Vitamin B12 and Folate Quiz

See Vitamin B12. Option E is correct. Dietary vitamin B12 is first freed from food protein and bound to haptocorrin from the saliva. In the small intestine it is passed on to intrinsic factor, a glycoprotein made by the parietal cells of the stomach. The B12-intrinsic factor complex travels intact through the small intestine and binds to specific receptors on the lining cells of the terminal ileum, the last stretch of the small intestine before the colon. This is receptor-mediated uptake: the cell engulfs the complex only after it docks with its receptor. Inside the cell, B12 is released and joins transcobalamin II, the carrier that takes it to the tissues, particularly the liver. This anatomy explains the causes of deficiency: lack of intrinsic factor (pernicious anaemia, the most common cause of severe deficiency in the UK, or gastrectomy) and disease or resection of the ileum, such as Crohn disease, which reduces absorption of the complex. A memory aid: the stomach makes the key, and the ileum has the lock.

• Carriers: B12 binds haptocorrin, then intrinsic factor made by the parietal cells of the stomach.
• Site: The complex binds receptors on the lining cells of the terminal ileum.
• Uptake: Receptor-mediated, as the cell engulfs the complex only after it docks with its receptor.
• Causes of deficiency: Lack of intrinsic factor, as in pernicious anaemia or gastrectomy, and ileal disease or resection such as Crohn disease.