Food-stuffs. Water; Inorganic salts; Proteids (or proteid-containing bodies); Albuminoids (a group of bodies resembling proteids, but having in some respects a different nutritive value); Carbohydrates; Fats. The main facts with regard to the specific nutritive value of each of these substances will be given later on, after the processes of digestion have been described. A few general remarks, however, at this place will serve to give the proper standpoint from which to begin the study of the chemistry of digestion and nutrition. Water and Salts.-Water and salts we do not commonly consider as foods, but the results of scientific investigation, as well as the experience of life, show that these substances are absolutely necessary to the body. The tissues must maintain a certain composition in water and salts in order to function normally, and, since there is a continual loss of these substances in the various excreta, they must continually be replaced in some way in the food. It is to be borne in mind in this connection that water and salts constitute a part of all our solid foods, so that the body gets a partial supply at least of these substances in everything we eat. Proteids. The composition and different classes of proteids are described from a chemical standpoint in the section on The Chemistry of the Body. Different varieties of proteids are found in animal as well as in vegetable foods. The chemical composition in all cases, however, is approximately the same. Physiologically, they are supposed to have equal nutritive values outside of differences in digestibility, a detail that will be given later. The essential use of the proteids to the body is that they supply the material from which the new proteid tissue is made or the old proteid tissue is repaired, although, as we shall find when we come to discuss the subject more thoroughly (p. 345), proteids are also extremely valuable as sources of energy to the body. Inasmuch as the most important constituent of living matter is the proteid part of its molecule, it will be seen at once that proteid food is an absolute necessity. Proteils contain nitrogen, and they are frequently spoken of as the nitrogenous foods; carbohydrates and fats, on the contrary, do not contain nitrogen. It follows immediately from this fact that fats and carbohydrates alone could not suffice to make new protoplasm. If our diet contained no proteids, the tissues of the body would gradually waste away and death from starvation would result. All the food-stuffs are necessary in one way or another to the preservation of perfect health, but proteids, together with a certain proportion of water and inorganic salts, are absolutely necessary for the bare maintenance of animal life-that is, for the formation and preservation of living protoplasm. Whatever else is contained in our food, proteid of some kind must form a part of our diet. The use of the other food-stuffs is, as we shall see, more or less accessory. It may be worth while to recall here the familiar fact that in respect to the nutritive importance of proteids there is a wide difference between animal and vegetable life. What is said above applies, of course, only to animals. Plants can, and for the most part do, build up their living protoplasm upon diets containing no proteid. With some exceptions that need not be mentioned here, the food-stuffs of the great group of chlorophyll-containing plants, outside of oxygen, consist of water, CO, and salts, the nitrogen being found in the lastmentioned constituent. Albuminoids.-Gelatin, such as is found in soups or is used in the form of table-gelatin, is a familiar example of the albuminoids. They are not found to any important extent in our raw foods, and they do not therefore usually appear in the analyses given of the composition of foods. An examination of the composition and properties of these bodies (see section on The Chemistry of the Body) shows that they resemble closely the proteids. Unlike the fats and carbohydrates, they contain nitrogen, and they are evidently of complex structure. Nevertheless, they cannot be used in place of proteids to build protoplasm. They are important foods without doubt, but their value is similar in a general way to that of the non-nitrogenous foods, fats and carbohydrates, rather than to the so-called "nitrogenous foods," the proteids. Carbohydrates. We include among carbohydrates the starches, sugars, gums, and the like (see Chemical section); they contain no nitrogen. Their physiological value lies in the fact that they are destroyed in the body and a certain amount of energy is thereby liberated. The energy of muscular work and of the heat of the body comes largely from the destruction or oxidation of carbohydrates. Inasmuch as we are continually giving off energy from the body, chiefly in the form of muscular work and heat, it follows that material for the production of this energy must be taken in the food. Carbohydrates form perhaps the easiest and cheapest source of this energy. They constitute the bulk of our ordinary diet. Fats. In the group of fats we include not only what is ordinarily understood by the term, but also the oils, animal and vegetable, that form such a common part of our food. Fats contain no nitrogen (see Chemical section). Their use in the body is substantially the same as that of the carbohydrates. Weight for weight, they are more valuable than the carbohydrates as sources of energy, but the latter are cheaper, more completely digested when fed in quantity, and more easily destroyed in the body. For these reasons we find that under most conditions fats are a subsidiary article of food as compared with the carbohydrates. From the standpoint of the physiologist, fats are of special interest because the animal body stores up its reserve of food material mainly in that form. The history of the origin of the fats of the body is one of the most interesting parts of the subject of nutrition, and it will be discussed at some length in its proper place. As has been said, our ordinary foods are mixtures of some or all of the food-stuffs, together with such things as flavors or condiments, whose nutritive value is of a special character. Careful analyses have been made of the different articles of food, mostly of the raw or uncooked foods. As might be expected, the analyses on record differ more or less in the percentages assigned to the various constituents, but almost any of the tables, published give a just idea of the fundamental nutritive value of the common foods. For details of separate analyses reference may be made to some of the larger works upon the composition of foods. The subjoined table is one compiled by Munk from the analyses given by König: An examination of this table will show that the animal foods, particularly the meats, are characterized by their small percentage in carbohydrate and by a relatively large amount of proteid or of proteid and fat. With regard to the last two food-stuffs, meats differ very much among themselves. Some idea of the limits of variation may be obtained from the following table, taken chiefly from König's analyses: The vegetable foods are distinguished, as a rule, by their large percentage in carbohydrates and the relatively small amounts of proteids and fats, as seen, for example, in the composition of rice, corn, wheat, and potatoes. Neverthe 1 König, Die Menschlichen Nahrungs und Genussmittel, 3d ed., 1889; Parke's Manual of Practical Hygiene, section on Food. 2 Atwater: The Chemistry of Foods and Nutrition, 1887. less, it will be noticed that the proportion of proteid in some of the vegetables is not at all insignificant. They are characterized by their excess in carbohydrates rather than by a deficiency in proteids. The composition of peas and other leguminous foods is remarkable for the large percentage of proteid, which exceeds that found in meats. Analyses such as are given here are indispensable in determining the true nutritive value of foods. Nevertheless, it must be borne in mind that the chemical composition of a food is not alone sufficient to determine its precise value in nutrition. It is obviously true that it is not what we eat, but what we digest and absorb, that is nutritious to the body, so that, in addition to determining the proportion of food-stuffs in any given food, it is necessary to determine to what extent the several constituents are digested. This factor can be obtained only by actual experiments. It may be said here, however, that in general the proteids of animal foods are more completely digested than are those of vegetables, and with them, therefore, chemical analysis comes nearer to expressing directly the nutritive value. The physiology of digestion consists chiefly in the study of the chemical changes that the food undergoes during its passage through the alimentary canal. It happens that these chemical changes are of a peculiar character. The peculiarity is due to the fact that the changes of digestion are effected through the agency of a group of bodies known as enzymes, or unorganized ferments, whose chemical action is more obscure than that of the ordinary reagents with which we have to deal. It will save useless repetition to give here certain general facts that are known with reference to these bodies, reserving for future treatment the details of the action of the specific enzymes found in the different digestive secretions. Enzymes.-Enzymes, or unorganized ferments, or unformed ferments, is the name given to a group of bodies produced in animals and plants, but not themselves endowed with the structure of living matter. The term unorganized or unformed ferment was formerly used to emphasize the distinction between these bodies and living ferments, such as the yeast-plant or the bacteria. "Enzyme," however, is a better name, and is coming into general use. Enzymes are to be regarded as dead matter, although produced in living protoplasm. Chemically, they are defined as complex organic compounds containing nitrogen. Their exact composition is unknown, as it has not been found possible heretofore to obtain them in pure enough condition for analysis. Although several elementary analyses are recorded, they cannot be considered reliable. It is not known whether or not the enzymes belong to the group of proteids. Solutions of most of the enzymes give some or all of the general reactions for proteids, but there is always an uncertainty as to the purity of the solutions. With reference to the fibrin ferment of blood, one of the enzymes, observations have recently been made which seem to show that it belongs to the group of combined proteids, nucleo-albumins (for details see the section on Blood). But even should this be true, we are not justified in making any general application of this fact to the whole group. Classification of Enzymes.-Enzymes are classified according to the kind of reaction they produce-namely: 1. Proteolytic enzymes, or those acting upon proteids, converting them to a soluble modification, peptone or proteose. As examples of this group we have in the animal body pepsin of the gastric juice and trypsin of the pancreatic juice. In plants a similar enzyme is found in the pineapple family (bromelin) and in the papaw (papaïn). 2. Amylolytic enzymes, or those acting upon the starches, converting them to a soluble form, sugar, or sugar and dextrin. As examples of this group we have in the animal body ptyalin, found in saliva, amylopsin, found in pancreatic juice, and in the liver an enzyme capable of converting glycogen to sugar. In the plants the best-known example is diastase, found in germinating seeds. This particular enzyme has been known for a long time from the use made of it in the manufacture of beer. In fact, the name diastase" is frequently used in a generic sense, "the diastatic enzymes," to characterize the entire group of starch-destroying ferments. 3. Fat-splitting enzymes, or those acting upon the neutral fats, breaking them up into glycerin and the corresponding fatty acid. The best-known example in the animal body is found in the pancreatic secretion; it is known usually as steapsin, although it has been given several names. Similar enzymes are known to occur in a number of seeds. 4. Sugar-splitting enzymes, or those having the property of converting the double into the single sugars-the di-saccharides, such as cane-sugar and maltose, into the mono-saccharides, such as dextrose and levulose. Two enzymes of this character are found in the small intestine of the animal body, one acting upon cane-sugar and one on maltose. The one acting on cane-sugar is known as invertine or invertase, while that acting on maltose is designated as maltase. 5. Coagulating enzymes, or those acting upon soluble proteids, precipitating them in an insoluble form. As examples of this class we have fibrin ferment (thrombin), formed in shed blood, and rennin, the milk-curdling ferment of the gastric juice. An enzyme similar to rennin has been found in pineapple-juice. These five classes comprise the chief groups of enzymes that are known to occur in the animal body. One or more examples of each group take part in the digestion of food at some time during its passage through the alimentary canal. From time to time other enzymes have been recognized in the liquids or tissues of the body. Thus in shed blood and indeed in other tissues an enzyme (glycolytic enzyme) that is capable of destroying sugar seems to be present. When sugar is added to shed blood it disappears as such, although the products formed have not been recognized. Similarly from many tissues of the body oxidizing enzymes have been extracted that are capable of causing energetic oxidation of organic bodies; for instance, they can convert salicylaldehyde to salicylic acid. It is possible that these oxidizing enzymes, 1 For a recent summary of facts and literature upon enzymes see Green: The Soluble Ferments and Fermentation, 1897. |