Ileum
The ileum is one of 3 parts of the small intestine[1]; the final and longest part that transitions into the large intestine[2]. Movement through the intestines is controlled by peristalic contractions, however since the inner walls of the illeum are relatively thin, mavement through this part of this intestine is significantly slower[3], aiding absorbtion of bile salts and B12 whish is the ileums main function .
Diseases effecting the ileum, such as Crohn's disease, can lead to defficiencies in B12 and cause diarrhea[4].