Protein

“ In addition to the use of protein as a growth food it has another use which no other kind of foodstuff can share. This is also because protein is the foundation material of living protoplasm. We do not know a great deal about what goes on in living protoplasm to make up what we call the life processes, but we do know that these processes are of a chemical nature, and that in connection with them there is a steady wastage of protein. ”

“ This protein is to be used by us for repairing our protoplasmic wastage or, in parts of us that are growing, for making new protoplasm. We have already seen that the protein which we eat must go through a process of digestion before we can use it for these purposes, and our present task is to explain just why this is necessary and to show how the protein is actually used in our bodies. ”

“ One of the great differences between animal protein and plant protein is that the percentages of the different amino acids are very different. Some of the amino acids that make up a large proportion of animal protein are very scantily represented in plant protein. To make human protein we must not only have all the amino acid ingredients, but we must also have enough of every one. On a purely vegetarian diet to get enough of those amino acids which are scantily present in plants we have to eat a large surplus of those amino acids which are specially abundant. ”

“ One of the most important powers of that cell body was its power of digesting and assimilating food, and its most important food was the nitrogenous food or protein from which it built up its own body substance. Now, foreign or food protein cannot be simply absorbed as such. Foreign protein is a poison and never tolerated in the blood. The foreign protein used as food must first [Pg 81] be changed into the special kind of protein that the body can use. The foreign protein is changed by splitting its molecule into its simplest parts and then recombining them in the desired form. ”

“ 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. ”

“ Proteins.—Protein substances contain the element nitrogen. Hence such foods are called nitrogenous foods. Man must form the protoplasm of his body (that is, the muscles, tendons, nervous system, blood corpuscles, the living parts of the bone and the skin, etc.) in part at least from nitrogenous food. Some of this he obtains by eating the flesh of animals, and some he obtains directly from plants (for example, peas and beans) . Proteins are the only foods available for tissue building. They may be oxidized to release energy if occasion requires it. ”

“ We have been accustomed to point to gelatin as the one exception to the rule, and have classed it as a protein-like substance, with as much nitrogen or even more than most proteins, but not truly a protein, since it can not support life. I fancy, however, that many true proteins may prove, when taken alone, unable to support life. As a matter of fact, few isolated proteins have been tested in this respect. Most of our feeding experiments have been made with mixtures of proteins, and consequently a considerable variety of protein cleavage products have been available for nutritive purposes. ”

“ There is never a period in life in which protein is not needed. During the early years it is essential that the proteins, or at least a goodly portion of them, be obtained from animal sources, milk and eggs in particular ”

“ The same cells of the plant that make sugar have the power to make protein, using as raw materials some of the sugar along with the substances brought in with the soil water. The energy for the manufacture of protein comes from the oxidation of some of the sugar or starch in the leaf. The finished protein has about the same energy value, weight for weight, as has the starch from which it was mainly derived.
When an animal eats a plant or part of one, he is eating for the sake of the sugar or sugar products which the plant has made. ”

“ The differences in proteins, especially in enzymes, account for differences among cells. It is now appropriate to ask what makes one protein different from another. We know that the structure of a protein depends upon several factors, such as the molecular weight. But the main differences among proteins depend upon the sequence, or order, of the amino acids that are linked together in the protein molecules. ”

“ Proteins undergo peptonization, but that does not change them sufficiently. The mere splitting into peptones, formerly thought sufficient, is now known to be quite inadequate. Recent researches have shown that the complex protein molecule has to be quite crushed into a heterogeneous assemblage of fragments, from which those suitable for building up into the body protein are selected for that purpose, the remainder being used as fuel; that is, as a source of energy. In the case of the other food principles, fats and starches, the chemical processes are much simpler. ”

“ The necessity of the protein compounds is emphasized when we realize that about one half of the body is composed of muscle, one fifth of which is protein, and the nitrogen in this protein can be furnished only by protein, since neither fats nor carbohydrates contain it. It is therefore evident that the protein-containing foods, such as beef, mutton, fish, eggs, milk, and others, are our most valued nutrients. Our daily diet must contain a due proportion. ”

“ To this complex combination, the nature of which has never been determined with exactness, the name of Protein has been applied. And if we use this term with such caution as may properly arise out of our comparative ignorance of the things for which it stands, it may be truly said, that all protoplasm is proteinaceous; or, as the white, or albumen, of an egg is one of the commonest examples of a nearly pure protein matter, we may say that all living matter is more or less albuminoid. ”

“ It is distinctly a tissue-building nutrient. It also enters into the composition of all the vital fluids of the body, as the blood, chyme, chyle, and the various digestive fluids. Hence it is that protein is required as a nutrient by the animal body, and it [Pg 23] cannot be produced from non-nitrogenous compounds. In vegetable bodies, the protein can be produced synthetically from amids, which in turn are formed from ammonium compounds. While protein is necessary in the ration, an excessive amount should be avoided. ”

“ The Best Dietary.—Inasmuch as all living substance contains nitrogen, it is evident that protein food must form a part of the dietary; but protein alone is not usable. If more protein is eaten than the body requires, then immediately the liver and kidneys have to work overtime to get rid of the excess of protein which forms a poisonous waste harmful to the body. We must take foods that will give us, as nearly as possible, the proportion of the different chemical elements as they are contained in protoplasm. ”

“ Now let's tackle the globin portion of the hemoglobin molecule. Globin is, as has been said, protein in nature, and proteins are by far the most important chemicals in living tissue. There is no question but that most or all of the secrets of life lie hidden in the details of protein structure. A biochemist who could learn the exact structure of some protein would be an awfully happy biochemist. ”

“ The substances that have nitrogen in them are called protein. They are very complex and difficult to analyze. In digestion these proteins are all torn to pieces and built up into other kinds of protein. Just as in tearing down an old house, only a portion of the material can be used in a new house, so it is with protein and laboratory analysis cannot tell us how much of the old house can be utilized in building the new one. ”

“ If, however, it is true that in normal digestion the protein molecule is completely broken down into relatively simple fragments, non-protein in nature, i. e., into amino-acids and polypeptides, and that from these fragments specific proteins are reconstructed, then it is plain that animals fed on a diet free from protein, but with a proper amount of these nitrogenous cleavage products, should live and thrive, assuming, of course, a sufficient addition of non-nitrogenous food. ”

“ In the present state of our knowledge concerning the functioning of the proteins, no significance in the physiology of plant life, or metabolism, is to be attached to the particular type of protein material which it contains, at least so far as the simple proteins of the cytoplasm are concerned. ”

“ We can well imagine that in the life and death of tissue cells autolytic decompositions are constantly taking place whereby cell protein is broken down into its component parts, while at the same time a synthesis of protein may be occurring from other amino-acids brought by blood or lymph with a possible utilization of some of the fragments liberated by the autolysis. In other words, our conception of nutritional changes to-day embodies the hypothesis of a synthesis of protein throughout the body ”

“ This controversy had led to a careful study of the different kinds of proteins that are found in foodstuffs. Through a brilliant series of chemical investigations for whose description we haven't time or space here, chemists had shown that every protein was built up of a collection of acids which were different in structure and properties, that there were some seventeen of these in all and that any given protein might have present all seventeen or be lacking in one or more and that the proportions present varied for every type of protein. ”

“ A minute study of the subject by Osborne, Mendel and numerous other physiologic chemists have shown that a perfect protein is composed of more than a dozen different bodies called amino-acids, each of which must be present in the right proportion to enable the body to use the protein in body building. Each plant produces its one peculiar kind of protein. The protein of milk, caseine, is a perfect protein. ”

“ Other forms of protein are globulin and myosin, which form the actual muscle-substance. These elements form perhaps three-fourths of the entire proteid of the animal, and are the most valuable substances of flesh food. A very small portion of meat proteids is formed by the free albumins of the blood, which are mechanically retained in the muscle-cells, the purpose of which is the nourishment of the animal, and therefore are not unwholesome as food. ”

“ Adults require protein for the repair of old tissues and to furnish material for the building of new cells, and again it is believed advisable to have at least a portion of this protein from animal sources, milk, meat or eggs. ”

“ When later I asked Portia what this protein was, she replied in a sort of chant, as though she were assisting at some mystic rite: ‘Next to water, protein is the largest ingredient in the human body, forming about eighteen per cent. It is similar to the white of egg, the lean of meat, the curd of milk, and the gluten of wheat.’ This and other intimations gave me to understand that protein is the sine qua non of dietetics.
“As we enjoyed the admirable omelet which followed—eight ounces: one of proteid, one of fat, one-half ounce of carbo-hydrate ”

“ Lean meat, as is well known, consists almost exclusively of protein matter and water. This protein matter is principally insoluble. Under the action of digestive ferments the protein of meat becomes broken up into more soluble bodies, known as albumoses, proteoses and peptones,—the latter being the final product of solution. These bodies are still true protein bodies containing the elements of sulfur as one of their essential constituents. ”

“ In all instances which have yet been investigated, the substance of this germ has a peculiar chemical composition, consisting of at fewest four elementary bodies, viz., carbon, hydrogen, oxygen, and nitrogen, united into the ill-defined compound known as protein, and associated with much water, and very generally, if not always, with sulphur and phosphorus in minute proportions. Moreover, up to the present time, protein is known only as a product and constituent of living matter. ”

“ Most of our proteids are of animal origin, but some protein material is also found in the vegetable world. This class of foods contains carbon, oxygen, and hydrogen, and in addition, two substances not found in carbohydrates or fats—namely, sulphur and nitrogen. Proteids always contain nitrogen, and hence they are frequently spoken of as nitrogenous foods. Since the proteids contain all the elements found in the two other classes of foods, they are able to contribute, if necessary, to the store of bodily energy ”

“ Proteins occur in nearly every part of all animals, and the "protoplasm" of the living cell is itself a protein. The keratins include the horny tissues of animals: the epidermis proper, the hair, horns, hoofs, nails, claws, the sebaceous and sudoriferous glands and ducts, and also the elastic fibres. The gelatins are obtained from the collagen of the skin fibres, the bones, tendons, ligaments, cartilages, etc. Fish bladders yield a strong gelatin. The albumins are obtained from the ova, blood, lymph, muscles and other internal organs of animals. ”

“ A few proteins lack certain essential amino-acids, but any ordinary diet supplies enough, and more than enough, of each and every amino-acid. Furthermore, since the proteins are completely disintegrated before absorption, it follows that any adventitious chemical substance that is bound to a protein taken in the food does not enter the blood and circulate in the body in the same protein compound. ”