Diet and the Gastrointestinal Tract

CHAPTER 10 Diet and the Gastrointestinal Tract




OBJECTIVES


By the end of this chapter you should be able to:



• summarize the main features of digestion and absorption


• describe the most common chronic diseases which affect the intestinal tract, their epidemiology and clinical presentation, aetiology and pathophysiology


• discuss the role of diet in their management


• discuss current understanding of food allergies and intolerance



10.1 INTRODUCTION


The primary function of the gastrointestinal tract is to facilitate digestion and the absorption of nutrients, although it also makes an important contribution to the body’s immune function. Intestinal function is modulated by gastrointestinal peptide hormones and an enteric nervous system (ENS). The GI tract comprises the mouth, pharynx, oesophagus, stomach, small intestine (duodenum, jejunum and ileum), the large bowel or colon, and the rectum and anus (Fig 10.1). Digestion is initiated in the mouth, continues in the stomach, and is completed in the small intestine. This process is aided by the presence of enzymes in saliva and gastric juices, and those secreted into the small intestine, as well as bile salts released by the gall bladder. Undigested material travels to the large bowel, where bacterial fermentation can occur, with the production of stool which is excreted via the rectum.


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Figure 10.1 The gastrointestinal tract (a) and movement of fluid through different regions (b). L = litres. (The daily amounts and sites of fluid secretion and absorption are from data in Ma & Verkman 1999.)


The immune system of the gastrointestinal tract has a number of different roles. Following ingestion of food or beverage the general trend is towards suppression of immunity to allow the digestion of substances foreign to the body, in other words, oral tolerance. However, active immunization may also follow the feeding of antigen and this is typically in the form of harmless secretory IgA antibody. In some circumstances there is, however, induction of potentially pathogenic immune reactions.


Gastrointestinal disorders can arise in a variety of circumstances, including exposure to pathogens, or particular food items, or nutrient deficiencies. Diet is often the only practicable therapy that patients are offered.



10.2 DIGESTION AND ABSORPTION


The gastrointestinal tract comprises different regions of activity in terms of digestion and absorption. Figure 10.2 depicts a diagrammatic representation of a cross-section across the intestinal wall, illustrating the relationship between the absorptive surface area and the blood and lacteal system that carry the products of digestion away from the gastrointestinal tract. The major components of the diet are starches, sugars, fats and proteins. These have to be hydrolysed to their constituent smaller molecules for absorption and metabolism. Starches and sugars are absorbed as monosaccharides; fats are absorbed as free fatty acids and glycerol (plus a small amount of intact triacylglycerol); proteins are absorbed as their constituent amino acids and small peptides. Table 10.1 summarizes the sites of nutrient absorption along the gastrointestinal tract.


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Figure 10.2 Cross-section of intestinal wall. (Redrawn with permission from Figure 24-18 in Tortora & Anagnostakos 1990. Reproduced with permission of John Wiley & Sons, Inc.)



Table 10.1 Regional anatomy of the intestine and sites of nutrient absorption

image


Digestive processes in the mouth


Chewing both grinds the food and also mixes it with saliva, which contains α-amylase and proteolytic enzymes that initiate digestion of dietary starch and protein. Lipid digestion also begins in the mouth, initiated by lipase secreted by the tongue (lingual lipase).



Digestive and absorptive processes in the stomach


Swallowing transfers a food bolus from the mouth to the oesophagus and thence to the stomach. Following a meal, gastric secretory activity follows three well-defined phases:



• the cephalic phase, during which the sight, aroma and anticipation of food stimulates the parasympathetic intestinal nervous system, leading to gastric acid and gastrin secretion


• the gastric phase, associated with increased acid and pepsinogen secretion


• the intestinal phase, which occurs towards the end of liquidization of food in the stomach.


Both dietary triacylglycerol and protein are hydrolysed by enzymes secreted in the gastric juice. Gastric lipase hydrolysis triacylglycerol to release free fatty acids. Gastric acid denatures dietary protein and facilitates hydrolysis by pepsins, which are generated from precursor pepsinogens.



Gastric emptying


The rate of gastric emptying is influenced by the amount of food eaten during a meal, and nutrients present in the food, but the control is exerted by neural or hormonal signals arising in response to nutrients in other parts of the gut. The effect is really one of feedback inhibition so that the rate at which the products of digestion in the stomach are allowed to move into the small intestine is regulated by the rate at which they are further digested and absorbed in the small intestine. Thus the presence of nutrients in the duodenum exerts an inhibitory effect on gastric emptying through the duodenal brake. Furthermore, the presence of nutrients in the ileum exerts a similar inhibitory effect on gastric emptying, known as the ileal brake. For example, fat in the meal that reaches the ileum inhibits gastric emptying, through effects on sensory receptors. Such an effect is mediated by neural and hormonal factors. Luminal lipid stimulates release of the regulatory peptide hormones cholecystokinin (CCK), neurotensin, peptide YY (PYY) and glucagon-like peptides (GLP), which collectively inhibit gastric motility.


In addition to the complex neural and hormonal signals, gastric distension is a very powerful inhibitory signal that increases feelings of fullness and satiety and slows down eating.



Digestive and absorptive processes in the small intestine


As the semi-liquid products of digestion in the stomach (called chyme) pass through the pyloric sphincter into the small intestine they are exposed to the digestive activity of intestinal secretions as well as the emulsifying activity of bile, produced in the gall bladder. Two hormones, secretin and cholecystokinin, are released into the duodenum, and these elicit the secretion of pancreatic enzymes and bile. Enzymes secreted by the pancreas act upon proteins, lipids and starch. The digestion of protein is achieved by proteases including trypsin, chymotrypsin, elastase and carboxypeptidases, and their combined actions lead to the production of free amino acids and short peptides.


The digestion of lipids, which was initiated in the stomach, continues in the small intestine with the formation of mixed micelles that have a hydrophobic core and hydrophilic outer surface. Pancreatic lipases hydrolyse lipids with the release of free fatty acids. The pancreatic secretions are relatively alkaline and thereby neutralize the acidity of the chyme. Intestinal motility facilitates the passage of food and the products of digestion down the gastrointestinal tract by the action of waves of contraction, called peristalsis. These waves of contraction begin in the oesophagus and run through the gastrointestinal tract.



Digestion and absorption of carbohydrate


In the small intestine carbohydrate digestion continues with the further hydrolysis of starch by pancreatic amylases to yield a mixture of glucose oligomers. These are further hydrolysed by the brush-border glucosidases to glucose, maltose and isomaltose. Disaccharides are hydrolysed to their constituent monosaccharides by disaccharidases on the brush border of the enterocytes. Uptake of glucose and galactose into the enterocytes takes place via the same active (sodium-linked) transporter (SGLT1), while fructose, some other monosaccharides and sugar alcohols are carried by passive transporters. Glucose can also be transported by a second glucose transporter, GLUT2, which responds to an increased dietary load of carbohydrate. Most carbohydrate absorption takes place in the duodenum, upper jejunum and proximal ileum.



Digestion and absorption of protein


The endopeptidases of gastric and pancreatic juice hydrolyse proteins to yield oligopeptides and hydrolysis continues by aminopeptidases secreted by the intestinal mucosa and pancreatic carboxypeptidases, to yield free amino acids and di- and tripeptides. Transport into the enterocytes occurs via transporters specific to particular amino acids. Intracellular peptidases complete the hydrolysis of small peptides, and the resultant amino acids pass into the bloodstream. Some relatively large peptides enter the bloodstream intact, either by passing between cells or by uptake into mucosal cells and this is the basis of food allergy.



Digestion and absorption of fat


The process of fat digestion is one of progressive emulsification of dietary fat and hydrolysis of triacylglycerol to free fatty acids and monoacylglycerols. The final product of fat digestion is the micelle which is taken up by the enterocyte. Short chain fatty acids enter the villus microcirculation, but most of the fatty acids are re-esterified to triacylglycerol in the enterocyte, packaged into chylomicrons and secreted into the lymphatic system. Cholesterol and fat-soluble vitamins are absorbed dissolved in the hydrophobic core of the micelles. Free fatty acids can diffuse freely across the cell membrane.



Events in the colon


The colonic microflora produce short chain fatty acids (SCFA), acetate, propionate and butyrate, by fermentation of resistant starch and non-starch polysaccharides. Butyrate is a preferential fuel for the colonic mucosa, and promotes colonocyte differentiation. It may thus have a significant role in preventing colon cancer. Water uptake is stimulated by SCFA absorption and this is likely to be one of the major mechanisms for water and electrolyte salvage in the large intestine.



Water and electrolytes


The human small intestine absorbs 6.5 to 7.5 litres of water each day. Water absorption is proportional to the amount of substrate and electrolyte that moves across the membrane and the direction of water movement is governed by solute movement. In cholera, excessive secretion of chloride into the colon is accompanied by water secretion and diarrhoea, leading to dehydration and eventually death, unless treated.


Sugar alcohols, used as sweeteners, such as xylitol, lactitol and sorbitol, are poorly absorbed and will enter the colon with water, for fermentation. If the colonic fermentation capacity is exceeded then diarrhoea ensues because the excess water carried into the colon with the sugars cannot be absorbed. Other causes of osmotic diarrhoea include dietary fibre such as guar gum, probiotics such as fructose oligosaccharide and beans that contain large quantities of stachyose, all of which are substrates for bacterial fermentation.



The role of the gastrointestinal tract in the regulation of feeding (see chapter 2, section 2.11)


Energy balance is maintained by both short- and long-term mechanisms to regulate energy intake and expenditure. Some reference has already been made to the ways in which the presence of food in different parts of the gastrointestinal tract can generate signals that feed back to regulate gastric emptying. Short-term control of appetite is regulated by the gastrointestinal tract as well as by the metabolic response to ingested nutrients. The gastrointestinal tract provides regulatory feedback signals that arise from direct effects of absorbed nutrients in the circulation, from neural signals from the gut and liver, and from hormonal signals. The taste of food, as well as the smell before eating, stimulates secretion of gastric juice and intestinal motility. The importance of taste in controlling sensations of hunger and satiety is seen in patients receiving long-term tube-feeding who experience constant feelings of hunger although nutritionally replete.


During eating, food stretches the stomach and induces a complex series of signals that lead to cessation of eating. The mechanism is due to stretch, not gastric pressure, and works through direct inhibition of the stimulating effect of pleasurable tastes on eating. Once in the small intestine nutrients in foods are detected by receptors in the intestinal mucosa which leads to the sending of signals to the brain that control eating. For example, the presence of fat in the intestinal lumen is sensed by receptors that lead to the secretion of the hormone cholecystokinin, which elicits inhibition of eating.


Absorbed nutrients may also elicit signals that modulate eating behaviour. For example, in adequately nourished subjects, the intravenous infusion of lipid stimulates dopamine activity and this is associated with increased satiety ratings and feelings of fullness. However, despite the existence of mechanisms to induce feelings of satiety and control eating these can be overridden centrally such that, for example, it is often possible to consume an appetizing dessert even following a heavy meal.



10.3 DISEASES OF THE GASTROINTESTINAL TRACT


Food has a complex relationship with the integrity and function of the gastrointestinal tract. Dietary inadequacies can impair normal development, structure and function, leading to malabsorption and secondary nutrient deficiencies. The gastrointestinal tract can also be profoundly adversely affected by exposure to certain foods or nutrients, sometimes in association with a genetic susceptibility. Additionally, the use of appropriate dietary measures may be central to treatment of gastrointestinal disease. This part of the chapter will consider chronic gastrointestinal diseases and the role of diet in their aetiology and treatment.



Coeliac disease

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Jun 13, 2016 | Posted by in ENDOCRINOLOGY | Comments Off on Diet and the Gastrointestinal Tract

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