CHAPTER I
FOODS AND THEIR DIGESTION
Topics: The purpose of nutrition. The food of man. Proteid foods. Carbohydrate foods. Fats. Food as fuel. Composition of foodstuffs. Availability of foods. Food as source of energy. Various factors in the nourishment of the body. Processes of digestion. Secretion of saliva. Function of saliva. Enzymes. Reversible action of enzymes. Specificity of enzymes. Mastication. Gastric secretion. Components of gastric juice. Action of gastric juice. Muscular movements of stomach. Time foods remain in stomach. Importance of stomach digestion. Processes of the small intestine. Secretion of pancreatic juice. Chemical changes in small intestine. Destruction of proteid food. Significance of the breaking down of proteid. Change of fatty foods and carbohydrates in intestine. Digestion practically complete at end of small intestine. Putrefaction held in check. Digestion a prelude to utilization of food.
One of the great mysteries of life is the power of growth, that harmonious development of composite organs and tissues from simple protoplasmic cells, with the ultimate formation of a complex organism with its orderly adjustment of structure and function. Equally mysterious is that wonderful power of rehabilitation by which the cells of the body are able to renew their living substance and to maintain their ceaseless activity through a period, it may be of fourscore years, before succumbing to the inevitable fate that awaits all organic structures. This bodily activity, visible and invisible, is the result of a third mysterious process, more or less continuous as long as life endures, of chemical disintegration, decomposition, and oxidation, by which arises the evolution of energy to maintain the heat of the body and the power for mental and physical work.
These three main functions constitute the purpose of nutrition. The growth of the adult man from the tiny cell or germ that marks his simple beginning is at the expense of the food material he absorbs and assimilates. The rehabilitation of the cells, or the composite tissues of the fully developed organism, is accomplished through utilization of the daily food, whereby cell substance is renewed and all losses made good. The energy which manifests itself in the form of heat and mechanical or mental work, i. e., the energy by which the vital machinery is maintained in ceaseless activity, comes from the breaking down of the food materials by means of which, as the saying goes, the body is nourished. The body thus becomes the centre of different lines of activity, the food serving as the material out of which new cells and tissues are constructed, old cells revivified, and energy for running the bodily machinery derived. Development, growth, and vital activity all depend upon the availability of food in proper amounts and proper quality.
The food of man is composed mainly of organic materials, for while, as Dr. Curtis[1] has expressed it, “the plant can make organic matter out of inorganic elements, just this the animal cannot do at all. The thing of legs and locomotion, of spine and speech, can build his organic walls only out of organic bricks ruthlessly ripped from existing walls of other animals or plants.” It is true that man has need of certain inorganic salts in his daily diet, but they are in the nature of aids to nutrition (aside from such as are necessary for the formation of bone and teeth), contributing in some measure toward regulation and control of nutritive processes rather than as a source of energy to the body. Inorganic substances, however, are an integral part of the essential tissues and organs of the body, being combined with the organic constituents of the living cells. Indeed, electrolytes are perhaps the substances that put life into the proteids of the protoplasm, and it is truly important for the integrity and functional power of living cells that the proportion of inorganic constituents therein be kept in a constant condition of quality and quantity. Still, the food of mankind is essentially organic in nature, and while it may be exceedingly varied in character, ranging from the simple vegetable dietary of the natives of India and the Far East to the voluminous admixture of varied forms of animal and vegetable foodstuffs so acceptable to the bon vivant of our western civilization, the principles contained therein are few in number.
The organic foodstuffs are of three distinct types and are classified under three heads, viz.: Proteids or Albuminous foodstuffs, Carbohydrates, and Fats. All animal and vegetable foods, whatever their nature and whatever their origin, are composed simply of representatives of one or more of these three classes of food principles.
Proteid substances are characterized by containing about 16 per cent of nitrogen. In addition, they contain on an average 52 per cent of carbon, 7 per cent of hydrogen, 23 per cent of oxygen, and 0.5–2.0 per cent of sulphur. A certain class of proteids, known as nucleoproteids because of their occurrence in the nuclei of cells, contain likewise a small amount of phosphorus in organic combination. Proteid or albuminous substances constitute the chemical basis of all living cells, whether animal or vegetable. This means, expressed in different language, that the organic substance of all organs and tissues, whether of animals or plants, is made up principally of proteid matter. Proteid substances occupy, therefore, a peculiar position in the nutrition of man and of animals in general. They constitute the class of essential foodstuffs without which life is impossible. For tissue-building and for the renewal of tissues and organs, or their component cells, proteid or albuminous foodstuffs are an absolute requirement. The vital part of all tissue is proteid, and only proteid food can serve for its growth or renewal. Hence, no matter how generous the supply of carbohydrates and fats, without some admixture of proteid food the body will weaken and undergo “nitrogen starvation.” It is to be noted, however, that while the element nitrogen (16 per cent) gives character to the proteid or albuminous foodstuffs, so that they are frequently spoken of or classified as the “nitrogenous foodstuffs,” it is not the nitrogen per se that is so essential for the nutrition of the body. Man lives in an atmosphere of oxygen and nitrogen. He can and does absorb and utilize the free oxygen of the air he breathes; indeed, it is absolutely essential for his existence, but the free nitrogen likewise drawn into the lungs at each inspiration is of no avail for the needs of the body. Further, there are many compounds of nitrogen, some of them closely allied to the proteid foodstuffs in chemical composition, which are just as useless as free nitrogen in meeting the wants of the body for nitrogenous foods.
Dame Nature is very discriminating; she demands a definite form of nitrogenous compound, some peculiar or specific grouping of the nitrogen element with other elements in the food that can make good the waste of proteid tissue. In the inactive and fibrous tissues of animals, such as are found in bones, tendons, and ligaments, there is present a substance known as collagen, which, when boiled with water, as in the making of soups, is transformed into gelatin. This body, because of its close chemical relationship to proteid or albuminous substances, is known as an albuminoid. Yet, though it has essentially the same chemical composition as ordinary albuminous substances and shows many of the reactions characteristic of the latter, it cannot take the place of true proteid in building up or repairing the tissues of the body. To quote again from Dr. Curtis: “Tissue is nitrogenous, so that, of course, only nitrogenous food can serve for its making; but of the two kinds of nitrogenous principles, proteids and albuminoids, behold, proteids only are of avail! Why this is so is unknown, since albuminoid is equally nitrogenous with proteid; but so it is—proteid and proteid alone can fulfil the high function of furnishing the material basis of life. Gelatin cannot even go to make the very kind of tissue of which itself is a derivative. Alongside of its brother proteid, gelatin stands as a prince of the blood whose escutcheon bears the ‘bend sinister.’ Such a one, though of royal lineage, may never aspire to the throne.” It is thus quite clear that the true proteid foods are tissue builders in the broadest sense of the term, and it is equally evident that they are absolutely essential for life, since no other kind or form of foodstuff can take their place in supplying the needs of the body. Every living cell, whether of heart, muscle, brain, or nerve, requires its due allowance of proteid material to maintain its physiological rhythm. No other foodstuff stands in such intimate relationship to the vital processes, but so far as we know at present any form of true proteid, whether animal or vegetable, will serve the purpose.
Carbohydrates include two closely related classes of compounds, viz., sugars and starches. They are entirely free from nitrogen, containing only carbon (44.4 per cent), hydrogen (6.2 per cent), and oxygen (49.4 per cent), and hence are classified as non-nitrogenous foods. Obviously, they cannot serve as tissue builders, but by oxidation they yield energy for heat and work. They constitute an easily oxidizable form of fuel, and when supplied in undue amounts they may undergo transformation within the body into fat, which is temporarily deposited in tissues and organs for future needs.
Fats, like carbohydrates, are free from nitrogen, but differ from them in containing a much larger percentage of carbon, and hence have greater fuel value per pound. Fats contain on an average 76.5 per cent of carbon, 11.9 per cent of hydrogen, and 11.5 per cent of oxygen. With their larger content of carbon and smaller proportion of oxygen, fats are less easily oxidizable than sugars, requiring a larger intake of oxygen for their combustion, but when oxidized they yield more heat per pound than carbohydrates.
Fats and carbohydrates are thus seen to be the natural fuel foodstuffs of the body. They cannot serve for the upbuilding or renewal of tissue, but by oxidation they constitute an economical fuel for maintaining body temperature and for power to run the bodily machinery. It should be remembered, however, that anything capable of being burned in the body may serve as fuel material; hence proteid food, though of specific value as a tissue builder, may likewise by its oxidation yield energy for heat and work, but its combustion, owing to the content of nitrogen, is never complete. Further, its use as fuel is uneconomical and undesirable for reasons to be discussed later, but it is well to know that its oxidation, though incomplete, is accompanied by the liberation of energy, as in the oxidation of non-nitrogenous foods. A portion of the carbon, hydrogen, and oxygen of the proteid molecule will burn within the body to gaseous products, as do sugars and fats, but there remains a nucleus of nitrogen, with some carbon, hydrogen, and oxygen, which resists combustion and must be gotten rid of by the combined labors of liver and kidneys. Fats and carbohydrates, on the other hand, undergo complete combustion to simple gaseous products, carbon dioxide and water, which are easily removed by the lungs, skin, etc.
These three classes of foodstuffs exist in a great variety of combinations or admixtures in nature. In many cases, noticeably in milk, all three occur together in fairly large quantities. In animal foods, such as meats, fish, etc., proteid and fat alone are found, while in perfectly lean meat proteid only is present, excepting a small amount of fat. Again, the white of the egg contains proteid alone. Hence, a meat and egg diet would be essentially a proteid diet. In vegetable foods, as in the cereals, there is found an admixture of proteid and starch, the latter predominating in many cases, as in wheat flour. The following table,[2] showing the chemical composition of various food materials, may be of service in throwing light on the relative distribution of the three classes of foodstuffs in natural products.
THE CHEMICAL COMPOSITION OF SOME COMMON FOOD MATERIALS
| Food Materials. | Proteid. | Carbo-hydrate. | Fat. | Water. | Mineral Matter. | Fuel Value per pound. |
|---|---|---|---|---|---|---|
| per cent | per cent | per cent | per cent | per cent | calories | |
| Fresh beef, loin, lean, edible portion | 24.2 | 0 | 3.7 | 70.8 | 1.3 | 615 |
| Fresh beef, round, lean, edible portion | 22.3 | 0 | 2.8 | 73.6 | 1.3 | 540 |
| Fresh Porterhouse steak, edible portion | 21.9 | 0 | 20.4 | 60.0 | 1.0 | 1270 |
| Fresh beef liver | 21.0 | 1.7 | 4.5 | 71.2 | 1.6 | 605 |
| Fresh beef tongue | 19.0 | 0 | 9.2 | 70.8 | 1.0 | 740 |
| Fresh sweetbreads | 16.8 | 0 | 12.1 | 70.9 | 1.6 | 825 |
| Fresh beef kidney | 16.9 | 0.4 | 4.8 | 76.7 | 1.2 | 520 |
| Cooked beef, roasted | 22.3 | 0 | 28.6 | 48.2 | 1.3 | 1620 |
| Cooked round steak | 27.6 | 0 | 7.7 | 63.0 | 1.8 | 840 |
| Broiled tenderloin steak | 23.5 | 0 | 20.4 | 54.8 | 1.2 | 1300 |
| Dried beef, canned | 39.2 | 0 | 5.4 | 44.8 | 11.2 | 960 |
| Stewed kidneys, canned | 18.4 | 2.1 | 5.1 | 71.9 | 2.5 | 600 |
| Fresh corned beef, edible portion | 15.3 | 0 | 26.2 | 53.6 | 4.9 | 1395 |
| Fresh breast of veal, lean | 21.2 | 0 | 8.0 | 70.3 | 1.0 | 730 |
| Fresh leg of lamb, edible portion | 19.2 | 0 | 16.5 | 63.9 | 1.1 | 1055 |
| Lamb chops, broiled | 21.7 | 0 | 29.9 | 47.6 | 1.3 | 1665 |
| Roast leg of lamb, edible portion | 19.4 | 0 | 12.7 | 67.1 | 0.8 | 900 |
| Roast leg of mutton, edible portion | 25.9 | 0 | 22.6 | 50.9 | 1.2 | 1420 |
| Fresh lean ham | 25.0 | 0 | 14.4 | 60.0 | 1.3 | 1075 |
| Smoked ham, fat, edible portion | 14.8 | 0 | 52.3 | 27.9 | 3.7 | 2485 |
| Chicken, broilers, edible portion | 21.5 | 0 | 2.5 | 74.8 | 1.1 | 505 |
| Turkey, edible portion | 21.1 | 0 | 22.9 | 55.5 | 1.0 | 1360 |
| Roast turkey, edible portion | 27.8 | 0 | 18.4 | 52.0 | 1.2 | 1295 |
| Fricasseed chicken, edible portion | 17.6 | 2.4 | 11.5 | 67.5 | 1.0 | 855 |
| Fresh cod, dressed | 11.1 | 0 | 0.2 | 58.5 | 0.8 | 215 |
| Fresh mackerel, edible portion | 18.7 | 0 | 7.1 | 73.4 | 1.2 | 645 |
| Fresh halibut, steaks | 18.6 | 0 | 5.2 | 75.4 | 1.0 | 565 |
| Fresh shad, edible portion | 18.8 | 0 | 9.5 | 70.6 | 1.3 | 750 |
| Fresh smelt, edible portion | 17.6 | 0 | 1.8 | 79.2 | 1.7 | 405 |
| Cooked bluefish, edible portion | 26.1 | 0 | 4.5 | 68.2 | 1.2 | 670 |
| Broiled Spanish mackerel, edible portion | 23.2 | 0 | 6.5 | 68.9 | 1.4 | 715 |
| Salt codfish, edible portion | 25.4 | 0 | 0.3 | 53.5 | 24.7 | 410 |
| Salt mackerel, edible portion | 22.0 | 0 | 22.6 | 42.2 | 13.2 | 1345 |
| Canned salmon, edible portion | 21.8 | 0 | 12.1 | 63.5 | 2.6 | 915 |
| Canned sardines, edible portion | 23.0 | 0 | 19.7 | 52.3 | 5.6 | 162 |
| Fresh round clams | 6.5 | 4.2 | 0.4 | 86.2 | 2.7 | 215 |
| Fresh oysters, solid | 6.0 | 3.3 | 1.3 | 88.3 | 1.1 | 230 |
| Fresh hen’s eggs | 13.4 | 0 | 10.5 | 73.7 | 1.0 | 720 |
| Boiled hen’s eggs | 13.2 | 0 | 12.0 | 73.2 | 0.8 | 765 |
| Butter | 1.0 | 0 | 85.0 | 11.0 | 3.0 | 3605 |
| Full cream cheese | 25.9 | 2.4 | 33.7 | 34.2 | 3.8 | 1950 |
| Whole cow’s milk | 3.3 | 5.0 | 4.0 | 87.0 | 0.7 | 325 |
| Corn meal, unbolted | 8.4 | 74.0 | 4.7 | 11.6 | 1.3 | 1730 |
| Oatmeal | 16.1 | 67.5 | 7.2 | 7.3 | 1.9 | 1860 |
| Rice | 8.0 | 79.0 | 0.3 | 12.3 | 0.4 | 1630 |
| Wheat flour, entire wheat | 13.8 | 71.9 | 1.9 | 11.4 | 1.0 | 1675 |
| Boiled rice | 2.8 | 24.4 | 0.1 | 72.5 | 0.2 | 525 |
| Shredded wheat | 10.5 | 77.9 | 1.4 | 8.1 | 2.1 | 1700 |
| Macaroni | 13.4 | 74.1 | 0.9 | 10.3 | 1.3 | 1665 |
| Brown bread | 5.4 | 47.1 | 1.8 | 43.6 | 2.1 | 1050 |
| Wheat bread or rolls | 8.9 | 56.7 | 4.1 | 29.2 | 1.1 | 1395 |
| Whole wheat bread | 9.4 | 49.7 | 0.9 | 38.4 | 1.3 | 1140 |
| Soda crackers | 9.8 | 73.1 | 9.1 | 5.9 | 2.1 | 1925 |
| Oyster crackers | 11.3 | 70.5 | 10.5 | 4.8 | 2.9 | 1965 |
| Ginger bread | 5.8 | 63.5 | 9.0 | 18.8 | 2.9 | 1670 |
| Sponge cake | 6.3 | 65.9 | 10.7 | 15.3 | 1.8 | 1795 |
| Lady fingers | 8.8 | 70.6 | 5.0 | 15.0 | 0.6 | 1685 |
| Apple pie | 3.1 | 42.8 | 9.8 | 42.5 | 1.8 | 1270 |
| Custard pie | 4.2 | 26.1 | 6.3 | 62.4 | 1.0 | 830 |
| Squash pie | 4.4 | 21.7 | 8.4 | 64.2 | 1.3 | 840 |
| Indian meal pudding | 5.5 | 27.5 | 4.8 | 60.7 | 1.5 | 815 |
| Tapioca pudding | 3.3 | 28.2 | 3.2 | 64.5 | 0.8 | 720 |
| Fresh asparagus | 1.8 | 3.3 | 0.2 | 94.0 | 0.7 | 105 |
| Fresh lima beans | 7.1 | 22.0 | 0.7 | 68.5 | 1.7 | 570 |
| Dried lima beans | 18.1 | 65.9 | 1.5 | 10.4 | 4.1 | 1625 |
| Dried beans | 22.5 | 59.6 | 1.8 | 12.6 | 3.5 | 1605 |
| Cooked beets | 2.3 | 7.4 | 0.1 | 88.6 | 1.6 | 185 |
| Fresh cabbage, edible portion | 1.6 | 5.6 | 0.3 | 91.5 | 1.0 | 145 |
| Green corn, edible portion | 3.1 | 19.7 | 1.1 | 75.4 | 0.7 | 470 |
| Dried peas | 24.6 | 62.0 | 1.0 | 9.5 | 2.9 | 1655 |
| Green peas | 7.7 | 16.9 | 0.5 | 74.6 | 1.0 | 465 |
| Raw potatoes, edible portion | 2.2 | 18.4 | 0.1 | 78.3 | 1.0 | 385 |
| Boiled potatoes | 2.5 | 20.9 | 0.1 | 75.5 | 1.0 | 440 |
| Fresh tomatoes | 0.9 | 3.9 | 0.4 | 94.3 | 0.5 | 105 |
| Baked beans, canned | 6.9 | 19.6 | 2.5 | 68.9 | 2.1 | 600 |
| Apples, edible portion | 0.4 | 14.2 | 0.5 | 84.6 | 3.0 | 290 |
| Bananas, yellow, edible portion | 1.3 | 22.0 | 0.6 | 75.3 | 0.8 | 460 |
| Fresh cranberries | 0.4 | 9.9 | 0.6 | 88.9 | 0.2 | 215 |
| Oranges, edible portion | 0.8 | 11.6 | 0.2 | 86.9 | 0.5 | 240 |
| Peaches, edible portion | 0.7 | 9.4 | 0.1 | 89.4 | 0.4 | 190 |
| Fresh strawberries | 1.0 | 7.4 | 0.6 | 90.4 | 0.6 | 180 |
| Dried prunes, edible portion | 2.1 | 73.3 | 0.0 | 22.3 | 2.3 | 1400 |
| Almonds, edible portion | 21.0 | 17.3 | 54.9 | 4.8 | 2.0 | 3030 |
| Peanuts, edible portion | 25.8 | 24.4 | 38.6 | 9.2 | 2.0 | 2560 |
| Pine nuts, edible portion | 33.9 | 6.9 | 49.4 | 6.4 | 3.4 | 2845 |
| Brazil nuts, edible portion | 17.0 | 7.0 | 66.8 | 5.3 | 3.9 | 3265 |
| Soft-shell walnuts, edible portion | 16.6 | 16.1 | 63.4 | 2.5 | 1.4 | 3285 |
In commenting on these figures, reference to which will be made from time to time in other connections, it may be wise to emphasize the large amount of water almost invariably present in natural foodstuffs. Further, it is to be noted that, in animal products especially, the variations in proteid-content are in large measure coincident with variations in the amount of water present. In other words, foods of animal origin if freed entirely of water would, as a rule, show essentially the same percentage of proteid matter. Fat is naturally variable, according to the condition of the animal at the time it was slaughtered. Among the vegetable products, carbohydrate, mainly in the form of starch, becomes exceedingly conspicuous, though proteid is by no means lacking. Indeed, in some cereals, as in oatmeal, in dried peas and beans, the content of proteid will average as high as in fresh beef, while in addition 50–70 per cent of the entire substance is made up of carbohydrate. Again, in the edible nuts, the content of proteid runs high, in some cases higher than in fresh beef, while at the same time carbohydrate and fat are noticeably large. Further, it is to be noted that in nuts there is here and there some striking individuality, as in pine nuts and Brazil nuts, both of which show a noticeable lack of carbohydrate as contrasted with peanuts, almonds, and walnuts; a fact of some importance in cases where a vegetable food rich in proteid is desired, but with freedom from starch.
Another generality, to be thoroughly understood, is that while the figures given for proteid express quite clearly and with reasonable degree of accuracy the relative amounts of proteid matter present in the foodstuffs in question, there may be important differences in availability of which the percentage figures give no suggestion. In other words, the analytical data deal solely with the total content of proteid, while there is needed in addition information as to the relative digestibility, or availability by the body, of the different kinds of proteid food. For example, roast mutton, cream cheese, and dried peas contain approximately the same amount of proteid. Are we then to infer that these three foods have the same nutritive value so far as proteid is concerned? Surely not, since no account is taken of the relative digestibility of the three foods. It is one of the axioms of physiology that the true nutritive value of any proteid food is dependent not alone upon the amount of proteid contained therein, but upon the quantity of proteid that can be digested and absorbed; or, in other words, made available for the needs of the body. The same rule holds good for both fats and carbohydrates, but as proteid is the more important foodstuff, and is as a rule taken more sparingly, the question of availability has greater import with the proteid foods.
The availability or digestibility of foods can be determined only by physiological experiment. By making a comparison for a definite period of time of the amount of a given food ingredient consumed and the amount that passes unchanged through the intestine, an estimate of its digestibility can be made. The result, to be sure, is not wholly free from error, since we cannot always distinguish between the undigested food and so-called metabolic products coming from the digestive juices and from the walls of the intestine; but the errors are not large, and results so obtained are full of meaning. In a general way it may be stated that with animal foods, such as meats, eggs, and milk, about 97 per cent of the contained proteid is digested and thereby rendered available for the body. With ordinary vegetable foods, on the other hand, as they are usually prepared for consumption, only about 85 per cent of the proteid is made available. This is partially due to the presence in the vegetable tissue of cellulose, which in some measure prevents that thorough attack of the proteid by the digestive juices which occurs with animal foods. With a mixed diet, i. e., with a variable admixture of animal and vegetable foods, it is usually considered that about 92 per cent of the proteid contained therein will undergo digestion.
Regarding differences in the availability of fats, it may be stated that, as a rule, the fatty matter contained in vegetable foods is less readily, or less thoroughly, digested than that present in foods of animal origin. In the latter, about 95 per cent of the fat is digested and absorbed. This figure, however, is generally taken as representing approximately the digestibility or availability of the fat contained in man’s daily dietary, since by far the larger proportion of the fat consumed is of animal origin. Carbohydrates, on the other hand, are much more easily utilized by the body. Naturally, sugars, owing to their great solubility and ready diffusibility, offer little difficulty in the way of easy digestion; but starches likewise, though not so readily assimilable, are digested, as a rule, to the extent of 98 per cent or more of the amount consumed. It is thus evident that in any estimate of the food value of a given diet, chemical composition is to be checked by the digestibility or availability of the food ingredients.
As has been stated several times, the proteid foodstuffs are the more important, since proteid matter is essential to animal life. Man must have a certain amount of proteid food to maintain the body in a condition of strength and vigor. The other essential is that the daily food furnish sufficient energy to meet the needs of the body for heat and power. This means that in addition to proteid, which primarily serves a particular purpose, there must be enough non-nitrogenous food (either carbohydrate or fat or both) to provide the requisite fuel for oxidation or combustion to meet the demands of the body for heat and for work; both of which are subject to great variation owing to differences in the temperature of the surrounding air, and especially because of variations in the degree of bodily activity. The energy which a given foodstuff will yield can be ascertained by laboratory experiment, in which a definite weight of the substance is burned or oxidized in a calorimetric bomb under conditions where the exact amount of heat liberated can be accurately measured. The fuel, or energy, value so obtained is expressed in calories or heat units. A calorie may be defined as the amount of heat required to raise 1 gram of water 1° C., or, to be more exact, the amount of heat required to raise 1 gram of water from 15° to 16° C. This unit is usually spoken of as the small calorie, to distinguish it from the large calorie, which represents the amount of heat required to raise 1 kilogram of water 1° C. Hence, the large calorie is equal to one thousand small calories. When burned in a calorimeter, 1 gram of carbohydrate yields on an average 4100 gram-degree units of heat, or small calories; 1 gram of fat yields 9300 small calories. Both of these non-nitrogenous foods burn or oxidize to the same products—viz., carbon dioxide and water—when utilized in the body as when burned in the calorimeter; hence, the figures given represent the physiological heat of combustion, per gram, of the two classes of foodstuffs. Obviously, the fuel values of different foods belonging to the same group or class will show slight variation, but the above figures represent average values.
Unlike fats and carbohydrates, proteids are not burned completely in the body; hence, the physiological fuel value of a proteid is less than the value obtained by oxidation in a bomb calorimeter. In the body, proteids yield certain decomposition products which are removed through the excreta, and which represent a certain quantity of potential energy thus lost to the economy. The average fuel value of proteids burned outside of the body is placed at 5711 calories per gram,[3] or 5.7 large calories. Deducting the heat value of the proteid decomposition products contained in the excreta, the physiological fuel value of proteids is reduced on an average to about 4.1 large calories per gram.[4] Rubner considers that the physiological fuel value of vegetable proteids is somewhat less than that of animal proteids; conglutin, for example, yielding 3.96 calories, as contrasted with 4.3 calories furnished by egg-albumin, or 4.40 calories from casein. On a mixed diet, where 60 per cent of the ingested proteid food is of animal origin and 40 per cent vegetable, the fuel value available to the body would be about 4.1 calories per gram of proteid, on the assumption that the physiological heat value of vegetable proteids averages 3.96 calories per gram and that of animal proteids 4.23 calories per gram (Rubner).
At present, we accept for all purposes of computation the following figures as representing the physiological or available (to the body) fuel value of the three classes of organic foodstuffs:
| 1 gram of proteid | 4.1 Large Calories |
| 1 gram of fat | 9.3 " " |
| 1 gram of carbohydrate | 4.1 " " |
From these data, it is evident at a glance that 1 gram of fat is isodynamic with 2.27 grams of either carbohydrate or proteid; and since carbohydrate and fat are of use to the body mainly because of their energy value, it is obvious that 50 grams of fat taken as food will be of as much service to the body as 113 grams of starch. In view of the relatively high fuel value of fats, it follows that the physiological heat of combustion of any given food material will correspond largely with the content of fat therein. This is quite apparent from the data given in the table showing chemical composition of food materials, where the fuel value per pound is seen to run more or less closely parallel with the percentage of fat. Experience, as well as direct physiological experiment, teaches us, however, that fat and carbohydrate cannot be interchanged indefinitely, because of the difficulty in utilization of fat when the amount is increased beyond a certain point. Personal experience provides ample evidence of the difference in availability between the two classes of foodstuffs. Carbohydrates are easily utilizable, fats with more difficulty. Palate, as well as stomach, rebels at large quantities of fat; a statement that certainly holds good for most civilized people, though exceptions may be found, as in the Esquimeaux and certain savage races.
In the nourishment of the body, the various factors that aid in the utilization of food are of great moment and must not be overlooked. It is not enough that the body be supplied with the proper proportion of nutrients, with sufficient proteid to meet the demand for nitrogen, and with carbohydrate and fat adequate to yield the needed energy; but all those physiological processes which have to do with the preparation of the foodstuffs for absorption into the circulating blood and lymph must be in effective working order. There is an intricacy of detail here which calls for careful oversight, and it is one of the functions of the nervous system to control and regulate both the mechanical and the chemical processes that are concerned in this seemingly automatic progression of foodstuffs from their entry into the mouth cavity to their final discharge from the alimentary tract, after removal of the last vestige of true nutritive material.
Mastication; deglutition; secretion of the various digestive juices, saliva, gastric juice, pancreatic juice, bile, intestinal juice, etc.; peristalsis, or the rhythmical movements of the muscular walls of the gastro-intestinal tract; the solvent action of the several digestive fluids on the different types of foodstuffs; the absorption of the products formed as a preliminary step in their transportation to the tissues and organs of the body, where they are to serve their ultimate purpose in nutrition; the interaction of these several processes one on the other; and, finally, the influence of the various nerve fibres and nerve centres concerned in the control of these varied activities,—all must work together in harmony and precision if the full measure of available nitrogen and energy-yielding material is to be extracted and absorbed from the ingested food, without undue expenditure of physiological labor. Further, the various processes of cell and tissue metabolism, by which the absorbed food material is built up into living protoplasm, and the chemical processes of oxidation, hydrolysis, reduction, etc., by which the intra and extra cellular material is broken down progressively into varied katabolic or excretory products, with liberation of energy; all these must move forward harmoniously and with due regard to the preservation of an even balance between intake and outgo, if the nutrition of the body is to be maintained at a proper level, and with that degree of physiological economy which is coincident with good health and high efficiency.
We may well pause here and consider briefly some of these processes which play so prominent a part in the proper utilization of the three classes of organic foodstuffs. The first digestive fluid which the ingested food comes in contact with is the saliva. Sensory nerve fibres, chiefly of the glossopharyngeal and lingual nerves which supply the mouth and tongue, are stimulated by the sapid substances of the food, and likewise by mere contact of the food particles with the mucous membrane lining the mouth cavity as the food is masticated and rolled about prior to deglutition. Impulses communicated in this way to the above sensory nerves are transmitted to certain nerve centres in the medulla oblongata, whence impulses are reflected back through secretory nerves going to the individual salivary glands, thereby calling forth a secretion. The production of saliva is thus a simple reflex act, in which the food consumed serves as a true stimulant or excitant. Pawlow,[5] indeed, claims a certain degree of adaptability of the secretion to the character of the food taken into the mouth. Thus, he finds that dry, solid food excites a large flow of saliva, such as would be needed to masticate it properly and bring it into a suitable condition for swallowing. On the other hand, foods containing an abundance of water cause only a scanty flow of saliva. The situation of this secretory centre in the medulla, and the many branchings of nerve cells in this locality would naturally suggest the possibility of salivary secretion being incited by stimuli from a variety of sources. This is indeed the case, and it is worthy of note that a flow of saliva may result from stimulation of the sensory fibres of the vagus nerves as well as of the splanchnic and sciatic, thus indicating how a given secreting gland may be called into activity by impulses or stimuli which come to the centre through very indirect and devious pathways. Further, the secretory centre may be stimulated, and likewise inhibited, by impulses which have their origin in higher nerve centres in the brain. These facts are of great importance in throwing light upon the ways in which a secretion like saliva is called forth and its digestive action thus made possible. The thought and the odor of savory food cause the mouth to water, the flow of saliva so incited being the result of psychical stimulation. Similarly, fear, embarrassment, and anxiety frequently cause a dry mouth and parched throat through inhibition of the secretory centre by impulses which have their origin in higher centres in the brain.
The application of these facts to our subject is perfectly obvious, since they suggest at once how the production or secretion of an important digestive fluid—upon which the utilization of a given class of foodstuffs may be quite dependent—is controlled and modified through the nervous system by a variety of circumstances. We might reason that the appearance, odor, and palatability of food are factors of prime importance in its utilization by the body; that the æsthetics of eating are not to be ignored, since they have an important influence upon the flow of the digestive secretions. A peaceful mind, pleasurable anticipation, freedom from care and anxiety, cheerful companionship, all form desirable table accessories which play the part of true psychical stimuli in accelerating the flow of the digestive juices and thus pave the way for easy and thorough digestion. Further, it is easy to see how thorough mastication of food may prolong mechanical stimulation of the salivary glands and thus increase the flow of the secretion, while the longer stay of sapid substances in the mouth cavity increases the duration of the chemical stimulation of the sensory fibres of the lingual and glossopharyngeal nerves. In this connection, we may cite the view recently advanced by Pawlow that the individual salivary glands respond normally to different stimuli. Thus, there are three pairs of salivary glands concerned in the production of saliva,—the submaxillary, parotid, and sublingual,—all of which pour their secretions through separate ducts into the mouth cavity. By experiment, Pawlow has found that in the dog the submaxillary gland yields a copious flow of saliva when stimulated by acids, the chewing of meats, the sight of food, etc., while the parotid gland fails to respond. On the other hand, the latter gland responds with an abundant secretion when dry food, such as dry powdered meat, dried bread, etc., is placed in the mouth. With this gland, the inference is that dryness is the active stimulus.
As a digestive secretion, saliva serves several important purposes. By moistening the food it renders mastication and deglutition possible; its natural alkalinity tends to neutralize somewhat such acidity as may be present in the food; it dissolves various solid substances, thus making a solution capable of stimulating the taste nerves; lastly, and most important, it has a marked digestive and solvent action on starchy foods. A large proportion of the non-nitrogenous food consumed by man—in most countries—is composed of some form of starch, and this the body cannot use until it has undergone conversion into soluble forms, such as dextrins and sugar. This it is the function of saliva to accomplish, and it owes its activity in this direction to the presence of a soluble ferment or enzyme known as ptyalin.
Enzymes, which play so important a part in all digestive processes, are a peculiar class of substances produced by the living cells which constitute the various secreting glands. They are of unknown composition, and are peculiar in that the chemical changes they induce are the result of what is termed catalysis, i. e., contact. That is, the enzyme or catalyzer does not enter into the reaction, it is not destroyed or used up, but by its mere presence sets in motion or accelerates a reaction between two other substances. The ordinary illustration from the inorganic world is spongy platinum, which, if placed in contact with a mixture of oxygen and hydrogen, causes the two gases to unite with formation of water, although the two gases alone at ordinary temperature will not so combine. In this reaction the platinum is not altered, neither does it apparently enter into the reaction; it is a simple catalyzer. The chemical nature of the change which most digestive enzymes produce is usually defined as hydrolytic, in which the substance undergoing transformation is made to combine with water, thus becoming hydrolyzed, this reaction generally being accompanied by a cleavage or splitting of the molecule into simpler substances. It is to be noted further that enzymes are specific in their action. An enzyme that acts upon starch, for example, cannot act on proteids or fats. Some digestive fluids have the power of producing changes in different classes of foodstuffs, but such diversity of action is always assumed to be due to the presence in the same fluid of different enzymes. Emil Fischer[6] has advanced the theory that the specificity of an enzyme is related to the geometrical structure of the substance undergoing change; i. e., that each enzyme is capable of acting upon or attaching itself only to such molecules as have a definite structure with which the enzyme is in harmony. Or, the enzyme may be considered as a key which will fit only into the lock (structure) of the molecule it acts upon.
One characteristic feature of enzymes is the incompleteness of their action. Thus, the enzyme of saliva transforms starch by a series of progressive changes into soluble starch, two or more dextrins, and the sugar maltose as the chief end-product. A mixture of starch paste and saliva under ordinary conditions, however, never results in the formation of a hundred per cent of maltose, but there always remains a variable amount of dextrin which appears to resist further change. This is apparently due to what is known as the reversible action of enzymes. Thus, the chemical reactions involved here are reversible actions, i. e., they take place in opposite directions. The catalyzer not only accelerates or incites a reaction in the direction of breaking down the substance acted upon, but it also aids in the recomposition of the products so formed into the original or kindred substance. With reversible reactions of this sort the opposite changes sooner or later strike an equilibrium, which remains constant until some alteration in the conditions brings about an inequality and the reactions proceed until a new equilibrium is established. In the body, however, where the circulating blood and lymph provide facilities for the speedy removal by absorption of the soluble products formed, the reaction may proceed until the original substance undergoing change is completely transformed into the characteristic end-product. This reversible action of enzymes is an important feature, and helps explain certain nutritional changes to be referred to later. Whether all enzymes behave in this way is not as yet determined.
Another peculiarity of digestive enzymes is their extreme sensitiveness to changes in their environment. Powerful in their ability to transform relatively large quantities of a given foodstuff into simple products better adapted for absorption and utilization by the body, they are, however, quickly checked in their action, and even destroyed, when the conditions surrounding them are slightly interfered with. They require for their best action a temperature closely akin to that of the healthy body, and any great deviation therefrom will result at once in an inhibition of their activity. Further, they demand a certain definite reaction of the fluid or mixture, if their working power is to be maintained at the maximum. Indeed, many enzymes, like the ptyalin of saliva, are quickly destroyed if the reaction is greatly changed. Enzymes are thus seen to be more or less unstable substances, endowed with great power as digestive agents, but sensitive to a high degree and working advantageously only under definite conditions. Many perversions of digestion and of nutrition are connected not only with a lack of the proper secretion of some one or more digestive enzyme, but also with the lack of proper surroundings for the manifestation of normal or maximum activity.
With these statements before us, we can readily picture for ourselves the initial results following the ingestion of starch-containing foods properly cooked; and it may be mentioned here that the cooking is an essential preliminary, for uncooked starch cannot be utilized in any degree by man. With the mind in a state of pleasurable anticipation, with freedom from care and worry, which are so liable to act as deterrents to free secretion, and with the food in a form which appeals to the eye as well as to the olfactories, its thorough mastication calls forth and prolongs vigorous salivary secretion, with which the food becomes intimately intermingled. Salivary digestion is thus at once incited, and the starch very quickly commences to undergo the characteristic change into soluble products. As mouthful follows mouthful, deglutition alternates with mastication, and the mixture passes into the stomach, where salivary digestion can continue for a limited time only, until the secretion of gastric juice eventually establishes in the stomach-contents a distinct acid reaction, when salivary digestion ceases through destruction of the starch-converting enzyme. Need we comment, in view of the natural brevity of this process, upon the desirability for purely physiological reasons of prolonging within reasonable limits the interval of time the food and saliva are commingled in the mouth cavity? It seems obvious, in view of the relatively large bulk of starch-containing foods consumed daily, that habits of thorough mastication should be fostered, with the purpose of increasing greatly the digestion of starch at the very gateway of the alimentary tract. It is true that in the small intestine there comes later another opportunity for the digestion of starch; but it is unphysiological, as it is undesirable, for various reasons, not to take full advantage of the first opportunity which Nature gives for the preparation of this important foodstuff for future utilization. Further, thorough mastication, by a fine comminution of the food particles, is a material aid in the digestion which is to take place in the stomach and intestine. Under normal conditions, therefore, and with proper observance of physiological good sense, a large proportion of the ingested starchy foods can be made ready for speedy absorption and consequent utilization through the agency of salivary digestion.
Nowhere in the body do we find a more forcible illustration of economical method in physiological processes than in the mechanism of gastric secretion. Years ago, it was thought that the flow of gastric juice was due mainly to mechanical stimulation of the gastric glands by contact of the food material with the lining membrane of the stomach. This, however, is not the case, as Pawlow has clearly shown, and it is now understood that the flow of gastric juice is started by impulses which have their origin in the mouth and nostrils; the sensations of eating, the smell, sight, and taste of food serving as psychical stimuli, which call forth a secretion from the stomach glands, just as the same stimuli may induce an outpouring of saliva. These sensations, as Pawlow has ascertained, affect secretory centres in the brain, and impulses are thus started which travel downward to the stomach through the vagus nerves, and as a result gastric juice begins to flow. This process, however, is supplemented by other forms of secretion, likewise reflex, which are incited by substances, ready formed in the food, and by substances—products of digestion—which are manufactured from the food in the stomach. Soups, meat juice, and the extractives of meat, likewise dextrin and kindred products, when present in the stomach, are especially active in provoking secretion. Substances which in themselves have less flavor, as water, milk, etc., are far less effective in this direction, while the white of eggs and bread are entirely without action in directly stimulating secretion. When the latter foods have been in the stomach for a time, however, and the proteid material has undergone partial digestion, then absorption of the products so formed calls forth energetic secretion of gastric juice. It is thus seen that there are three distinct ways—all reflex—by which gastric juice is caused to flow into the stomach as a prelude to gastric digestion. Further, it has been shown by Pawlow that there is a relationship between the volume and character of the gastric juice secreted and the amount and composition of the food ingested, thus suggesting a certain adjustment in the direction of physiological economy well worthy of note. A diet of bread, for example, leads to the secretion of a smaller volume of gastric juice than a corresponding weight of meat produces, but the juice secreted under the influence of bread is richer in pepsin and acid, i. e., it has a greater digestive action than the juice produced by meat. The suggestion is that gastric juice assumes different degrees of concentration, with different proportions of acid and pepsin, to meet the varying requirements of a changing dietary.
As has been indicated, pepsin and hydrochloric acid are the important constituents of gastric juice. It is noteworthy, however, that it is the combination of the two that is effective in digestion. Pepsin without acid is of no avail, and acid without pepsin can accomplish little in the digestion of food. Pepsin and acid are secreted by different gland cells in the stomach, and gastric insufficiency, or so-called indigestion, may arise from either a condition of apepsia or from hypoacidity. It is worthy of comment that the amount of hydrochloric acid secreted during 24 hours by the normal individual, under ordinary conditions of diet, amounts to what would constitute a fatal dose of acid if taken at one time in concentrated form. At the outset of gastric secretion, the fluid shows only a slight degree of acidity, but as secretion proceeds, the acidity rises to 0.2–0.3 per cent of hydrochloric acid. The main action of gastric juice is exerted on proteid foods, which under its influence are gradually dissolved and converted into soluble products known as proteoses and peptones. It is a process of peptonization, in which the proteid of the food is gradually broken down into so-called hydrolytic cleavage products. The enzyme, like the ptyalin of saliva, is influenced by temperature, maximum digestive action being manifested at about 38° C., the temperature of the body. Further, a certain degree of acidity is essential for procuring the highest degree of efficiency. Ordinarily, it is stated that digestive action proceeds best in the presence of 0.2 per cent hydrochloric acid, but what is more essential for vigorous digestion is a certain relationship between the acid, pepsin, and proteid undergoing digestion. As pepsin and the amount of proteid are increased, the amount of acid, and its percentage somewhat, must be correspondingly increased if digestion is to be maintained at the maximum.
Another important function of gastric juice is that of curdling milk, due to the presence in the secretion of a peculiar enzyme known as rennin. The latter ferment acts upon the casein of milk,—the chief proteid constituent,—transforming it into a related substance commonly called paracasein. This then reacts with the calcium salts present in milk, forming an insoluble curd or calcium compound. From this point on, the digestion of milk-casein by gastric juice is the same as that of any other solid proteid, it being gradually transformed by the pepsin-acid into soluble cleavage products. Why gastric juice should be provided with this special enzyme, capable of acting solely on the casein of milk, can only be conjectured, but we may assume that it has to do with the economical use of this important food. As the sole nutriment of the young, milk occupies a peculiar position as a foodstuff, and being a liquid, its proteid constituent might easily escape complete digestion were it to pass on too hastily through the gastro-intestinal tract. Experiment has shown that when liquid food alone is taken into the stomach it is pushed forward into the small intestine in a comparatively short time. Curdled as it is by rennin, however, casein must stay for a longer period in the stomach, like any other solid food, and its partial digestion by gastric juice thereby made certain. For the reasons above stated, it is apparent why milk should not be treated as a drink in our daily diet. Remembering that when milk reaches the stomach it is converted into a solid clot or curd, there is obvious reason for sipping it, instead of taking it by the glassful, thereby favoring the formation of small, individual clots instead of one large curd, and thus facilitating instead of retarding digestion.
Among other factors in gastric digestion, the muscular movements of the stomach walls are to be emphasized, since we have here a mechanical aid to digestion of no small moment, and likewise a means of accomplishing the onward movement of the stomach contents. The outer walls of the stomach are composed of a thick layer of circular muscular fibres, especially conspicuous at the pyloric end of the organ, where the latter is joined on to the intestine; a smaller, less conspicuous layer of longitudinal muscle fibres, and some oblique fibres. At the pylorus, the circular fibres are so arranged as to form a structure which, aided by a peculiar folding of the inner mucous membrane, serves as a sphincter, closing off the stomach from the duodenum, the beginning of the small intestine. The movements of the stomach were first made the subject of careful investigation by Dr. Beaumont in his study of the celebrated case of Alexis St. Martin, a French Canadian, who, in 1822, was accidentally wounded by the discharge of a musket, with the resultant formation of a permanent fistulous opening in the stomach. Dr. Beaumont, in the description[7] of his observations, writes that “by the alternate contractions and relaxations of these bands (of muscle) a great variety of motion is induced on this organ (the stomach), sometimes transversely, and at other times longitudinally. These alternate contractions and relaxations, when affecting the transverse diameter, produce what are called vermicular or peristaltic motions. . . . When they all act together, the effect is to lessen the cavity of the stomach, and to press upon the contained aliment, if there be any in the stomach. These motions not only produce a constant disturbance, or churning of the contents of this organ, but they compel them, at the same time, to revolve around the interior, from point to point, and from one extremity to the other.” Of more recent investigations, the most important are those made by Cannon,[8] with the X-ray apparatus. From these later studies, it is evident that Dr. Beaumont’s view of the entire stomach being involved in a general rotary movement is not correct, since in reality the movements are confined mainly to the pyloric end of the stomach, the fundus or portion nearer the œsophagus not being directly involved. This means that when food material passes into the stomach, it may remain at the fundic end for some time more or less undisturbed before admixture with the gastric juice occurs, and under such conditions, until acidity creeps in, the salivary digestion of starch can continue.
According to the observations of Cannon, the contractile movements of the stomach commence shortly after the entrance of food, the contractions starting from about the middle of the stomach and passing on toward the pylorus. These waves of contraction follow each other very closely, certainly not more than one or two minutes apart, and perhaps less, while the resulting movements bring about an intimate commingling of food and gastric juice in the pyloric portion of the stomach; followed by a gradual diffusion of the semi-fluid mixture into the fundus accompanied by a gradual displacement of the more solid food in the latter region. These movements of the stomach are more or less automatic, arising from stimuli—the acid secreted—originating in the stomach itself, although it is considered that the movements are subject to some regulation from extrinsic nerve fibres, such as the vagi and the splanchnics. As digestion proceeds and the mass in the stomach becomes more fluid, the pyloric sphincter relaxes and a certain amount of the fluid material is forced into the intestine by the pressure of the contraction wave. This is repeated at varying intervals, depending presumably in some measure upon the consistency of the mass in the stomach, until after some hours of digestion the stomach is completely emptied.
Especially interesting and suggestive are the experiments made by Cannon[9] on the length of time the different types of foodstuffs remain in the stomach. Using cats as subjects, he found that fats remain for a long period in the stomach; they leave that organ slowly, the discharge into the intestine being at about the same rate as the absorption of fat from the small intestine or its passage into the large intestine. Carbohydrate foods, on the other hand, begin to leave the stomach soon after their ingestion. They pass out rapidly, and at the end of two hours reach a maximum amount in the small intestine almost twice the maximum for proteids, and two and a half times the maximum for fats, both of which maxima are reached only at the end of four hours. Carbohydrates remain in the stomach about half as long as proteids. Proteids, Cannon finds, frequently do not leave the stomach at all during the first half-hour after they are eaten. After two hours, they accumulate in the small intestine to a degree only slightly greater than that reached by carbohydrates an hour and a half earlier. The departure of proteids from the stomach is therefore slower at first than that of either fats or carbohydrates. When a mixture of equal parts of carbohydrates and proteids is fed, the discharge from the stomach is intermediate in rapidity. When fat is added to either carbohydrates or proteids it retards the passage of both foodstuffs through the pylorus.
It is evident from what has been stated that the gastric digestion of proteid foods is a comparatively slow process, involving several hours of time; and further, that food material in general remains in the stomach for varying periods, dependent upon its chemical composition. It would appear further, that relaxation of the pyloric sphincter, allowing passage of chyme into the intestine, must depend somewhat upon chemical stimulation, as this offers the most plausible explanation of the diversity of action seen with the different foodstuffs. As has been pointed out, gastric digestion is primarily a process for the conversion of proteid food into soluble products. It would be a mistake, however, to assume that the digestion of proteid foods is complete in the stomach. Stomach digestion is to be considered more as a preliminary step, paving the way for further changes to be carried forward by the combined action of intestinal and pancreatic juice in the small intestine. The importance of gastric digestion is frequently overrated. It is unquestionably an important process, but not absolutely essential for the maintenance of life. Dogs have lived and flourished with their stomachs removed, the intestine being joined to the œsophagus. The intestine is a much more important part of the alimentary tract; it is likewise far more sensitive to changing conditions than the stomach, and undoubtedly one function of the latter organ is to protect the intestine and preserve it from insult. The stomach may be compared to a vestibule or reservoir, capable of receiving without detriment moderately large amounts of food, together with fluid, in different forms and combinations, with the power to hold them there until by action of the gastric juice they are so transformed that their onward passage into the intestine can be permitted with perfect safety. Then, small portions of the properly prepared material may be discharged from time to time through the pylorus without danger of overloading the intestine, and in a form capable of undergoing rapid and complete digestion. Further, the stomach as a reservoir is very useful in bringing everything to a proper and constant temperature before allowing its entry into the intestine. Another fact of some importance is that, contrary to the general view, absorption from the stomach of the products of digestion is not very rapid under ordinary conditions. Even water and soluble salts pass very slowly into the circulation from the stomach. Like the partially digested food material, they are carried forward through the pyloric sphincter into the intestine, where absorption of all classes of material is most marked.
It is in the small intestine that both digestion and absorption are seen at their best. It is here that all three classes of foodstuffs are acted upon simultaneously through the agency of the pancreatic juice, intestinal juice, and bile. Here, too, are witnessed some of the most complicated and interesting reactions and changes occurring in the whole range of digestive functions. Especially noteworthy is the peculiar mechanism by which the secretion of pancreatic juice is set up and maintained. On demand, pancreatic juice is manufactured in the pancreas and poured into the intestine just beyond the pylorus through a small duct—the duct of Wirsung. Secretion is started by contact of the acid contents of the stomach with the mucous membrane of the small intestine, so that as soon as the acid chyme passes through the pyloric sphincter there commences an outflow of pancreatic juice into the intestine. While acid is plainly the inciting agent in this secretory process, its action is indirect. It does not cause secretion through reflex action on nerve fibres, but it acts upon a substance formed in the mucous membrane of the intestine, transforming it into secretin, which is absorbed by the blood and carried to the pancreas, where it excites secretory activity. As would be expected from the foregoing statements, the secretion of pancreatic juice commences very soon after food finds its way into the stomach, and naturally increases in amount with the onward passage of acid chyme into the intestine, the maximum flow being obtained in the neighborhood of the third or fourth hour, after which the secretion gradually decreases. In man, it is estimated on the basis of one or two observations that the amount secreted during 24 hours is about 700 cc., or a pint and a half. Careful experiments, however, tend to show that the quantity of secretion depends in some measure at least upon the character of the food, and also that the composition of the secretion varies with the character of the food. Thus, on a diet composed mainly of meat, the proteid-digesting enzyme is especially conspicuous, while on a bread diet, with its large content of starch, the starch-digesting enzyme is increased in amount. In other words, there is suggested the possibility of an adaptation in the composition of the secretion to the character of the food to be digested.
Pancreatic juice is an alkaline fluid, rather strongly alkaline in fact, from its content of sodium carbonate, and is especially characterized by the presence of at least three distinct enzymes; viz., trypsin, a proteid-digesting ferment; lipase, a fat-splitting enzyme; and amylopsin, a starch-digesting enzyme. It has already been pointed out how dependent the secretion of pancreatic juice is upon the co-operation of the intestinal mucous membrane. A similar dependence is found when the digestive activity of the secretion is studied. As just stated, pancreatic juice contains a proteid-digesting enzyme. This statement, however, is not strictly correct, for if the secretion is collected through a cannula so that it does not come in contact with the mucous membrane of the intestine, it is found free from any digestive action on proteids. The secretion is activated, however, by contact with the duodenal membrane. Expressed in different language, pancreatic juice as it is secreted by the gland does not contain ready-formed trypsin; it does contain, however, an inactive pro-enzyme, which, under the influence of a specific substance contained in the intestinal mucous membrane, known as enterokinase, is transformed into the active enzyme trypsin. There is thus seen another suggestive example of the close physiological relationship between the small intestine and the activity of the pancreatic gland, or its secretion.
The chemical changes taking place in the small intestine are many and varied. The acid chyme, with its admixture of semi-digested food material, as it passes through the pyloric sphincter into the small intestine, is at once brought into immediate contact with bile, pancreatic juice, and intestinal juice, all of which are more or less alkaline in reaction. As a result, the acidity of the gastric juice is rapidly overcome, and the enzyme pepsin, which up to this point could exert its characteristic digestive action, is quickly destroyed by the accumulating alkaline salts. Pepsin digestion thus gives way to trypsin digestion,—most effective in an alkaline medium,—and the proteids of the food, already semi-digested by pepsin-acid, are further transformed by trypsin; aided and abetted by another enzyme, known as erepsin, secreted by the mucous membrane of the intestine. These two enzymes are much more powerful agents than pepsin. It is true that they begin work where pepsin left off, but most striking is the character of the end-products which result from their combined action, since they are small molecules and there is a surprising diversity of them. In other words, while gastric digestion breaks down the proteid foodstuffs into soluble bodies, such as proteoses and peptones closely related to the original proteids, in pancreatic digestion as it takes place in the intestine there is a profound breaking down, or disruption of the proteid molecule into a row of comparatively simple nitrogenous fragments, many of them crystalline bodies; such as leucin, tyrosin, glutaminic acid, aspartic acid, arginin, lysin, histidin, etc., known chemically as monoamino-acids and diamino-acids. We have no means of knowing to how great an extent these more profound disruptive changes of the proteid molecule take place in the intestine. Whether practically all of the ingested proteid food is broken down into these relatively simple compounds prior to absorption, or whether only a small fraction suffers this change, cannot be definitely stated.
A few years ago, the majority of physiologists held to the view that in the digestion of proteid food all that was essential was its conversion into soluble and diffusible forms which would permit of ready absorption into the blood. The belief was prevalent that, since the proteid of the food was destined to make good the proteid of the blood and through the latter the proteids of the tissues, any change beyond what was really necessary for absorption of the proteid would be uneconomical and indeed wasteful. On the other hand, due weight must be given to the fact that in trypsin digestion, proteid can be quickly broken down into simple nitrogenous compounds, and that in the enzyme erepsin, present in the mucous membrane of the intestine, we have an additional ferment very efficient in bringing about cleavage of proteoses and peptone into amino-acids. From these latter facts it might be argued that, in the digestion of proteid foodstuffs by the combined action of gastric and pancreatic juice in the alimentary tract, a large proportion of the proteid is destined to undergo complete conversion into amino-acids, and that from these fragments the body, by a process of synthesis, can construct its own peculiar type of proteid.
This latter suggestion is worthy of a moment’s further consideration. As is well known, every species of animal has its own particular type of proteid, adapted to its particular needs. The proteids of one species directly injected into the blood of another species are incapable of serving as nutriment to the body, and frequently act as poisons. Man in his wide choice of food consumes a great variety of proteids, all different in some degree from the proteids of his own tissues. Is it not possible, therefore, that it is the true function of pancreatic and intestinal digestion to break down the different proteids of the food completely into simple fragments, so that the body can reconstruct after its own particular pattern the proteids essential for its nourishment? Or, we can follow the suggestion contained in the work of Abderhalden,[10] who finds that in the long continued digestion of various proteids by pancreatic juice there results in addition to the amino-acids a very resistant residue, non-proteid in nature, which is termed polypeptid. In other words, Abderhalden believes that pepsin, trypsin, and erepsin are not capable of bringing about a complete breaking down of proteids into amino-acids, but that there always remains a nucleus of the proteid not strictly proteid in nature, though related thereto,—polypeptid,—which may serve as a starting-point for the synthesis or construction of new proteid molecules, the various amino-acids being employed to finish out the structure and give the particular character desired. This view, however, is rendered somewhat untenable by the more recent experiments of Cohnheim,[11] who claims that proteids can be completely broken down by pepsin, trypsin, and erepsin, and consequently polypeptids would hardly be available for the synthesis of proteids. Moreover, Bergell and Lewin[12] have ascertained that there is present in the liver an enzyme or ferment which has the power of digesting or breaking down certain dipeptids and polypeptids into amino-acids. Hence, it follows that if any polypeptids are absorbed from the intestine, they would naturally be carried to the liver, where further cleavage into fragments suitable for synthetical processes might occur. In any event, there is good ground for the belief that the more or less complete disruption of the proteid molecule into small fragments renders possible a synthetical construction of new proteid to meet the demands of the organism; a fact of great importance in our conception of the possibilities connected with this phase of proteid nutrition.
Fatty foods undergo little or no chemical alteration until they reach the small intestine. During their stay in the stomach they naturally become liquid from the heat of the body, and there is more or less liberation of fat from the digestive action of gastric juice on cell walls, connective tissues, etc. Most food fat is in the form of so-called neutral fat, which must undergo hydrolysis or saponification before it can be absorbed and thus made available for the body. This is accomplished by the enzyme lipase, or steapsin, of the pancreatic juice, aided indirectly by the presence of bile. Under the influence of this fat-splitting enzyme all neutral fats, whether animal or vegetable, are broken apart, through hydrolysis, into glycerin and a free fatty acid; the latter reacting in some measure with the sodium carbonate of the pancreatic juice to form a sodium salt, or soluble soap, while perhaps the larger part of the fatty acid is held in solution by the bile present. Soap, free acid, and glycerin are then absorbed from the intestine and are found again combined in the lymph as neutral fat. In this way the fats of the food are rendered available for the nourishment of the body.
The next important chemical change taking place in the small intestine is that induced by the amylopsin of the pancreatic juice, which, acting in essentially the same manner as the ptyalin of saliva, converts any unaltered starch into dextrins and sugar. The latter substance, maltose, is exposed to the action of another enzyme contained in the intestinal secretion termed maltase, which transforms it into dextrose, a monosaccharide.
In these ways the proteids, fats, and carbohydrates of the food are gradually digested, so far as conditions will admit, digestion being practically completed by the time the material reaches the ileocæcal valve at the beginning of the large intestine. Throughout the length of the small intestine absorption proceeds rapidly; water, salts, and the products of digestion passing out from the intestine into the circulating blood and lymph. At the ileocæcal valve, however, the contents of the intestine are practically as fluid as at the beginning of the small intestine, due to the fact that water is continually being secreted into the intestine. In the large intestine, the contents become less and less fluid through reabsorption of the water, and as the propulsive movements of the circular and longitudinal muscle fibres of the intestinal wall carry the material onward toward the rectum, the last portions of available nutriment are absorbed. Finally, in varying degree, certain putrefactive changes are observed in the large intestine involving a breaking down of some residual proteid matter, through the agency of micro-organisms almost invariably present, with formation of such substances as indol, skatol, phenol, fatty acids, etc. These processes, however, in health are held rigidly in check, and count for little in fitting the food for absorption. Digestion, on the other hand, extending as we have seen from the mouth cavity to the ileocæcal valve, is the handmaiden of nutrition, preparing all three classes of organic foodstuffs for their passage into the circulating blood and lymph, and thus paving the way for their utilization by the hungry tissue cells.