The primary function of the small intestine is the absorption of nutrients, minerals and vitamins released from food through the process of digestion. This presents a paradox to the intestine, which is specifically designed to stop molecules released from pathogens such as viruses, bacteria and parasites from crossing into the circulation. Initially it was thought that the process of absorption was highly non-specific such that all molecules simply diffused through the cells that line the small intestine and from there enter the circulation. More recently it has been discovered that the process of absorption is a highly specific process that requires specific pores and transporters to enable more specific uptake of molecules such as water soluble vitamins, fats, fat soluble vitamins, minerals, amino acids, sugars and there are even pores involved in uptake of water (aquaporins). Even small sugar molecules such as hexose and glucose are transported via the sodium-glucose transporters. There are transporters for individual amino acids, for dipeptides and nucleotides, as well as organic cation transporters, organic anion transporters, bile acid transporters and fatty acid transporters, and there are even transporters for small atoms such as copper. Basically there appear to be some sort of transporter for all small dietary molecules (1).
That is fine for very small molecules, but what about larger molecules such as the larger water soluble vitamins (folate, biotin, riboflavin and vitamin B12) and proteins such as insulin, IGF1, lactoferrin, and even very large proteins such as antibody molecules, how do they get across the gut? It turns out that this problem is not unique to the gut, but also occurs in the alveolar epithelium, the vascular endothelium, the renal proximal tubular endothelium and the blood brain barrier.
It is apparent, though that very large molecules do in fact cross the intestinal cells as can be attested to by allergic reactions to dietary allergens and the response to many diarrhea-causing gut pathogens. It has been established that entry of these larger molecules requires that they either bind to, or are bound by some sort of recognition molecule on the intestinal wall. Generally this is a receptor that is specific for the molecule. Once the receptor has bound to the molecule to be taken into the cell, the cell envelopes the molecule in a process called endocytosis and then subsequently transports the molecule across the gut.
Transport in the small intestine https://www.youtube.com/watch?v=8Fp91-ostBE
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26. Russell-Jones G. J. 2000 Use of the vitamin B12 transport system to enhance the oral bioavailability of peptides and proteins. In “Drug Targeting in the Gastrointestinal Tract” Ed. Josef Tukker. Harwood Academic Publishers.
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The statements on this site compose a compendium of generally recognized signs of vitamin B12 deficiency, and problems that can then ensue They also are formulated from a summary of relevant scientific publications. In addition they may contain some forward looking statements of a general nature.
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