THE WEIGHT OF EVIDENCE in man and in several species of animals, including the dog, indicates that the major portion of orally administered vitamin B12 is absorbed from the distal ileum.1-4 Other investigators, however, have variously indicated that the maximal absorption of vitamin B12 takes place in the duodenum,5 in the jejunum,6 or throughout the entire small intestine.7
Studies also indicate that a substance from gastric mucosa, probably a mucopolysaccharide termed intrinsic factor, forms a complex with vitamin B12, thereby enhancing its absorption by the intestine. It has been suggested that a possible explanation for the conflicting evidence as to the site of absorption of vitamin B12 may be the inconstant availability of endogenous intrinsic factor during transit from stomach to ileum. This might result from the binding of vitamin B12 by proteins without intrinsic factor activity, making it unavailable
Drapanas T, Williams JS, McDonald JC, Heyden W, Bow T, Spencer RP. Role of the Ileum in the Absorption of Vitamin B12 and Intrinsic Factor (NF). JAMA. 1963;184(5):337–341. doi:10.1001/jama.1963.73700180001009
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