The Chemistry of Digestion

THE CHEMISTRY OF DIGESTION. BY DR. JOHN A. WIDTSOE, DIRECTOR EXPERIMENT STATION, STATE AGRICULTURAL COLLEGE, LOGAN, UTAH.

No questions can be of greater importance to man than those that deal with foods and eating. Every young man and woman should be able to explain in an intelligent manner the general composition of food; the value of special foods; the changes that foods undergo when they are taken into the system; the proper combination of food materials for the best health; the economy of different foods; and the other general facts that bear on this sub- ject. Were the relation of man to the food he eats more clearly understood, and were this knowledge applied in our daily lives,

much of the weakness and sickness of the race would disappear; the improved methods of living would result in greater physical and mental vigor; it would lead to greater economy in the household,

and would encourage a more rational system of agriculture than is now in vogue. Much of the success of the past century was made possible, no doubt, through the greater knowledge gained concern- ing the real value of food to man. The young people of Zion, especially, who desire to be receptive to every prompting of the Spirit, should inform themselves of every fact, the possession of which will enable them to keep their bodies in such a condition that the Spirit can work its will without hindrance. As an aid toward this object, this paper has been written. It attempts to outline, in general terms, the changes that foods undergo during digestion. To understand the processes of digestion, it is necessary to review some of the elementary facts bearing on the composition

THE CHEMISTRY OF DIGESTION. 565

of foods. Nearly all materials, used for eating, contain a great number of different substances. Each one of these substances is of particular value to the body; and, to know the value of any food, it is necessary to know its composition. Fortunately for the food-chemist, nearly all substances, so far as their food value is

concerned, may be grouped into a few great classes; to know the food value of any given food, it is, then, sufficient to determine the amount of each one of the great food principles that it contains. Water is found in all foods, without exception, and forms the first great food principle. Fats, of all kinds, form the second. There is a great variety of fats in nature; some are hard, resembling the

hard beef tallow; others are liquid, as, for instance, olive oil.

Between these two extremes, there is almost every gradation. Still, all these fats have the same chemical structure, and, very

nearly, the same chemical composition, and are used in the same manner by the animal body. The third great food principle is known as the carbohydrates. It is composed of starches, sugars, gums and related substances. Starch in fact may be changed into sugar, by treatment with acids, and, in the plant, sugars are readily changed into gums and starch. The fourth food principle is known as protein. It is represented by the white of egg, the lean of meat, and the gluten of flour. It is extremely important to the human body, because it makes muscle; which fats and car- bohydrates cannot do. The fifth and last food principle is mineral matter. Every plant contains a certain amount of material that remains as an ash when the plant is burned. This is essential to the body in the formation of bones, blood, and other parts of the body. To digest a food is really to make it soluble, so that it can pass through the walls of the stomach and intestines, and thus be brought into the blood without hindrance. The study of digestion is, therefore, a study of the means by which food is made soluble within the body. Digestion begins in the mouth. The food is chewed in order that it may be reduced to fine particles that are more readily acted upon by the digestive juices than larger particles would be. In the mouth, the food is also mixed with the first digestive fluid, the saliva, which]iis a watery, tasteless liquid secreted by three dis-

666 IMPROVEMENT ERA.

tinct glands. Volume for volume, saliva is slightly heavier than water; when perfectly pure, it is transparent, colorless and posses- es a faint alkaline reaction. Normal saliva consists of water holding in suspension and solution small quantities of solids in about the following proportion: Water - - - 99.47 parts. Solids - . - .53 parts.

100.00 The solids are made up of an organic part, that may be burned away, and of an inorganic part that remains as ash. The ash amounts to only 0.1 per cent of the saliva; it consists of lime, soda, magnesia and phosphoric acid. The organic solids contain small quantities of imperfectly known substances, and in addition a very small quantity of an unorganized ferment called ptyalin. More will be said about this substance, presently. The quantity of saliva secreted by a healthy person is very large. It is said that, during eating, each ounce of gland-matter will secrete thirteen ounces of saliva in an hour. The material required for so much saliva is, of course, taken, chiefly, from the blood, which, when the nerves controlling the salivary secretion are stimulated by agreeable odors, drugs, or chewing, rushes with great force into the glands. Saliva is of value in various ways. By moistening the mouth, it renders distinct articulation possible; by mixing with the food, it makes swallowing easy and agreeable, and prevents the food from sticking to the mouth. While these are verv important functions, the solvent action of saliva must not be overlooked. Starch forms a large portion of our daily diet, as shown by the high starch con- tent of many of our staple foods. Bread, for instance, contains fifty per cent of starch; potatoes, about eighteen per cent; beans, about forty-eight per cent, and so on. In its natural condition, however, starch cannot be used by the body; even when boiled, it is not directly serviceable to the human system; before it can enter the blood, it must be changed into a soluble form. The power of accomplishing this change is possessed by saliva. As already stated, saliva contains as unorganized ferment, ptyalin. This substance may be taken as a type of a number of

THE CHEMISTRY OF DIGESTION. 567

bodies that are specially concerned in the process of digestion. An unorganized ferment is the product of a living organism, but does not itself possess life, that is to say, it cannot grow or reproduce itself. In this respect an unorganized ferment differs from one that is organized, as for instance, yeast, which eats and grows and reproduces its kind. The characteristic power of the unorganized ferments is, that by their mere presence they are able to produce deep- going changes in other substances. For instance, the ferment ptyalin, of the saliva, when brought into contact with starch, will immediately begin to change the insoluble starch into soluble sugar. Similar ferments are found in all the digestive juices, and lie at the bottom of most of the changes that occur in the diges- tive tract. Though it is true that the food does not remain long enough in the mouth for the ptyalin to transform all the starch into sugar, yet, if the food be thoroughly masticated and mixed with saliva, when it passes into the stomach in a lump, the ptyalin can

exert its action on the inside of the mass until the antagonistic juices of the stomach work their way in. Incidentally, this empha- sizes the good advice that food should be chewed thoroughly; for, unless this is done, it is not mixed thoroughly with saliva, and a great deal of starch, more than designed by nature, passes out of the stomach undigested. Ptyalin can make starch soluble only in a faintly alkaline solution, such as saliva; if neutralized by vine- gar, or any other acid, the action at once ceases. This furnishes an argument against the use of sour pickles as a constant article of diet. The food, broken down into small particles, and thoroughly mixed with saliva, is swallowed, and passed down the oesophagus into the stomach, where it is mixed with gastric juice, and under- goes many complex changes. Gastric juice is a thin, colorless liquid, possessed of a very acid reaction, and of peculiar taste and

odor. Its approximate composition is:

Water - - - 97.30. Solids - - - 2.70.

100.00. The solids are composed partly of inorganic and partly of

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568 IMPROVEMENT ERA.

poorly known organic substances. The characteristic ingredients are hydrochloric acid, and an unorganized ferment called pepsin. Of these substances, pepsin is by far the more important, because it aids in the digestion of the important food principle, protein. The substances constituting protein are very inactive in their nature, and are quite insoluble. When they are brought into contact with the pepsin of the gastric juice, they begin to break down immedi- ately into simpler forms that are more soluble, until perfectly soluble compounds, called peptones, are obtained. The peptones pass through the walls of the stomach, into the blood, and are used in replacing worn-out muscular tissue or in building new. When, for any reason, the stomach fails to secrete enough pepsin to dissolve the protein that is eaten, indigestion results. In such cases, it is often customory to swallow pepsin taken from the stom- achs of animals; the ferment thus taken into the stomach mixes with the food, and helps to make it soluble. Many of the pre- parations for the cure of indigestion, now found on the market, are mixtures of pepsin with other more or less inert substances. It is possible also to gather the pepsin from the stomach of ani- mals, and by dissolving it in water to make an artificial gastric juice. When this preparation is mixed with meat, milk, or other foods containing protein, digestion sets in, and the so-called pre- digested foods are produced. Pepsin will exert its peculiar action only in an acid solution. Hence we find that the gastric juices contains from two-tenths to three-tenths per cent of free hydrochloric acid. It occurs, occa- sionally, that the stomach does not secrete enough acid for the proper action of the pepsin. The food then remains undigested in the stomach, ferments, and a sour stomach is the result. In such cases, physicians frequently prescribe small doses of hydrochloric acid, to enable the pepsin to do its work. Now, it must not be supposed that the stomach is simply a bag into which are poured food and gastric juice. The blood cir- culating in and about the stomach, keeps it at a favorable temper- ature for digestion; and the stomach is further provided with ar- rangements which enable it to alter its capacity according to the mass which it contains, which it grasps, and kneads, and rotates, a perfectly contrived arrangement for continually mixing the food

THE CHEMISTRY OF DIGESTION. 56&

to be dissolved with the solvent juice. As the food is made soluble, a part of it is absorbed by the stomach and hurried on to be mixed with the blood. The digestive process in the stomach continues from three to five hours; at the end of that period, all the food that has not been digested, together with the gastric juice holding in solution part of the digested materials, has passed from the stomach into the small intestines, there to be further acted upon. The mixture thus leaving the stomach is called chyme. During the time digestion is going on in the mouth, a small quantity of starch was digested; in the stomach, much of the pro*,

tein was made soluble; there remain then in the chyme a large quantity of undigested starchy matters, nearly all the fat, and whatever remains undigested of the protein. In fact, the most active digestion must occur after the food has left the stomach. The pancreas is a gland which exists in all air-breathing ani- mals, and in many fishes. It secretes an alkaline juice that is

emptied into the upper portion of the small intestines. This juice is of great importance in the process of digestion. The pancreatic juice is a somewhat viscid, gluey liquid, possessing a strong alka- line reaction and a saltish taste. Its composition is: Water - - - 90.00. Solids - - - 10.00.

100.00. The solids are composed chiefly of at least three ferments that act in various ways upon food, and certain mineral matters that are necessary for the proper activity of the ferments. The most abundant unorganized ferment of the pancreatic juice has been called trypsin. In common with the pepsin of the stomach, it possesses the power of dissolving the protein, or flesh- forming element of our food, by converting it into peptones. How- ever, trypsin is much more powerful in this respect than is pep- sin; for, in water solution, it dissolves, almost instantaneously, many hundred times its own weight of protein. It differs from pepsin in that it may act in neutral, alkaline, or feebly acid solu- tion. The second most abundant ferment of the pancreatic juice is nearly related to the ptyalin of the saliva. It possesses a remark-

570 IMPROVEMENT ERA.

able power of changing starch and starch-like substances into sugar. It is said that one part by weight of this ferment will

transform, in a few minutes, about forty thousand parts of starch. As far as is known, fats are not at all digested in the mouth and stomach. The pancreatic juice, however, contains a third fer- ment of rather uncertain properties which is said by many to de- compose fats. Fats, as is well known, are composed of glycerine and various organic acids. When a fat is decomposed, it is split into these two constituents, both of which are soluble and may pass into the blood. Recent researches indicate that a portion of the fat eaten, is taken into the blood without undergoing any chemical change; and some three or four years ago, it was dis- covered that a ferment exists in the blood, which has the power of decomposing fats. If fats are at all changed in the digestive

tract, the change occurs in the small intestines, immediately after the food has left the stomach. The bile, which is a secretion of the liver, enters the intes- tines near the entrance for the pancreatic juice. The bile is a chemically highly complex substance that serves many important purposes in the animal economy, among which is a general furth- ering of the digestive process. As the bile contains no unorgan- ized ferments, it does not make the food soluble directly, but in some indirect manner, it is essential to the processes that go on in the intestines. It has been observed in animals, whose livers failed to secrete bile, that fats were not at all digested; and this knowledge has been applied from time immemorial, in removing grease stains with bile. The facts that have been stated concerning digestion by the pancreatic juice in the small intestines, show that the greatest part of the food is digested after it has passed through the stom- ach. In the mouth and stomach, the more easily digestible por- tions of the food are made soluble, but the resistant portions are overcome only by the pancreatic juice and the. bile that flow into the small intestines. The last of the secretions that appear to take part in the digestion of food is the so-called intestinal juice which is produced by the glands found in the mucus membrane of the small intes- tines. It is a pale yellow, usually somewhat turbid liquid of

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powerfully alkaline reaction, which latter is due to the presence of a considerable quantity of soda. Only one unorganized ferment is known, with certainty, to exist in the intestinal juice; it has the property of converting starch into sugar, and appears to finish the conversion of starch into sugar; on protein, and on fats, this juice seems to have no solvent action. The food, after having been subjected to the processes that are characteristic of the mouth, the stomach, and the small intes- tines, is very soluble; the protein has been changed to peptones, the starch to sugar, and the fats either into glycerine and acid or into such forms that can pass readily into the blood. The portions that have not by this time been digested, are fit only to be thrown off by the body. The yellowish, almost odorless mixture of water, digested and undigested food, passes now from the small into the large intes- tine, where rapid absorption of the water and digested matters, by the walls of the intestines, occurs. The undigested food is hurried on, to be removed from the body. The food is then digested and absorbed, and the process is finished. This, then, is the brief and rapid sketch of the process that occurs in our bodies after every meal that we eat; and on the proper performance of which our lives depend. It is as wonderful

as the romances of childhood; and even more wonderful is the story of the methods, pursued by untiring lovers of truth, by which the facts, the outline of which is here presented, have been obtained.