I found something very interesting: The enterohepatic circulation of vitamin B-12 is very important in vitamin B-12 economy and homeostasis (27). Nonvegetarians normally eat 2-6 mcg of vitamin B-12/d and excrete from their liver into the intestine via their bile 5-10 mcg of vitamin B-12/d. If they have no gastric, pancreatic, or small bowel dysfunction interfering with reabsorption, their bodies reabsorb ~3-5 mcg of bile vitamin B-12/d. Because of this, an efficient enterohepatic circulation keeps the adult vegan, who eats very little vitamin B-12, from developing vitamin B-12 deficiency disease for 20-30 y (27) because even as body stores fall and daily bile vitamin B-12 output falls with body stores to as low as 1 mcg, the percentage of bile vitamin B-12 reabsorbed rises to close to 100%, so that the whole microgram is reabsorbed.http://www.ajcn.org/content/59/5/1213S.long Basically, vit b-12 is excreted via bile and the reabsorbed in the gut. The degree of excretion and reabsorption seems to be adjusted with regards to b-12 intake and b-12 status. Now I don't now how fast this adjustment is, but I would imagine it has some lag would explain for me why some changes in nutrition have only temporary improvement, as one might end up with an temporary increased b-12 status after such a change.