GASTROINTESTINAL FUNCTION

14.2 GASTROINTESTINAL FUNCTION

14.2.1 O RAL C AVITY

In the oral cavity, foods are masticated and lubricated with saliva. This process initiates the breakdown of food in a manner that will allow penetration and action of digestive enzymes. Saliva contains alpha-amylase, which can initiate the hydrol- ysis of starch. Given the time that most food is in the oral cavity, it is unlikely that much starch hydrolysis occurs; however, the enzyme can remain active in the gastric contents until the pH is lowered significantly due to secretion of gastric acid. Foods rich in dietary fiber as a part of cell wall structures are likely to take longer to chew; 2,3 however, cooking plant foods will soften the cell wall structure, making mastication easier. It is possible that polysaccharides with a high water-holding capacity might stimulate more fluid secretion into the mouth during mastication, but such an effect has not been tested directly.

14.2.2 S TOMACH

After swallowing, the food bolus passes down the esophagus to the stomach. The stomach can hold several liters, allowing humans to consume food in meals. In the stomach, chyme is mixed with gastric juices, which contain acid as well as pro- teolytic and lipolytic enzymes. Salivary amylase can continue to hydrolyze starch until the pH is lowered, and the gastric secretions and motility will continue to solubilize and disperse food components and the polysaccharide matrix that is a part of foods containing NSPs and digestible carbohydrates. The rate of gastric filling is determined by the rate of food consumption, and, in turn, the rate of gastric emptying controls the rate of nutrient digestion and absorption from the small intestine. Properties such as water-holding capacity, nondigestible bulk, and viscosity can alter gastric distension and the rate of gastric emptying and contribute to feelings of

fullness. 4 Carbohydrates that increase the viscosity of gastric contents have been shown to slow the rate of gastric emptying. 5,6 This effect of viscous polysaccharides on gastric emptying, leading to a slower small intestinal transit time, is associated with the ability of certain sources of NSPs to blunt glycemic and insulin responses, as well as contributes to lowering plasma cholesterol. 7–12

When the food bolus or chyme leaves the stomach, the NSPs are relatively intact, some hydration of these compounds has occurred, starch hydrolysis has been initi- ated, and sugars have probably been solubilized. In addition, the pH of the bolus is relatively low due to mixing with gastric acid.

Carbohydrates and Gastrointestinal Tract Function

14.2.3 S MALL I NTESTINE

In the small intestine, more fluid is mixed with the chyme to continue the process of digestion and facilitate mobility of compounds for absorption. The macromole-

cules in food are broken down by digestive enzymes to substrates that can be absorbed by the enterocytes. The presence of protein and fat as well as acid in the duodenum stimulates the secretion of pancreatic juice and bile into the small intes- tine. The presence of NSPs appears to facilitate the secretion of enymes into the small intestine; however, the effect may be indirect by slowing the digestion of lipids and proteins. 13,14 Pancreatic amylase hydrolyzes the alpha-1,4 linkages found in starch; in addition, saccharides are hydrolyzed by enzymes associated with the brush border of intestinal enterocytes. The net effect of the carbohydrate-digesting enzymes in the small intestine is to hydrolyze digestible carbohydrates to sugars that can be readily absorbed by the small intestine cells while leaving the nondigestible carbo- hydrates intact. During this process not all starch is digested; a certain amount

remains in the small intestine and is referred to as resistant starch. 15 In the small intestine, carbohydrates are separated into those that are absorbed for use as energy substrates in the body and those that remain as bulk within the intestinal contents and pass as a part of the residue into the large intestine.

Thus, one of the important characteristics of carbohydrates that determine func- tion within the small intestine is the nature of the chemical bonds and the ability of mammalian enzymes to hydrolyze these bonds. The digestible carbohydrates are absorbed for energy, but also promote adaptive changes in the digestive enzyme

profile of the small intestine and pancreas. 13 Thus, a high-starch diet results in elevated specific activity of pancreatic amylase, and diets high in specific disaccha- rides can result in elevated activity of enzymes that hydrolyze these sugars. For those carbohydrates that are not digested by mammalian enzymes, their physical presence in the small intestine has physiologic importance. Digestive enzyme activity can be inhibited by interaction with certain polysaccharides or by specific enzyme inhibitors found in plant products that are also rich in dietary fiber. 16,17 In addition, the nondi- gestibility of NSPs contributes to an increase of material in the bulk phase and in the volume of the aqueous phase of the intestinal contents, which is likely to alter mixing and diffusion in the gut contents. 18,19 Certain polysaccharides, such as pectin and gums, can increase viscosity of the intestinal contents. Viscosity has been shown to delay absorption of sugar from the small intestine, in part due to alterations in mixing and diffusion. 20–23 In addition to interactions with digestive enzymes in the small intestine, certain NSPs can bind or adsorb bile acids and increase their excre- tion. 14,24,25 The effects of viscous polysaccharides on gastric emptying, transit and absorption from the small intestine, and on bile acids have been associated with the ability of these polysaccharides to blunt glucose and insulin responses and lower plasma cholesterol concentrations.

The ability of viscous polysaccharides to slow the rate of absorption from the small intestine is likely to alter the pattern of hormone release from the GIT. 4 Following consumption of a meal containing beta-glucan-enriched barley, cholecys- tokinin (CCK) concentrations remained elevated above baseline levels for a longer

period of time than the concentrations from a meal without a fiber source. 26 CCK

Functional Food Carbohydrates

concentrations were twice as high after consuming a meal containing beans than a test meal lower in fiber. 27 Adding viscous polysaccharides to a low-fat test meal

significantly increased the CCK response in women. 28 CCK plays a central role in orchestrating the GIT response to eating by delaying gastric emptying and stimu-

lating pancreatic secretion and gall bladder contraction, and it is associated with satiety. The results suggest that slower lipid absorption from the intestine may be associated with prolonging the CCK response to a meal. Because ghrelin release may be suppressed by carbohydrates, it would be interesting to investigate the effect of NSPs on the release of this hormone from the stomach and the time course over which release is suppressed by the presence of NSPs, especially since elevated ghrelin levels are associated with feelings of hunger. In addition, hormones associ- ated with regulation of food intake are released from the ileum, an effect that has been referred to as the ileal brake. 29,30 The presence of nondigested polysaccharides, as well as nutrients that are associated with this fraction of food in the ileum, can promote release of these peptides. Because of the unique chemical and physical properties of the NSPs, research is needed on their role in regulating the GIT response. In many cases, their effects on GIT function may be indirect due to viscosity and the ability to carry nutrients further down the small intestinal tract.

14.2.4 L ARGE I NTESTINE

From the small intestine, the residual diet material passes into the large intestine. A key function of the large intestine is reabsorption of water and electrolytes that have been secreted into the gut during digestion. Within the large intestine is a large and complex microflora that utilizes the residual diet, of which major components are NSPs and resistant starch, and digestive secretions for growth and metabolism. Cummings and coworkers estimated that approximately 15 to 60 g of carbohydrates that can be fermented enters the large intestine daily. 31,32 From this fermentation, approximately 200 to 700 mmol of short-chain fatty acids (SCFAs) can be produced. The primary SCFAs include acetate, propionate, and butyrate. Butyrate is utilized as an energy source by colonocytes, propionate is cleared from the portal blood by the liver, and acetate can be utilized by muscle cells and other peripheral tissues. In addition to the production of SCFAs, microbial activity in the large bowel results in decreased pH, modifications in microbial enzyme activity, and an increase in

microbial cell mass. 33 SCFA production has been investigated as a source of energy as well as for its effects on mucosal cell proliferation. 33–36 The significant component of large bowel function, in which nondigested car- bohydrates have an important function, is the process of laxation and elimination of fecal material. The major components of fecal material are water, undigested diet residue, and microbial mass. 37–39 NSP is the only dietary component known to increase stool weight, which it can do either directly as undigested residue or by

supporting the growth of microorganisms. 31 In both human and animal studies, stool weight is significantly correlated with consumption of NSPs. 40

Carbohydrates and Gastrointestinal Tract Function