Home > Encyclopedia of Food & Culture > Digestion

DIGESTION. Digestion can occur at many levels in the body; generally, it refers to the breakdown of macro-molecules or a matrix of cells, or tissues, into smaller molecules and component parts. This particular section will focus on digestion of food in the gastrointestinal tract: the process that is required to obtain essential nutrients from the food we eat. The gastrointestinal tract (GIT) is a highly specialized organ system that allows humans to consume food in discrete meals as well as in a very diverse array of foodstuffs to meet nutrient needs. Figure 1 contains a schematic of the GIT and illustrates the organs of the body with which food comes into contact during its digestion. These organs include the mouth, esophagus, stomach, small intestine, and large intestine; in addition, the pancreas and liver secrete into the intestine. The system is connected to the vascular, lymphatic, and nervous systems; however, the function of these systems in gastrointestinal physiology is beyond the scope of this article, which focuses primarily on the process of breaking down macromolecules and the matrix of food.

Mechanical Aspects of Digestion

Food is masticated in the mouth. Chewing breaks food into smaller particles that can mix more readily with the GIT secretions. In the mouth, saliva lubricates the food bolus so that it passes readily through the esophagus to the stomach. The sensory aspects of food stimulate the flow of saliva, which not only lubricates the bolus of food but is protective and contains digestive enzymes. Swallowing is regulated by sphincter actions to move the bolus of food into the stomach. The motility of the stomach continues the process of mixing food with the digestive secretions, now including gastric juice, which contains acid and some digestive enzymes. The action of the stomach continues to break down food into smaller particles prior to passage to the intestine. The mixture of food and digestive juices is referred to as digesta, or chyme. The stomach, which after a meal may contain more than a liter of material, regulates the rate of digestion by metering chyme into the small intestine over several hours. Several factors can slow the rate of gastric emptying; for example, solids take longer to empty than liquids, mixtures relatively high in lipid take longer to empty, and viscous, or thick, mixtures take longer to empty than watery, liquid contents.

In the upper part of the small intestine, the duodenum, receptors appear to influence the rate of gastric emptying either through hormonal or neural signals. Peristaltic motor activity in the small intestine propels chyme along the length of the intestine, and segmentation allows mixing with digestive juices in the intestine, which include pancreatic enzymes, bile acids, and sloughed intestinal cells. Digestion of macronutrients, which began in the mouth, continues in the small intestine, where the intestinal surface provides an immense absorptive surface to allow absorption of digested molecules into circulation. While the intestine from the outside appears to be a tube, the lining of the inner surface contains tissue folds and villi that are lined with intestinal cells, each with microvilli, or a brush border, which greatly amplify the absorptive surface. The intestinal cells can absorb compounds by several cell membrane–mediated transport mechanisms and then transform them into compounds, or complexes, that can enter circulation through the blood, or lymphatic, system.

What is not digested and absorbed passes into the large intestine. In this organ, water and electrolytes are reabsorbed, and the movements of the large intestine allow mixing of the contents with the microflora of bacteria and other microbes that are naturally present in the large intestine. These microbes continue the process of digesting the chyme. Eventually the residue enters the rectum and the anal canal, and stool is formed, which is defecated. Transit time of a non-digestible marker from mouth to elimination in the stool varies considerably: normal transit time is typically twenty-four to thirty-six hours, but can be as long as seventy-two hours in otherwise healthy individuals.

Breakdown of Macromolecules in Foods

Foods are derived from the tissues of plants and animals as well as from various microorganisms. For absorption of nutrients from the gut to occur, the cellular and molecular structure of these tissues must be broken down. The mechanical actions of the GIT help disrupt the matrix of foods, and the macromolecules, including proteins, carbohydrates and lipids, are digested through the action of digestive enzymes. This digestion produces smaller, lower molecular weight molecules that can be transported into the intestinal cells to be processed for transport in blood, or lymph.

Proteins are polymers of amino acids that in their native structure are three-dimensional. Many cooking or processing methods denature proteins, disrupting their tertiary structure. Denaturation, which makes the peptide linkages more available to digestive enzymes, is continued in the stomach with exposure to gastric acid. In addition, digestion of the peptide chain begins in the stomach with the enzyme pepsin. Once food enters the small intestine, enzymes secreted by the pancreas continue the process of hydrolyzing the peptide chain either by cleaving amino acids from the C-terminal end, or by hydrolyzing certain peptide bonds along the protein molecule. The active forms of the pancreatic enzymes include trypsin, chymotrypsin, elastase, and carboxypeptidase A and B. This process of protein digestion produces small peptide fragments and free amino acids. The brush border surface of the small intestine contains peptidases, which continue the digestion of peptides, either to smaller peptide fragments or free amino acids, and these products are absorbed by the intestinal cells.

Carbohydrates are categorized as digestible or non-digestible. Digestible carbohydrates are the various sugar-containing molecules that can be digested by amylase or the saccharidases of the small intestine to sugars that can be absorbed from the intestine. The predominant digestible carbohydrates in foods are starch, sucrose, lactose (milk sugar), and maltose. Glycogen is a glucose polymer found in some animal tissue; its structure is similar to some forms of starch. Foods may also contain simple sugars such as glucose or fructose that do not need to be digested before absorption by the gut. Alpha amylase, which hydrolyzes the alpha one to four linkages in starch, is secreted in the mouth from salivary glands and from the pancreas into the small intestine. The action of amylase produces smaller carbohydrate segments that can be further hydrolyzed to sugars by enzymes at the brush border of the intestinal cells. This hydrolysis step is closely linked with absorption of sugars into the intestinal cells.

Non-digestible carbohydrates cannot be digested by the enzymes in the small intestine and are the primary component of dietary fiber. The most abundant polysaccharide in plant tissue is cellulose, which is a glucose polymer with beta one to four links between the sugars. Amylase, the starch-digesting enzyme of the small intestine, can only hydrolyze alpha links. The non-digestible carbohydrates also include hemicelluloses, pectins, gums, oligofructose, and inulin. While non-digestible, they do affect the digestive process because they provide bulk in the intestinal contents, hold water, can become viscous, or thick, in the intestinal contents, and delay gastric emptying. In addition, non-starch polysaccharides are the primary substrate for growth of the microorganisms in the large intestine and contribute to stool formation and laxation. Products of microbial action include ammonia, gas, and short-chain fatty acids (SCFA). SCFA are used by cells in the large intestine for energy and some appear in the circulation and can be used by other cells in the body for energy as well. Thus, while dietary fiber is classified as non-digestible carbohydrate, the eventual digestion of these polysaccharides by microbes does provide energy to the body. Current research is focused on the potential effect of SCFA on the health of the intestine and their possible role in prevention of gastrointestinal diseases.

For dietary lipids to be digested and absorbed, they must be emulsified in the aqueous environment of the intestinal contents; thus bile salts are as important as lipolytic enzymes for fat digestion and absorption. Dietary lipids include fatty acids esterified to a glycerol backbone (mono-, di-or triglycerides); phospholipids; sterols, which may be esterified; waxes; and the fat-soluble vitamins, A, D, E, and K. Digestion of triglycerides (TG), phospholipids (PL), and sterols illustrate the key factors in digestion of lipids. Lipases hydrolyze ester bonds and release fatty acids. In TG and PL, the fatty acids are esterified to a glycerol backbone, and in sterols, to a sterol nucleus such as cholesterol. Lipases that digest lipids are found in food, and are secreted in the mouth and stomach and from the pancreas into the small intestine. Lipases in food are not essential for normal fat digestion; however, lipase associated with breast milk is especially important for newborn infants. In adults the pancreatic lipase system is the most important for lipid digestion. This system involves an interaction between lipase, colipase, and bile salts that leads to rapid hydrolysis of fatty acids from TG. An important step in the process is formation of micelles, which allows the lipid aggregates to be miscible in the aqueous environment of the intestine. In mixed micelles, bile salts and PL function as emulsifying agents and are located on the surface of these spherical particles. Lipophilic compounds such as MG, DG, free sterols, and fatty acids, as well as fat-soluble vitamins, are in the core of the particle. Micelles can move lipids to the intestinal cell surface, where the lipids can be transported through the cell membrane and eventually packaged by the intestinal cells for transport in blood or lymph. Most absorbed lipid is carried in chylomicrons, large lipoproteins that appear in the blood after a meal and which are cleared rapidly in healthy individuals. Bile salts are absorbed from the lower part of the small intestine, returned to the liver, and resecreted into the intestine, a process referred to as enterohepatic circulation. It is important to note that bile salts are made from cholesterol, and drugs such as cholestyramine or diet components such as fiber that decrease the amount of bile salt reabsorbed from the intestine help to lower plasma cholesterol concentrations.

Regulation of Gastrointestinal Function

Regulation of the gastrointestinal response to a meal involves a complex set of hormone and neural interactions. The complexity of this system derives from the fact that part of the response is directed at preparing the GIT to digest and absorb the meal that has been consumed in an efficient manner and also at signaling short-term satiety so that feeding is terminated at an appropriate point. Traditionally, physiologists have viewed the regulation in three phases: cephalic, gastric, and intestinal. In the cephalic phase, the sight, smell, and taste of foods stimulates the secretion of digestive juices into the mouth, stomach, and intestine, essentially preparing these organs to digest the foods to be consumed. Experiments in which animals are sham fed so that food consumed does not actually enter the stomach or intestine demonstrate that the cephalic phase accounts for a significant portion of the secretion into the gut. The gastric and intestinal phases occur when food and its components are in direct contact with the stomach or intestine, respectively. During these phases, the distension of the organs with food as well as the specific composition of the food can stimulate a GIT response.

The GIT, the richest endocrine organ in the body, contains a vast array of peptides; however, the exact physiological function of each of these compounds has not been established. Five peptides, gastrin, cholecystokinin (CCK), secretin, gastric inhibitory peptide (GIP), and motilin are established as regulatory hormones in the GIT. Multiple aspects have been investigated to understand their release and action. For example, CCK is located in the upper small intestine; protein and fat stimulate its release from the intestine, while acid inhibits its secretion. Once released, it can inhibit gastric emptying and stimulate secretion of acid and pancreatic juice and contraction of the gall bladder. In addition, it stimulates motility and growth in the GIT and regulates food intake and insulin release. Among the other established gastrointestinal peptides, secretin stimulates secretion of fluid and bicarbonate from the pancreas, gastrin stimulates secretion in the stomach, GIP inhibits gastric acid secretion, and motilin stimulates the motility of the upper GIT. In addition to investigating the various factors causing release of these hormones and the response to them, physiologists are also interested in the interactions among hormones as well as those with the nervous system, since the response to a meal involves release of many factors.

Obtaining food and digesting it efficiently are paramount to survival. The human GIT system most likely evolved during the period when the species acquired its food primarily through hunting and gathering. The over-lapping regulatory systems, combined with an elevated capacity to digest food and absorb nutrients, insured that humans used food efficiently during periods in which scarcity might occur.

See also Eating; Intestinal Flora; Microbiology.

Barbara O. Schneeman