Normal absorption of vitamin B12

• acidic environment of the stomach facilitates the breakdown of vitamin B12 that is bound to food
  
  
  • intrinsic factor, which is released by parietal cells in the stomach, binds to vitamin B12 in the duodenum
  • vitamin B12-intrinsic factor complex subsequently aids in the absorption of vitamin B12 in the terminal ileum
    
    
• in addition to this method of absorption, evidence supports the existence of an alternate system that is independent of intrinsic factor or even an intact terminal ileum
  
  
  • approximately 1 percent of a large oral dose of vitamin B12 is absorbed by this second mechanism (1)
    • this pathway is important in relation to oral replacement

Once absorbed, vitamin B12 binds to transcobalamin II and is transported throughout the body

Notes:

• in humans, only two enzymatic reactions are known to be dependent on vitamin B12
  
  
  • methylmalonic acid is converted to succinyl-CoA using vitamin B12 as a cofactor
    • vitamin B12 deficiency can lead to increased levels of serum methylmalonic acid
      
      
  • homocysteine is converted to methionine by using vitamin B12 and folic acid as cofactors
    • therefore a deficiency of vitamin B12 or folic acid may lead to increased homocysteine levels

Reference:

1. Sukumar N et al. Investigating vitamin B12 deficiency. BMJ. 2019 May 10;365:l1865;