Under normal circumstances, vitamin B12 is found in animal products. Upon ingestion, stomach acid releases the B12 from the food, and it binds to intrinsic factor, a glycoprotein that binds to and protects B12 from further degradation. The B12-intrinsic factor then travels down the intestine as usual, and is absorbed at the terminal ileum.
A B12 deficiency can therefore result from two general categories. Either:
- not enough is being eaten - e.g. strict vegans, or
- not enough is being absorbed - intrinsic factor deficiency (from chronic atrophic gastritis, or stomach surgery), disease or resection of the terminal ileum (e.g. Crohn's), or bacterial overgrowth of the bowel (bacteria also like B12!)
The Shilling test helps to shed some light as to the B12 deficiency's aetiology. As I mentioned, there are two parts to it.
In the first phase, the aim is to determine whether or not there is an absorption problem (i.e. one of the second category's causes). Firstly, a B12 injection is administered intramuscularly, so that the body's stores are saturated. Thereafter radiolabeled B12 is given orally. Because the body's stores are full, the additional oral B12 should be absorbed but not well utilised by the tissues, and so a significant proportion (> 5%) should end up in the urine. If this figure is less than 5%, it can be assumed that not enough of the oral B12 was absorbed.
The second part of the test seeks to determine whether this malabsorption is due to intrinsic factor deficiency. The protocol is the same, except that intrinsic factor is administered along with the oral B12. If the problem was a shortage of intrinsic factor, this should remedy things. On the other hand, if B12 is still not well absorbed despite this, it implies that the malabsorption was not due to an intrinsic factor deficiency. You obviously only need to do the second part of the Schilling test if the first one showed a pathological result.