthesiae). In pernicious anaemia, treatment is by B12 injections, which bypass the gut.
The stomach wall has different cells, each paired with its own product:
- Chief cells make pepsinogen, the inactive form of pepsin.
- G cells release the hormone gastrin, which increases acid secretion.
- D cells release somatostatin, which inhibits acid and gastrin secretion.
- Mucous cells make the mucus that protects the lining from acid.
Key facts
- Source
- Parietal cells in the stomach lining secrete intrinsic factor as well as acid.
- Job
- Intrinsic factor is a protein needed to carry vitamin B12 to its site of absorption.
- Release and handover
- Acid and pepsin help to release B12 from food, and the freed B12 is then handed to intrinsic factor in the duodenum.
- Absorption site
- The B12 and intrinsic factor are absorbed in the terminal ileum.
- Without intrinsic factor
- B12 deficiency follows, and with it a macrocytic anaemia and nerve damage.
- Pernicious anaemia
- An autoimmune attack on the parietal cells and intrinsic factor, which causes B12 deficiency and a macrocytic anaemia.
Exam traps
- Chief cells do not make intrinsic factor, because they secrete pepsinogen, the inactive form of pepsin.
- G cells do not make intrinsic factor, because they release gastrin, which increases acid secretion.
- Mucous cells do not make intrinsic factor: they make the mucus that protects the lining, and loss of these cells would predispose to ulcers, not B12 deficiency.
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