Food and You

Dallas E. Boggs, PhD

CHAPTER III

The Digestion of Food

Introduction

HOW IS IT POSSIBLE for the body to so easily digest the great variety of foods available to man? The most important factor in the digestive process is the action of enzymes.

       A most interesting class of compounds, enzymes have the special property of accelerating certain chemical reactions—they behave like catalysts. For example, the stomach produces the enzyme pepsin, which accelerates the breakdown of food proteins to simpler compounds. To accomplish the same change in a test tube, it is necessary to boil the food protein for several hours in very strong acid. The enzymes enable the body to change and reconstruct the foodstuffs to suit its own needs—without resorting to the use of high temperature or strong chemical treatment.The effect of an enzyme on a chemical reaction may be compared with the effect of snow on a toboggan slide. The toboggan will go down a steep slide even in the summer; but it will go very much faster in the winter when there is snow on the slide.

      There are hundreds of different enzymes, and every living cell has its own characteristic assortment; life, as we know it, would be impossible without them. When foods first enter the body, they are subjected to enzymatic action in the mouth; and from that time until their final burning they are subjected to a series of enzyme assisted reactions. It can truly be said that life is one enzymatic reaction after another.

Digestion in the Mouth

        When food is eaten with good appetite, the mouth is prepared with a generous supply of saliva. This wets the food for easier chewingand supplies ptyalin—an enzyme that splits starch to maltose, a simple sugar. The “watering" of the mouth at the sight, smell or even the thought of savory food is a well known phenomenon. The salivary glands produce saliva as the result of nerve impulses that originate in the brain. A whiff of steak and onions stirs the memory of previous delectable experiences of the same kind—and, if the person is hungry, his mouth waters. This flow of saliva before food enters the mouth is the end result of a conditioned reflex—so called because it is conditioned by previous experience and involves the memory. The mere presence of anything in the mouth stimulates the nerves in the lining of the mouth; and impulses are sent to the brain, which in turn sends impulses back to the salivary glands. This action is automatic and unconditional because it does not involve mem­ories (either pleasant or unpleasant). Thus, there is always a supply of saliva whenever food is eaten. Without saliva, it would be very difficult to chew dry foods; and it would be almost impossible to swallow. It also contains mucin, a substance that serves to lubricate each portion of food and makes it slippery before it is swallowed. The water in saliva tends to dissolve the flavoring substances from food as it is chewed—so that the taste buds are stimulated, inhancing the enjoyment of food.

      Chewing is more important for good digestion than most people realize. Unless the larger particles of food are crushed in the mouth, the stomach must assume the extra burden of reducing the swallowed food to proper size for passage into the small intestine. Thorough chewing not only brings about stimulation of many taste buds; it also results in the release of pleasant odors that are most important in the appreciation of good food. This physiological fact is easily observed by holding the nose while chewing a favorite food—without the sense of smell, even the most delectable morsel tastes rather flat.

      Some extremists believe that their nutrition will be improved by continued chewing of each mouthful of food long beyond the normal time—but this idea cannot be supported by scientific fact. At the other extreme are the people who consider a meal an unwarranted interruption of the working day and something to be disposed of quickly. When food is bolted without chewing, there is no real enjoy­ment of it except for the distention of the stomach—the cavity is filled, but nothing more. The happy medium lies somewhere between the extremes. Most people would regard it as silly to spend a large amount of time chewing their food unnecessarily; but, on the other hand, they wish to get the maximum pleasure from their meals. The latter is important because, when food is eaten with anticipation and enjoyment, the stomach (like the mouth) "waters". As soon as food arrives, the stomach already contains enough gastric juice to begin the next step in digestion.

Swallowing

      Once a particle of food is pushed toward the back of the mouth and started on its way to the stomach, it is beyond fur­ther voluntary control; the events that follow are automatic and do not stop until the food has reached the stomach. The esophagus, the muscular tube that connects the mouth with the stomach, is normally a "one‑way street". Anything that enters from the mouth must inevitably go the whole distance to the stomach. This orderly movement of contents is called peristalsis, and it is quite characteristic of the whole alimentary canal. It is accomplished by a ring of contraction that travels away from the mouth and carries the lump (or bolus) of food ahead of it. This progression is assisted by a ring of relaxation that precedes the ring of contraction. It is something like removing an orange from the Christmas stocking—when the stocking is squeezed close to the orange on the toe side, the orange will soon pop out at the other end.

Digestion in the Stomach

      The stomach lining contains thousands of small glands that produce gastric juice, which owes its activity to the presence of hydrochloric acid and the enzyme pepsin. The stomach is the only organ of the body that is capable of manu­facturing these substances, and the mechanisms by which it is accomplished remain some of the most intriguing mysteries of physiology. The stomach is ready to go to work as soon as food arrives from the mouth—because some gastric juice is secreted while the food is being chewed and before it has reached the stomach. The juice secreted under such conditions is often referred to as "appetite juice". It is another example of the operation of a conditioned reflex and is quite similar to "watering of the mouth."

      The most important function of gastric juice is the partial digestion of the dietary proteins. The mass of swallowed food is well kneaded and mixed with gastric juice by means of the muscular movements of the stomach. Pepsin and hydrochloric acid make a combined attack upon the food proteins and split them into smaller, more soluble fragments called proteoses and peptones. As di­gestion proceeds, more gastric juice is secreted, and the stomach contents become more fluid. The muscular movements that sweep over the stomach at regular intervals tend to drive the liquid portions downward into the small intestine—another example of the peristalsis (one‑way movement) first encountered in the esophagus. The fats and carbohydrates pass through the stomach almost without alteration.

      The stomach, by means of persistent peristalsis, is able to pass on its contents to the small intestine in the space of two to four hours. The emptying of the stomach is governed by the composition of the meal being digested and by the activity of the small intestine. The greater the fat content of the meal, the slower its exit from the stomach. This seems to be a physiologi­cal safety device to prevent the stomach from overloading the small intestine with too much fat before it has disposed of what is already present. When fat arrives from the stomach, enterogastrone, a hormone, is formed in the wall of the small intestine. It enters the blood and reaches the stomach to decrease both the secretion of gastric juice and the peristalsis of that organ.

      When the upper small intestine becomes sufficiently filled to cause slight distention, nerve impulses originating in the stretched gut wall are relayed, by way of the brain, to the stomach; and its activity is decreased or sus­pended until the intestine disposes of the load. When the intestine has disposed of its fat, the stomach resumes its normal activity.

      During the few hours required to empty the stomach, there must be some provision for maintaining a flow of gastric juice. The appetite juice mentioned earlier lasts only a short time; and it stops about 20 minutes after the last mouthful of food is swallowed. Digestion is begun by the appetite juice; and the proteoses and peptones resulting from this preliminary digestion of protein come in contact with the stomach lining, which then produces a hormone called gastrin. This hormone finds its way into the blood stream— and to all the gastric glands, which are stimulated to produce more gastric juice. This process continues until the stomach is empty. Therefore, in the stomach, the digestive juice is under double control: (1) Nervous, through a conditioned reflex, and (2) Chemical, through the hormones gastrin and enterogastrone.

      When the stomach becomes empty, it gradually develops stronger and stronger contractions until they enter conscious­ness as the familiar hunger contractions and serve to remind the individual that more food is needed. The stomach thus contains several automatic mechanisms—it provides itself with a continuing supply of gastric juice, empties itself and supplies the cue that will almost certainly result in a refill.

Digestion in the Small Intestine.

      In the small intestine, the food encounters three more diges­tive juices; and their combined actions result in splitting all of the remaining foodstuffs into fragments small enough to be ab­sorbed. One of these juices comes from the pancreas, a large gland that pours its secretion through a duct into the small intestine near its connection with the stomach. The main regulator of the flow of pancreatic juice is another hormone, secretin; it is produced in the wall of the small intestine and carried from there to the pancreas by way of the blood stream. Secretin has the property of stimulating the pancreatic cells to produce large quantities of pancreatic juice, and this process continues as long as any partially digested food remains in the small intestine.

           

     The liver is another large gland found in the abdominal cavity, and it contributes bile to assist in digestion. It does not possess digestive power by itself, but it contains an activator for two fat‑splitting enzymes. Bile flows continu­ously from the liver—between meals it is concentrated and stored temporarily in the gall bladder. Another hormone, with the tongue‑twisting name cholecystokinin, is formed when food reaches the intestine from the stomach; it causes the gall bladder to contract and squeeze its bile into the small intestine. The third digestive juice concerned in intestinal digestion is the intestinal juice proper, which is produced by millions of tiny glands that lie in the gut wall. These glands are stimulated to activity by direct contact with partially di­gested food as well as by a hormone, enterocrinin, pro­duced by the intestine for its own use. Thus, the pancreatic juice, the bile and the intestinal juice to­gether form a combination with so much digestive power that little, if any, food escapes complete disintegration into particles or molecules that are easily absorbed.

      The job of digesting the proteins, which was begun in the stomach, is completed in the small intestine by the action of trypsin in the pancreatic juice and erepsin in the intestinal juice. The end result of the action of these enzymes is the breaking down of all proteins to amino acids—or simple combi­nations of them. It will be recalled that amino acids are the building blocks from which proteins are made. After digestion is complete, therefore, the body is presented with an assortment of amino acids from which it can select the ones required for the synthesis of its own proteins.

      The first attack on dietary starch is made by the enzyme pty­alin found in the saliva. The time spent in the mouth is too short to permit much digestion, but ptyalin continues to act in the stomach until the mass of food becomes saturated with hy­drochloric acid. The starch that remains on arrival in the small intestine is digested by the action of amylase, a starch splitting enzyme that is included in both the pancreatic and the intestinal juices. The end result of ptyalin and amylase action is the production of maltose from starch. Maltose, sucrose (table sugar) and lactose (milk sugar) are all double sugars (disaccharides) that in turn must be split into the simpler sugars of which they are composed. The intestinal juice contains the enzyme maltase,which splits maltose to two molecules of glucose (grape sugar), sucrase that converts sucrose to one molecule each of glucose and fructose (fruit sugar), and lactase that di­gests lactose to glucose and galactose. The various enzymes mentioned in this paragraph are capable of digesting all dietary carbohydrate to simple sugars that can then be absorbed and used as fuel.

      The fats are not much affected by passage through the mouth and stomach. The fat‑splitting enzymes, called lipases, are found in large amounts only in the pancreatic and intesti­nal juices—and, hence, fat digestion is confined to the small intes­tine. The action of lipase is to split food fat into glycerin and fatty acids, and this can be accomplished only in the presence of bile. Therefore, the liver, gall bladder, pancreas and small bowel are all involved in the digestion of fat. Unlike the proteins and carbohydrates, the fats can be absorbed without being completely split into their component parts. Partial digestion liberates some fatty acids, which (in combination with bile) are capable of forming such a fine emulsion of the remaining fat that it can be absorbed without further change.

      The vitamins and mineral salts (as such) do not require digestion; they are absorbed unchanged. In their natural state, they may be combined with other foodstuffs; but it seems probable that   after the proteins, fats and carbohydrates are digested and ready for absorption, the vitamins and mineral salts will have been liberated so that their absorption is assured.

     The cellulose fibers of vegetables, fruit seeds and skins, and the tough connective tissue of meat (examples of indigestible and unabsorbable substances present in all diets) are passed along to the large intestine, which removes most of the water and then discards what re­mains as the feces. The fecal material also contains residues of the various digestive juices and millions of bacteria both dead and alive.

 

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