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 peristaltic contractions, however since the inner walls of the ileum are relatively thin, movement through this part of this intestine is significantly slower[3], aiding absorption of bile salts and B12 which is the ileums main function.
Diseases effecting the ileum, such as Crohn's disease, can lead to deficiencies in B12 and cause diarrhoea[4].