Editing Intrinsic factor (section)

== Site of secretion ==
Intrinsic factor is secreted by [[parietal cell]]s within the stomach, and so is present in the gastric juice as well as in the [[Gastric mucosa|gastric mucous membrane]].<ref name="Sharma_2016">{{cite book | vauthors = Sharma K | chapter = Gastrointestinal System | title = Textbook Of Biochemistry, Biotechnology, Allied And Molecular Medicine | pages = 632 | date = 2016 | veditors = Talwar G, Hasnain SE, Sarin SK | publisher = PHI Learning Private Limited | isbn = 978-81-203-5125-7 | edition = 4th | chapter-url = https://books.google.com/books?id=mVuNCwAAQBAJ&q=gastric+intrinsic+factor+secreted++parietal+cells&pg=PA632 | department = (secondary) }}</ref> The optimum pH for its action is approximately 7.<ref name="Shum_1971">{{cite journal | vauthors = Shum HY, O'Neill BJ, Streeter AM | title = Effect of pH changes on the binding of vitamin B12 by intrinsic factor | journal = Journal of Clinical Pathology | volume = 24 | issue = 3 | pages = 239–243 | date = Apr 1971 | pmid = 5103294 | pmc = 476962 | doi = 10.1136/jcp.24.3.239 }}</ref> Its concentration does not correlate with the amount of [[hydrochloric acid|HCl]] or [[pepsin]] in the gastric juice, e.g., intrinsic factor may be present even when pepsin is largely absent.<ref name="Poliner_1958">{{cite journal | vauthors = Poliner IJ, Spiro HM, Pask BA, Trocchio N | title = The independent secretion of acid, pepsin, and intrinsic factor by the human stomach | journal = Gastroenterology | volume = 34 | issue = 2 | pages = 196–209 | date = Feb 1958 | pmid = 13512593 | doi = 10.1016/S0016-5085(58)80102-X | department = (primary) }}</ref> The site of formation of the intrinsic factor varies in different species. In pigs it is obtained from the [[pylorus]] and beginning of the [[duodenum]];<ref name="Heatley_1954">{{cite journal | vauthors = Heatley NG, Florey H, Turnbull A, Jennings MA, Watson GM, Wakisaka G, Witts LJ | title = Intrinsic factor in the pyloric and duodenal secretions of the pig | journal = Lancet | location = London, England | volume = 267 | issue = 6838 | pages = 578–580 | date = Sep 1954 | pmid = 13193076 | doi = 10.1016/S0140-6736(54)90355-4 | department = (primary) }}</ref>  in human beings it is present in the [[Fundus (stomach)|fundus]] and body of the [[stomach]].<ref name="Howard_1996">{{cite journal | vauthors = Howard TA, Misra DN, Grove M, Becich MJ, Shao JS, Gordon M, Alpers DH | title = Human gastric intrinsic factor expression is not restricted to parietal cells | journal = Journal of Anatomy | volume = 189 | issue = Pt 2 | pages = 303–313 | date = Oct 1996 | pmid = 8886952 | pmc = 1167747 | department = (primary) }}</ref>

The limited amount of normal human gastric intrinsic factor limits normal efficient absorption of B<sub>12</sub> to about 2&nbsp;μg per meal, a nominally adequate intake of B<sub>12</sub>.<ref>{{cite journal | vauthors = Watanabe F | title = Vitamin B12 sources and bioavailability | journal = Experimental Biology and Medicine | location = Maywood, N.J. | volume = 232 | issue = 10 | pages = 1266–1274 | date = Nov 2007 | pmid = 17959839 | doi = 10.3181/0703-MR-67 | s2cid = 14732788 | department = (review) }}</ref>