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Intestinal Absorptive Function Gut: First Published As 10.1136/Gut.35.1 Suppl.S5 on 1 January 1994

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Intestinal Absorptive Function Gut: First Published As 10.1136/Gut.35.1 Suppl.S5 on 1 January 1994 Gut 1994; supplement 1: S5-S9 S5 Intestinal absorptive function Gut: first published as 10.1136/gut.35.1_Suppl.S5 on 1 January 1994. Downloaded from R C Spiller Abstract causing them to lose some of their ordered The normal gut is adapted to intermittent array permitting better access of salivary feeding with complex macromolecular amylase.1 Grinding by the teeth and later by substrates of low sodium content. The the action of the 'antral pump', combined with high permeability of the upper small the action of salivary amylase, reduces the food intestine to sodium, together with sodium to an homogenous chyme, which is pumped rich saliva and pancreaticobiliary secre- into the duodenum at a rate of about 1-2 tions results in large sodium fluxes into kcallmin. Once within the duodenum, starch is the lumen. These substantial sodium hydrolysed extremely rapidly because of a influxes are matched by equally large superabundance of pancreatic amylase, which effluxes from the ileum and proximal cleaves the ot 1-4 glucocidic bonds in starch, colon, which are comparatively imperme- reducing the polymer to maltose, maltotriose, able to sodium and capable of active and ao limit dextrins. Attached to the brush sodium absorption. Resection of these border, facing into the gut lumen are the brush distal, sodium absorbing regions of the border hydrolases, large glycoproteins now intestine, lead to problems with sodium sequenced, which include maltase, isomaltase- depletion. Controliled transit of chyme is sucrase, and glucosidases capable of reducing essential to permit time for optimium ot limit dextrins and other polymers containing digestion and absorption and a range less than six glucose monomers, to glucose of feedback control mechanisms exist. monomer. This then binds to the glucose/ Partially digested nutrients, both in the sodium cotransporter, a 72 kilo dalton protein, duodenum and ileum, exert inhibitory whose affinity for glucose is appreciably feedback to delay delivery of further increased when it combines with sodium. nutrients and here again surgery may Depending upon exactly where in the body compromise these reflexes. Brush border the cell membrane is derived, either one or hydrolase values are strongly influenced two sodium molecules bind to the transport by luminal nutrient concentrations, being protein. This then undergoes conformational impaired by malnutrition and total paren- change, which transports the glucose and teral nutrition, but restored by enteral sodium across the cell membrane to be feeding. Viscous fibre slows absorption released into the cytoplasm.2 In effect this http://gut.bmj.com/ and may delay transit through mech- system uses the chemical gradient of sodium anisms that are as yet uncertain. Whether across the enterocyte brush border membrane, and how novel substrates activate normal generated by the 'pumping' action of the baso- control mechanisms will be important lateral Na+/K+ ATPase, to energise active factors determining their effectiveness glucose transport. and patient acceptability. on September 25, 2021 by guest. Protected copyright. (Gut 1994; supplement 1: S5-S9) Delivery ofnutrients to the small intestine Absorption in its most restricted sense consists Emptying of gastric chyme is to a considerable of the process whereby nutrients pass from the extent controlled by feedback inhibition, gut lumen either across the brush border cell whereby nutrients and hyperosmolar solutions membrane after binding to a specific transport within the duodenum inhibit antral peristalsis protein, or through the intercellular tight and reduce fundal tone, causing food to be junction, in effect crossing the barrier between redistributed more proximally within the the external melieu and the organism. This stomach. This 'duodenal brake' is the first of process requires a very extensive 'preprocess- many such control loops whereby the arrival ing' of food to permit the intimate contact of excess nutrients distally delays further between substrate and enterocyte needed to delivery, hence permitting more time for permit the binding of nutrient to the various digestion and absorption. Such feedback loops transport proteins. are vital because of the very variable nutrient content and digestibility of food. Detectors positioned distally can assess how successful Preprocessing of food before absorption digestion has been and moderate the flow of Food is usiWlly composed of macromolecules, chyme to minimise malabsorption. The best Queen's Medical often part of other organisms and hence has example of what happens if these feedback Centre, University as walls that are lost is the malabsorption that Hospital, Nottingham complex structures such cell loops R C Spiller require breaking down before the nutrients can commonly accompanies the preciptious and be accessed. This is best illustrated by con- uncontrolled gastric emptying seen in patients Correspondence to: Dr R C Spiller, Queen's sidering the digestion of starch. Mechanical with the 'dumping syndrome' after vagotomy Medical Centre, University and chemical degradation begins with cooking, and pyloroplasty, or to a lesser degree in most Hospital, Nottingham NG7 2UH. which hydrates the long starch polymers patients with a gastroenterostomy. S6 Spiller sodium per 24 hours, which it reduces to about 6 Input 200,4 the colon absorbing 950/o of this to result in stool output of 10-20 mmol/24 h. The EE .Ea ileum is adapted to absorbing sodium against Gut: first published as 10.1136/gut.35.1_Suppl.S5 on 1 January 1994. Downloaded from 7- its electrochemical gradient and this requires that the ileum be impermeable to sodium. It is, 6- however, more permeable to chloride ions and 5- so does not generate much of a lumen-serosa E potential difference when exposed to an 4- isotonic salt solution. By contrast the colon is 0 c highly impermeable to both sodium and c. 3- Jejunal chloride and generates a high electrochemical 2- gradient by active electrogenic sodium trans- Ileal port, lowering the luminal sodium concentra- tion to 20-30 mmol and the luminal-serosa 0- 1 3 5 7 9 11 13 15 17 19 potential difference to -35 mV.7 Hours Figure 1: Flow through the upper gastrointestinal tract studied over 20 hours by means of a slow marker infusion technique combined with aspiration from the upperjejunum and distal SIGNIFICANCE OF REGIONAL DIFFERENCES IN ileum.4 ABSORPTION These differences become of great importance Enteral nutrition to a large extent avoids when the functions of various regions of the such problems by in effect doing most of the .gut are lost by disease or surgical resection. mechanical and chemical preprocessing for the Ileal resection results in a doubling of the patient. Thus commercial enteral diets are sodium load to the colon, although this can homogenised, isotonic, and often partially usually be compensated for by increased digested. Furthermore by using a pump to colonic absorption,8 stool output does increase deliver nutrients at 1 cal/min problems with in most cases.9 The major problem is when the abnormal gastric emptying can be avoided colon is also resected when the vast sodium except in exceptional cases such as severe head flow from the jejunum becomes stoma effluent. injury, when gastric stasis can be profound.3 This point is well made by the studies on the short bowel patients (Fig 2). Patients with less than 100 cm of jejunum remaining could not Intestinal absorption absorb sufficient salt to avoid desalination and Before considering absorption from enteral were dependent on daily intravenous saline.10 diets it is worth considering the normal In these patients salt balance is highly precari- response to feeding as these are the conditions ous but vital to control. The situation can be that the gut has evolved to cope with. Figure 1 much improved by careful choice of fluids http://gut.bmj.com/ shows that although input into the organism is pulsatile, the stomach reduces this very consid- erably so that the fluctuation in flow is less 600r in the jejunum, and the ileum is exposed to c 0 400 H still smaller variations.4 In general there are ._ important differences between the jejunum, o0 H 200 on September 25, 2021 by guest. Protected copyright. which is exposed to high flow and high con- .0U) I centrations of nutrients compared with the ileum, which receives a smaller load and must E 1 = = - - absorb from lower concentrations. These 0 -200 o0 o differences in load result in quite considerable (I) a differences in absorptive characteris- 0 1 o regional CllnI{ tics as is clearly seen with sodium absorption. 40 80 120 160 80r SODIUM ABSORPTION 0 U . Most of our food has a low sodium concentra- 60 H- U tion comparative with blood. A normal beef- .0o. 0 burger meal, if homogenised, has a sodium 0 40 F- oCu 0 concentration of only 35 mmol. Sodium rich n1 a H 0 saliva raises this somewhat but in the stomach QCL 20 acid secretion lowers the sodium content still to U further. The duodenum is therefore exposed c a sodium concentration of 20-30 mmol. The UJi 0 -zul jejunum cannot, however, maintain such a 0 20 40 60 80 100 120 140 160 gradient being highly permeable to sodium, Jejunal length (cm) which is rapidly secreted into the gut lumen.5 Figure 2: Influence ofjejunal length on salt and energy This secretion, combined with the very rapid balance in patients with jejunostomy. Fourteen patients simultaneous absorption of water, means that, with extensive small bowel resection and a highjejunostomy and one patient with ajejunorectal anastomosis had by the time the chyme reaches the ileum, the balance studies. No patient with less than 100 cm of sodium concentration has reached 120 mmol. jejunum could maintain a positive sodium balance by the The ileum receives approximately 400 mmol of enteral route alone. 10 Intestinal absorptive function S7 once the implications of the higih jejunal per- TABLE II Flow and motility during continuous enteral nutrition.
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