Lipid

“ While it is nice that they are beginning to concede the importance of lipids, this has every appearance of a desperate attempt to save a totally untenable hypothesis from fatal criticism, rather than a well-thought out theory. ”

“ Protein, fats, carbohydrates, mineral salts, and water are the compounds we need in our foods, and they are found in flesh foods and vegetables. Protein, the most important element, is derived principally from meat, eggs, and milk. It is also furnished by some vegetables, as beans, peas, and the gluten of wheat; but in these it is mixed with too much extraneous matter, as husks, bulbs, woody fiber, etc., to be useful in the diet of the sick. Extractives, as beef-tea, are included in the nitrogen compounds, but they neither build tissue nor furnish energy; they are appetizers and stimulants. ”

“ The formation of fat is interesting, both from a chemical and a physiological point of view. The relation of lignine starch and sugar to alcohol, afforded reasons for Liebig’s theory of the formation of fat in the body. Recent experiments by Liebig, Bopp, Guckelberger, Keller and others, on the formation of the lower terms of the series of fatty acids by the oxidation and putrefaction of the blood-forming substances, rendered possible the formation of the higher members, from albumen, fibrin and caseine, by similar means, [1] for example, by a less intense degree of oxidation. ”

“ You know that if you shake up oil and water together, though by violent shaking you may mix them a good deal, directly you leave off they separate again, and all the oil is seen floating on the top of the water. If, however, you shake up oil with pancreatic juice and bile, the oil does not separate. You get a sort of creamy mixture, and will have to wait a very long time before the oil floats to the top. Milk, you know, contains fat, the fat which is generally called butter. If you examine milk under the microscope, you will find that the fat is all separated into the tiniest possible drops. ”

“ Such substances are common salt, potash, lime, magnesia, phosphoric acid, and, above all, iron, without which the blood-corpuscles would lose their vitality. But none of these groups is, by itself, sufficient for nutrition; they must all be combined. Blood-formers, heat-producers, and nutritive salts, are not separately foods, but only factors of food, each as indispensable for the vital processes as air and water, but each incapable by itself of supporting life. One cannot live on albumen alone or on fat alone. ”

“ A trifling amount of urea is also eliminated in the sweat and in the breath as well as in the feces. Proteins increase nitrogenous metabolism and also the metabolism of other foods, but the amount of nitrogen eliminated is just equal to that taken in. Probably some comes from the tissues themselves and not from the food. The oxidation of carbohydrates and fats is measured by the amount of carbon excreted. At first as much is given off as is taken in, but after a while the carbohydrate is stored up as glycogen in the liver and the fats are stored as fat. ”

“ Compared to the protein, the carbohydrates and fats are relatively simple chemical substances, fairly stable and, hence, easy to study chemically. Proteins are excessively changeable, to which character they owe their name. Some proteins are called albumins. Egg albumin, egg white, is a typical protein possessing all of the essential protein characteristics. The carbohydrates and fats serve chiefly as fuels, while the proteins, besides providing energy, build or rebuild the living tissues of the body. A diet may be deficient in either fat or carbohydrate, [Pg 19] and still remain adequate. ”

“ What happens is that all the digestion products, except the fats, pass through the capillaries of the liver, but the fat gets into the blood stream by another way and does not go through the liver en route; for some reason or other it seems to be to the advantage of the body that the fat should not be allowed to circulate through the liver capillaries. It is, as we shall see in a later chapter, a distinct advantage to have the glucose pass through the liver, and there is probably also a good reason for having the digestion products of protein take that course. ”

“ Its melting point varies from 114° to 145°, there being, according to Duffy, three modifications of this substance.
The fats and oils are lighter than water. They contain far more carbon and hydrogen, and less oxygen, than are found in the sugars and starches. They all consist of acids (stearic, palmitic, &c.) united with glycerine. On being boiled with potash or soda, the latter take the place of the glycerine, which is set free, and a soap is produced. The fatty acids strongly resemble the fats. In nutritive power, one part of fat is equal to 21⁄2 parts of starch or sugar. ”

“ Butter, the fat of meats, olive oil, and the oil of nuts are the fats of greatest importance as foods. Fats, like the carbohydrates, are composed of carbon, hydrogen, and oxygen. They are rather complex chemical compounds, though not so complex as proteids. Since neither fats nor carbohydrates contain nitrogen, they are frequently classed together as non-nitrogenous foods.
Purpose Served by Carbohydrates, Fats, and Albuminoids.—These classes of nutrients all serve the common purpose of supplying energy. By uniting with oxygen at the cells, they supply heat and the other forms of bodily force. ”

“ Now, let us consider what ultimately becomes of these substances—how Nature utilizes them in the physical economy. Protein is used to build up the solid tissues of the body, the muscles and tendons. It is also a source of nutrition for brain and nerve substance, and partially serves as fuel. Fats simply form fatty tissue and serve as fuel to maintain the heat of the body, by combustion or oxidation. Carbohydrates mainly serve as fuel, owing to the large percentage of carbon they contain, which readily unites with the oxygen. ”

“ There are present in the blood, however, substances closely related to the peptones, maltose, glycerine, etc.; substances which have in fact been formed from them. During their transfer from the food canal, the dissolved nutrients undergo changes, giving rise to the materials in the blood. Thus are the serum albumin and serum globulin of the blood derived from the peptones and proteoses; the dextrose, from the maltose and other forms of sugar; and the fat droplets, from the glycerine, fatty acid, and soluble soap. ”

“ Fats are absorbed practically entirely by the lacteals. They enter the cells covering the villi, travel thence to the lymph spaces, and so into the lacteal or main lymph channel, whence they are carried to the thoracic duct and the general circulation. From the blood they are absorbed as fat and stored up as adipose or fatty tissue, which is found throughout the body in connective tissue about the organs. Organic salts and water are for the most part absorbed by the portal system, which they reach through the capillaries of the villi and through which they go to the liver. ”

“ Actually, the liver itself produces cholesterol; it's presence in the blood is an important part of the body chemistry. Cholesterol only becomes a problem because of deranged body chemistry due to the kind of overall malnutrition Americans usually experience on their junk food diets. Avoiding cholesterol in foods does little good, but eating a low-fat, low-sugar, complex-carbohydrate (whole foods) diet high in minerals does lower blood cholesterol enormously.
Actually, high quality fresh (not rancid) butter in moderate quantities is about the finest fat a person could eat. ”

“ Examples: milk, cheese, eggs, meat, fish, cereals, peas, beans, and lentils. The principal constituent of proteid food is albumen. Albumen as found in food takes different names, but has the same chemical composition; as, albumen in eggs, fibrin in meat, casein in milk and cheese, vegetable casein or legumen in peas, beans, and lentils; and gluten in wheat. To this same class belongs gelatin.
The chief office of the carbohydrates is to furnish energy and maintain heat. They contain carbon, hydrogen, and oxygen, and include foods containing starch and sugar. ”

“ Fatty tissue (not muscle) serves as fuel, therefore the value of such foods as butter, cream, oils, etc., is apparent. Carbohydrates form fat and serve as fuel and force producers; these come in the form of starches, sugars,—vegetables and grains being the most important. In being themselves burned to yield energy, the nutrients protect each other from being consumed. The protein and fats of body tissue are used like those [13] of foods. An important use of the carbohydrates and fats is to protect protein (muscle, etc.) ”

“ Now, proteids are, when cooked, insoluble in water (see p. 49) ; and fat, you know, will not mingle with water. Both these parts of meat, both these food-stuffs, must be acted upon before they can pass from the inside of the alimentary canal, through the epithelium of the mucous membrane, into the blood capillaries.
Besides meat we eat bread. Bread is chiefly composed of starch; but besides starch we find in it a substance containing nitrogen, exceedingly like the proteid matter of muscle or of blood. ”

“ 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. ”

“ Animals in running water leave a fat after three years, which is more abundant in the intestines than in the muscles, and more fat is formed in stagnant, than in running water.
Chevreul, 1812, found the fat of church yards to contain margaric and oleic acids, combined with yellow colouring and odorous matters, also lime, potash, oxide of iron, lactic acid salts and azotized matter. He supposes the fatty acids are liberated from their glycerine by ammonia, which subsequently itself escapes, and that adipocire is thus formed from the original fat of the body. ”

“ The larger part of man’s food is used for the production of energy, and it is physiologically economic that this energy be supplied by the non-nitrogenous nutrients, particularly the carbohydrates, and to allocate to protein, so far as practicable, its special rôle of building material.
Nitrogenous Waste Products.—The end-product of proteins is urea, which is formed from protein decomposition products in relatively large amounts in the liver cells, and, being readily soluble and diffusible, is easily eliminated by the kidneys. ”

“ First, that fats as such may be translocated from cell to cell, since cell-walls and cell protoplasm seem to be permeable to oil if it is a sufficiently fine emulsion; or they may be hydrolyzed into glycerol and fatty acids and translocated from cell to cell in these forms and recombined into fats in the new location. Second, that fats are formed from glucose in some plants, from sucrose and from starch in others, and from mannite and similar compounds in still other species. ”

“ Here the bile and the pancreatic juice, breaking up the fat into tiny{122} particles, mix fat, and broken meat, and empty wrappings, and salts, and water, all together into a thick, dirty, yellowish cream. Squeezed along the intestine by the contraction of the muscular walls, the goodness of this cream is little by little sucked up. The fat goes drop by drop, particle by particle, into the lacteals, and so away into the blood. The proteids, more and more dissolved the further they travel along the canal, soak away into blood-vessel or into lacteal. ”

“ Fate of the Proteins.—After utilization in the body, the proteins, like the other foods, leave certain waste products which indicate to a greater or lesser extent the completeness with which the organism has made use of the food materials. The end-products of protein metabolism are: urea, ammonium salts, purin bodies, and creatinin. These products leave the body chiefly in the urine. The chief end-product in man is urea. This substance represents from 82-88% of the total nitrogen excreted by the kidneys. ”

“ Proteids, as the student will remember, contain carbon, hydrogen, oxygen, and nitrogen, and sometimes small quantities of sulfur, phosphorus, or iron. These forms of proteids are now known to be chemically changed, by the digestive enzyms of the intestines, into simpler compounds containing these same elements.
How proteids may form body-fats
These simple nitrogenous substances pass into the liver. Just as the liver regulates the supply of blood sugar, so it regulates the supply of nitrogenous compounds in the blood. ”

“ The red corpuscles contain the oxygen which goes to combine with the superabundant carbon, and develop heat; the fatty matters probably repair the fatty tissues, and glands that are of a fatty nature; and the various salts contribute to the bones, and to the chemical properties of those secretions which are formed by the glands, &c., while the great proportion of water is employed in cleansing, softening, and cooling the whole, or the living edifice, and it is the medium through which all the nutrition of the body is distributed. ”

“ Many cells throughout the body contain this ferment. The soluble soaps which are probably conveyed by the blood will be quickly taken up by such cells, synthetized into neutral fats, and stored in a non-diffusible form till required. The fat in this condition is readily recognized by the usual microchemical and staining reactions. As fat is a food element essential to the carrying out of the vital energies of the cell, a certain amount of fatty matter must be present, in a form, however, unrecognizable by our present microchemical and staining methods. ”