Animal protein

“ It is generally agreed that vegetable protein is not so digestible as animal protein, but this disadvantage is offset by the fact that it does not bring about so much intestinal trouble as does the protein of animal foods and is less likely to cause disturbances that are usually attributed to foods high in this substance. Vegetable protein is affected by heat in much the same way as other protein. ”

“ The agency which breaks down protein into these simpler substances is called an enzyme, and protein must always be so liquefied or digested by enzymes before it can be absorbed through membranes; hence the necessity of digestion in the stomach of animals preparatory to absorption of food through the intestines. Now, animals, including fish, require a certain amount of new protein to support the body activities, which, failing, the animal would immediately perish. But the hazards in the existence of any animal often make it obligatory to do without food for a shorter or longer period. ”

“ 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 larger portion of the protein of meats and cereals is in insoluble forms. The meat juices, which contain the soluble portion of the proteins, constitute less than 5 percent of the nitrogenous compounds. Meats contain less amid substances than plants, in which the amids are produced from ammonium compounds and are supposed to be intermediate products in the formation of proteids, while in the animal body they are derived from the proteids supplied in the food and, it is generally believed, cannot form proteids. ”

“ Experiments made by F. H. Hall and W. P. Wheeler, at the New York Agricultural Experiment Station, regarding the best food for "chicks, pullets, cockerels, and ducklings," seem to indicate conclusively that part of the protein must be drawn from animal sources if we are to get the best results. Rations in which from forty to fifty per cent of the protein was supplied by animal food produced more rapid growth and at less cost of production. ”

“ If the stomach became empty because of temporary shortage of food or an injured mouth, the animal would die unless special provision were made to supply protein from some other source. But nature has provided a means whereby the proteins in the less important parts of the body can be used for the time being to support the activities of the absolutely necessary vital parts. The stored protein is within cells and could not possibly be carried by the blood stream to the point of need unless it could get out. ”

“ The composition of vegetable protein is only slightly different from that of animal protein. In fact, the experiments of scientists show that animal protein may be readily replaced by vegetable protein. One of these proteins is sometimes called vegetable albumin, but the chief protein of vegetables containing the largest amount of this substance, namely, beans, peas, and lentils, is called legumin, from the term legumes, the name of this class of vegetables. ”

“ 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 ”

“ Lindeborg (1952) and Dice (1922) fed mice on a [486] mixture of rolled oats, meat scraps, dry skimmed milk, wheat germ, etc. described by Dice (1934) . Their data on water consumption in P. maniculatus indicate that this mixture probably is lower in protein content than Purina Laboratory Chow, that was used in my experiments and those of Williams' (tables 8 and 9) .
The amount of dietary protein consumed under natural conditions is not known for most wild animals. One index of the minimum amount of protein necessary is the amount required for an animal to maintain its weight. ”

“ 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. ”

“ It would appear that in the chemical composition of the proteins there are all the constituents, or simpler products, of the non-nitrogenous compounds, and these are in chemical combination with amid radicals and nitrogen in various forms. The nitrogen of many proteids appears to be present in more than one form or radical. The proteids take an important part in life processes. They are found more extensively in animal than in plant bodies. The protoplasm of both the plant and animal cell is composed mainly of 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. ”

“ When substances, that are out of harmony with the plasma, are introduced into the plasma or serum of a given animal, we always find developed a special power of decomposing bodies belonging to the protein series, especially protein itself and its peptones. A specific activity of the injected substrates could be traced only so far as that, after the injection of proteins and peptones, ferments appeared in the plasma which were able to chemically reduce the derivatives of this group, but not, for instance, fats or carbohydrates. ”

“ There are, however, even amongst unicellular organisms, some that are very fastidious and will only thrive in the presence of certain peptones. Others even require certain forms of protein from which to obtain their derivatives. ”

“ One of the most useful and valuable of animal food proteins is obtained from hen eggs. The "white" of eggs is almost pure albumin, and there is much protein in the yolk also. Eggs are now produced and imported by the million, and form a most important item in the country's dietary, the protein being in a very easily digestible form. ”

“ 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. ”

“ 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 ”

“ The proteins are all complex chemical compounds, which in nutrition become reduced to simple forms, and are then built up again into flesh. The animal foods are in the main the best of the protein compounds, for they are rich in nitrogenous matter, are easily digested, and from their composition and adaptability are most valuable in maintaining the life of the body. ”

“ Zein, the principal protein of corn contains no glycine, lysine, or [Pg 178] tryptophane. It is not sufficient to support animal life and promote growth, if used as an exclusive source for protein for food.
THE EXTRACTION OF PROTEINS FROM PLANT TISSUES
Since proteins are indiffusible, it is essential that the cell-walls of the tissue shall be thoroughly ruptured as the first step in any process for the extraction of these compounds from plant tissues. ”

“ The remaining material is supposed to be also composed of amino-acids which are linked together in some arrangement which is not broken apart by any method of hydrolysis which has yet been devised. This view is borne out by the fact that substances which exhibit all the characteristic properties of proteins have been artificially synthetized, by using only amino-acid compounds. Animal proteins often show a much larger proportion of unhydrolyzable material than do plant proteins. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ When we eat lean beef or pork we get exactly the same eighteen amino acids that are in our own proteins, but not put together in precisely the same way. What we have to do with these proteins is to break them up into the amino acids of which they are composed and then put these together again in the combination which makes up human protein. ”

“ There are on the average about fifteen different kinds of these amino acids in the proteins, which are especially valuable in supplying building material for the tissues of the human body. These amino acids are united in long chains to form the protein molecule, and in this respect can be compared to cars in a train. By the work of digestion, the proteins are broken down into these comparatively simple building stones, which, when absorbed into the circulation, are used by the body in building working tissues as they are needed. ”

“ 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. ”

“ 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. ”

“ Dogs, for example, fed on a mixture of protein cleavage products with suitable addition of fat and carbohydrate, maintained nitrogen equilibrium and even stored up nitrogen, presumably in the form of protein, thus indicating that the animals were able to utilize these end products of protein decomposition in much the same way as protein food would be utilized. ”

“ Of the proteins noted in our Introduction, the keratins have no value as foods; the gelatins have some value as culinary material, but little actual food value; whilst the albumins comprise practically all the useful animal food proteins. Whilst the actual flesh of animals is the principal source of food proteins—both as to quantity and food value—other parts of animals, e.g. kidneys, liver, blood, brains, tongue, are used and relished. ”

“ 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. ”