Intrinsic factor (IF) and haptocorrin (transcobalamin I) are necessary for the absorption of vitamin B12

Intrinsic factor (IF), cobalamin binding intrinsic factor, also known as gastric intrinsic factor (GIF), is a glycoprotein produced by the parietal cells (in humans) or chief cells (in rodents) of the stomach. It is necessary for the absorption of vitamin B₁₂ later on in the distal ileum of the small intestine. In humans, the gastric intrinsic factor protein is encoded by the CBLIFgene.

• Pocock G, Richards C (2006). Human Physiology: The Basis of Medicine (3rd ed.). Oxford University Press. p. 230. ISBN 978-019-856878-0
• “CBLIF – Cobalamin binding intrinsic factor precursor – Homo sapiens (Human) – CBLIF gene & protein”. www.uniprot.org. Retrieved 15 March 2022.PMID 22116708.

Haptocorrin (transcobalamin I) is another glycoprotein secreted by the salivary glands which binds to vitamin B₁₂. Vitamin B₁₂ is acid-sensitive and in binding to haptocorrin it can safely pass through the acidic stomach to the duodenum.

• Fedosov SN (2012). “Physiological and molecular aspects of cobalamin transport”. Water Soluble Vitamins. (review). Subcellular Biochemistry. Vol. 56. pp. 347–67. doi:10.1007/978-94-007-2199-9_18. ISBN 978-94-007-2198-2. 

In the less acidic environment of the small intestine, pancreatic enzymes digest the glycoprotein carrier and vitamin B₁₂ can then bind to intrinsic factor. This new complex is then absorbed by the epithelial cells (enterocytes) of the ileum. 

• Fedosov SN (2012). “Physiological and molecular aspects of cobalamin transport”. Water Soluble Vitamins. (review). Subcellular Biochemistry. Vol. 56. pp. 347–67. doi:10.1007/978-94-007-2199-9_18. ISBN 978-94-007-2198-2. PMID 22116708.

Inside the cells, vitamin B₁₂ dissociates once again and binds to another protein, transcobalamin II; the new complex can then exit the epithelial cells to be carried to the liver.

• Alpers DH, Russell-Jones G (May 2013). “Gastric intrinsic factor: the gastric and small intestinal stages of cobalamin absorption. A personal journey”. (review). Biochimie. 95 (5): 989–94. doi:10.1016/j.biochi.2012.12.006. PMID 23274574.