Читать книгу Dietetics for Nurses - Fairfax T. Proudfit - Страница 12

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Upon investigation it was found that neither the fats nor carbohydrates were the chief constituents of the active tissues. It was found, in fact, that the carbohydrates occurred in very small quantities only in the muscles, and that frequently the quantity of fat was likewise limited. Other substances, containing nitrogen and sulphur in addition to carbon, oxygen, and hydrogen, which were invariably present, and which are essential constituents of all tissues and cells, both in animals and in plants, must be necessary to all known life. To these substances, believed at the time to be the fundamental constituents of all tissues, Mulder gave the name Protein, from the Greek, meaning “to take first place.” Later investigations proved that, while the proteins were essential to the building and repairing of the tissues and cells in general, they were not the only factors concerned in the work; that certain mineral salts were necessary constituents of all tissues, and must be present in order for any normal growth and development to occur.[4]

Composition of Proteins.—The average nitrogen content of common proteins is about 16%; that is, in 100 grams of protein there will be approximately 16 grams of nitrogen, or in 6.25 grams of protein there will be 1 gram of nitrogen. To estimate the protein content of a food when the percentage of nitrogen is known, it is necessary simply to multiply the percentage of nitrogen present, by the nitrogen factor, 6.25; or, if the amount of nitrogen is desired, when the percentage of protein is given, to divide by same factor.

Construction of Proteins.—In plant structure the building up of the proteins is accomplished by the plants from inorganic substances existing in the soil and air; but in the animal body this is not possible, because the construction of the tissues requires the use of other proteins—the most available ones being found in food. Each animal (or species) forms the proteins characteristic of its own tissues—while the proteins of food are similar to those found in the body, they cannot be utilized in their original form, but must be split into simpler substances from which the cells of the various tissues throughout the body may select those particularly adapted for their purpose. These transformed substances are known as amino acids, the production of which is a result of digestion in the digestive tract. There are about seventeen of these acids entering into the construction of the common proteins. One scientist has likened these units to letters of the alphabet, which, being combined, spell many proteins. When a protein contains all of the essential units, it may be said to be “complete,” the best example of which may be seen in milk, eggs, and meat. When a protein lacks some of the essential elements, or letters of the protein alphabet, it is said to be incomplete. Gelatin is the best example of this type of protein, but the cereals and beans must likewise be supplemented by other substances; milk being the one most generally used for this purpose. For the purpose of building young tissues, and maintaining those already mature, it is logical to use foods containing the foodstuffs in their best form; that is, those that not only contain the complete protein, but also the requisite mineral salts and vitamines. Foods lacking in some of these respects become adequate when supplemented by these foods which can supply the missing constituents; hence, the use of such incomplete protein foods need not necessarily be abandoned, for, as in the case of cereals, the foods are both economical and palatable, and, when used in addition to milk, furnish valuable adjuncts to the dietary.

Classification of Proteins.—A brief description of some of the more important proteins with which we are chiefly concerned will serve to simplify the formulation of a diet. Those assuming the most important position in nutrition and food are globulins, albumens, nucleoproteins, phosphoproteins, hemoglobins, and derived proteins such as proteoses and peptones. The albumens and globulins associated together occur in the tissues of both animals and plants. The albumens are richer in sulphur than the globulins and are found more abundantly in the animal fluids, such as the blood, while the globulins predominate in the more solid tissues of animals and in plants. The close association of these two proteins is particularly noticeable in the blood and cells. They have different characteristics, however.

Albumins.—The best examples are found in egg albumin (white of egg), lactalbumin (milk), serum albumin (blood), leucosin (wheat), legumelin (peas). Albumins are all soluble in pure water, and are coagulable by heat. Coagulation, due to the action of the ferments in the body, takes place in milk, blood, and muscle plasma. Certain albumens are particularly adapted for the building and repairing of tissues. Among those that have been used in feeding experiments to determine whether or not they were capable, when used as the sole protein in the diet, of maintaining animals in normal nutrition, and of supporting normal growth in the young animal—may be cited lactalbumin and egg albumin. These experiments provided diets adequate in other respects, the object being to determine the value of the various proteins. It was found that the albumin from milk was more efficient in this respect than the egg albumin.[5]

In the invalid dietary the solubility of the albumins in water makes them of especial value as reinforcing agents, since they may be introduced into fluids without materially altering either their flavor or their bulk.

Globulins.—Simple proteins, insoluble in pure water, but soluble in neutral salt solutions; examples, muscle globulin, serum globulin (blood), edestin (wheat), physelin (beans), legumin (beans and peas), tuberin (potatoes), amandin (almonds), arachin, and conarachin (peanuts).

Alcohol-Soluble Proteins.—Simple proteins soluble in alcohol of from 70–80% strength. Insoluble in absolute alcohol, water and other neutral solvents. Examples of these proteins may be seen in the gliadin of wheat, zein of corn, and hordein of barley.

Albuminoids.—These substances represent one group of incomplete proteins, inasmuch as they cannot alone support protein metabolism. However, they are classed with the proteins and may be substituted for at least a part of these compounds in the daily dietary, since they are able to do much of the work of the pure proteins. The best example of this group is seen in gelatin. This substance contains many of the structural units of meat protein but in very different relative amounts. It has not, therefore, the chemical units necessary to repair the worn-out parts of cell machinery.[6]

Conjugated Proteins:—Nucleoproteins, Phosphoproteins and Hemoglobin.

(a) Nucleoproteins.—This type of protein is characteristic of all cell nuclei, and is particularly abundant in the highly nucleated secreting cells of the glandular organs, such as the liver, pancreas, and the thymus gland. The nucleoproteins are composed of simple proteins and nuclein. Nucleic acid is rich in phosphorus and upon decomposition yields some of the purin bases (xanthin, adenin, guanin), a carbohydrate and phosphoric acid.[7]

(b) Phosphoproteins.—Compounds in which the phosphorus is in organic union with the protein molecule otherwise than a nucleic acid or lecithin. Examples: caseinogin (milk), ovovitellin (egg yolk).

(c) Hemoglobin.—Much of the greater part of the iron existing in the body occurs as a constituent of the hemoglobin of the red blood cells. When the intake of iron is not sufficient to cover the output, there must be a consequent diminution in the hemoglobin of the blood with a corresponding development of anemia.

The importance of knowing these characteristic proteins is apparent. Not only will such knowledge lead to a more intelligent use of protein foods in the normal dietary, but it will prove of the greatest assistance in the adjusting of the foodstuffs in diet for individuals suffering from certain abnormal conditions.

In abnormal conditions this knowledge of the various proteins—their composition, source, and behavior in the body assumes a position of the greatest importance; since it represents the means for safeguarding a patient from the results caused by the wrong kind of food. In certain types of nephritis, for example, it is perfectly safe to give milk where the ingestion of meat and eggs might cause serious, if not fatal, results. In treating gout, when it is deemed advisable to limit the purin foods in order to control in a measure the retention of uric acid in the body, the realization that certain of the nucleoproteins, upon being broken down in the body, yield the purins, which in turn give rise to the production of uric acid, will permit the nurse to adjust the diet so as to eliminate such foods entirely (see Gout). The importance of keeping the hemoglobin content of the blood normal has already been mentioned.

The Effect of Heat upon Proteins.—The fact that certain proteins are most susceptible to heat has already been stated, but the application of this knowledge in the preparation of protein foods is important. In milk, for example, whole raw milk forms a large hard curd; whereas boiled milk curdles in a much finer and softer form. Pasteurized milk shows smaller curds than raw whole milk, but larger than the boiled whole milk.[8]

An egg cooked by the application of a long-continued high temperature (212° F.) has a tough white; whereas an egg cooked until hard at a temperature under the boiling point shows a tenderness in the white which renders it distinctly more palatable. Soft-cooked eggs leave the stomach in less time than is required for hard cooked ones; poached (cooked in water under the boiling point), shirred eggs (cooked in hot dish), and soft-cooked eggs are among the most readily digestible forms of eggs. Raw eggs are slightly less stimulating to acid secretion in the stomach and require a longer time to leave the stomach than boiled eggs. Thus it is seen that in many cases the difference in preparation of the protein foods may make a difference in the way in which the digestive tract handles them. Necessarily, this point is emphasized more in abnormal than in normal conditions; for example, albuminized orange juice gives rise to a distinct gastric secretion, and leaves the stomach rapidly—a great advantage in certain abnormal conditions, and especially in those requiring liquid diet of high nutriment value.

The knowledge of the coagulation of proteins by heat points out the advantage of using cold water over hot in the preliminary cleansing of utensils in which protein foods have been prepared. Certain members of this group are soluble in pure water, and will readily dissolve; whereas, if the water is heated, their coagulation would prevent this taking place so readily.

Functions of Protein in the Body.—The proteins serve two distinct uses in the body; first, that of building and repairing tissues and furnishing, in conjunction with other substances, material for growth; second, that of producing energy for the internal and external work of the body. For this latter function a large percentage of the proteins ingested is used; consequently, since the carbohydrates and fats are primarily the energy furnishing material most readily used by the organism, it is clearly demonstrated that the average individual takes more protein into the body than is necessary for its maintenance. Except during the period when an allowance for growth must be made, it is probable that a much smaller daily consumption of protein could be made without disadvantage to the organism, leaving the bulk of the work, in so far as the running of the engine is concerned, to the other organic foodstuffs.