Part 13

No one can explain all the different intricate changes which a particle of food undergoes from the moment it enters the mouth until its final transformation into tissue or some form of energy; but by comparing the income with the outgo, ideas may be gained of what goes on in the economy of the body, and of the proportion of nutrients used, and some of the intricate and complex chemical changes which the different food principles undergo in the various processes of digestion, assimilation, and use. Probably hundreds of changes take place in the body, in its various nutritive functions, of which nothing is known, or they are entirely unsuspected, so that if we do our utmost with the present lights which we possess for guidance to health, we shall still fall far short of completeness. The subject of food and nutrition, viewed in the light of bacteriology and chemistry, is one of the most inviting subjects of study of the day, and is worthy of the wisest thought of the nation.

The body creates nothing of itself, either of material or of energy; all must come to it from without. Every atom of carbon, hydrogen, phosphorus, or other elements, every molecule of protein, carbohydrate, or other compounds of these elements, is brought to the body with the food and drink it consumes, and the air it breathes. Like the steam-engine, it uses the material supplied to it. Its chemical compounds and energy are the compounds and energy of the food transformed (Atwater). A proof of this is seen in the fact that when the supply from without is cut off, the body dies. The raw material which the body uses is the air and food which it consumes, the greater portion of which is digested and distributed, through the medium of the blood, to all parts of the body, to renew and nourish the various tissues and to supply the material for the different activities of life.

=Ways in which Food Supplies the Wants of the Body.= Food supplies the wants of the body in several ways--(1) it is used to form the tissues of the body--bones, flesh, tendons, skin, and nerves; (2) it is used to repair the waste of the tissues; (3) it is stored in the body for future use; (4) it is consumed as fuel to maintain the constant temperature which the body must always possess to be in a state of health; (5) it produces muscular and nervous energy. The amount of energy of the body depends upon two things--the amount in the food eaten, and the ability of the body to use it, or free it for use.

With every motion, and every thought and feeling, material is consumed, hence the more rapid wearing out of persons who do severe work, and of the nervous--those who are keenly susceptible to every change in their surroundings, to change of weather, even to the thoughts and feelings of those about them.

We easily realize that muscular force or energy cannot be maintained without nutriment in proper quality and amount. An underfed or starving man has not the strength of a well-fed person. He cannot lift the same weight, cannot walk as far, cannot work as hard. We do not as easily comprehend the nervous organism, and generally have less sympathy with worn-out or ill-nourished nerves than muscles, but the sensibilities and the intellectual faculties, of which the nerves and brain are but the instruments, depend upon the right nutrition of the whole system for their proper and healthful exercise.

So many factors enter into the make-up of a thought that it cannot be said that any particular kind of food will ultimately produce a poem; but of this we may be sure, that the best work, the noblest thoughts, the most original ideas, will not come from a dyspeptic, underfed, or in any way ill-nourished individual.

The classification of foods has been usually based upon the deductions of Prout that milk contains all the necessary nutrients in the best form and proportions, viz., the nitrogenous matters, fat, sugar, water, and salts; the latter being combinations of magnesium, calcium, potassium, sodium, and iron, with chlorin, phosphoric acid, and, in smaller quantities, sulphuric acid.

These different classes seem to serve different purposes in the body, and are all necessary for perfect nutrition. Some of them closely resemble each other in composition, but are quite different in their physiological properties, and in the ends which they serve. For instance, starch (C{6}H{10}O{5}) has almost the same chemical formula as sugar (C{12}H{22}O{11}), and yet the one cannot replace the other to its entire exclusion.

=The Protein Compounds.= In general it may be said that the carbohydrates are changed into fats, and are used for the production of force, and that the fats are stored in the body as fat and used as fuel. The protein compounds do all that can be done by the fats and carbohydrates, and in addition something more; that is, they form the basis of blood, muscle, sinew, skin, and bone. They are, therefore, the most important of all the food compounds. The terms "power-givers" and "energy-formers" are sometimes applied to them, because wherever power and energy are developed they are present, though not by any means the only substances involved in the evolution of energy. Probably the fats and carbohydrates give most of the material for heat and the various other forces of the body. In case of emergency, where these are deficient, the proteins are used; but protein alone forms the basis of muscle, tendons, skin, and other tissues. This the fats and carbohydrates cannot do (Atwater). The different tissues are known from analysis to contain this complex nitrogenous compound, protein. Now, since the body cannot construct this substance out of the simpler chemical compounds which come to it, it becomes perfectly evident that the diet must have a due proportion of protein in order to maintain the strength of the body. We get most of our proteins from the flesh of animals, and they in turn get it from plants, which construct it from the crude materials of earth and air.

=The Extractives=, usually classed with the protein compounds, such as meat extract, beef tea, etc., are not generally regarded as direct nutrients, but, like tea and coffee, are valuable as accessory foods, lending savor to other foods and aiding their digestion by pleasantly exciting the flow of the digestive fluids. They also act as brain and nerve stimulants, and perhaps also in some slight degree as nutrients.

The principal proteins or nitrogenous substances are albumen in various forms, casein both animal and vegetable, blood fibrin, muscle fibrin, and gelatin. All except the last are very much alike, and probably can replace one another in nutrition.

Modern chemists agree that nitrogen is a necessary element in the various chemical and physiological actions which take place in the body to produce heat, muscular energy, and the other powers. Every structure in the body in which any form of energy is manifested is nitrogenous. The nerves, muscles, glands, and the floating cells in the various liquids are nitrogenous. That nitrogen is necessary to the different processes of the system, is shown by the fact that if it be cut off, these processes languish. This may not occur immediately, for the body always has a store of nitrogen laid by for emergencies which will be consumed first, but it will occur as soon as these have been consumed. The energy of the body is measured by its consumption of oxygen. Motion and heat may be owing to the oxidation of fat, or of starch, or of nitrogenous substances; but whatever the source, the direction is given by the nitrogenous structure--in other words, nitrogen is necessary to all energy generated in the body.

Protein matter nourishes the organic framework, takes part in the generation of energy, and may be converted into non-nitrogenous substances. 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.

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.

A diet of lean meat alone serves to build up tissue. If nothing else be taken, the stored-up fat of the body will be consumed, and the person will become thin. Athletes while in training take advantage of this fact, and are allowed to eat only such food as shall furnish the greatest amount of strength and muscular energy with a minimum of fat. The lean of beef and mutton, with a certain amount of bread, constitute the foundation of the diet.

=Fats.= Most of the fatty substances of food are liquefied at the temperature of the body. When eaten in the form of adipose tissue, as the fat of beef and mutton, the vesicles or cells in which the fat is held are dissociated or dissolved, the fat is set free, and mingles with the digesting mass. This is done in the stomach, and is a preparation for its further change in the intestines.

Fats are not dissolved--that is, in the sense in which meats and other foods are dissolved--in the process of digestion; the only change which they undergo is a minute subdivision caused principally by the action of the pancreatic juice. In this condition of fine emulsion they are taken up by the lacteals; they may also be absorbed by the blood-vessels.

It has been found that fat emulsions pass more easily through membranes which have been moistened with bile, and it is probable that the function of bile is partly to facilitate the absorption of fat. That the pancreatic juice is the chief agent in forming fats into emulsion was discovered in 1848. Bile is, however, essential to their perfect digestion, and we may therefore say that they are digested by the united action of the pancreatic juice and the bile.

Fat forms in the body fatty tissues, and serves for muscular force and heat; it is also necessary to nourish nerves and other tissues,--in fact, without it healthy tissues cannot be formed. A proper amount of fat is also a sort of albumen sparer.

It is probable that the fat which is used in the body either to be stored away or for energy, is derived from other sources than directly from the fat eaten. From experiments made by Lawes and Gilbert on pigs, it is evident that the excess of fat stored in their bodies must be derived from some other source than the fat contained in their food, and must be produced partly from nitrogenous matter and partly from carbohydrates, or, at least, that the latter play a part in its formation. It would appear from this that life might be maintained on starch, water, salts, and meat free from fat; but although the theory seems a good one, practically it is found in actual experiment that nutrition is impaired by a lack of fat in the diet. The ill effects were soon seen, and immediate relief was given when fat was added to the food. Besides, in the food of all nations starch is constantly associated with some form of fat; bread with butter; potatoes with butter, cream, or gravy; macaroni and polenta with oil, and so forth. A man may live for a time and be healthy with a diet of albuminoids, fats, salts, and water, but it has not yet been proved that a similar result will be produced by a diet of albuminoids, carbohydrates, salts, and water without fat. Fat is necessary to perfect nutrition. Health cannot be maintained on albuminoids, salts, and water alone; but, on the other hand, cannot be maintained without them.

Probably the value of fats, as such, is dependent upon the ease with which they are digested. The fats eaten are not stored in the body directly, but the body constructs its fats from those eaten, and from other substances in food,--according to some authorities from the carbohydrates and proteids, and according to others from proteids alone.

Fats are stored away as fat, furnish heat, and are used for energy; at least, it is probable that at times they are put to the latter use. The fats laid by in the body for future use last in cases of starvation quite a long time, depending, of course, upon the amount. At such times a fat animal will live longer than a lean one.

Doubtless in the fat of food the body finds material for its fats in the most easily convertible form. Of the various fatty substances taken, some are more easily assimilated than others. Dr. Fothergill, in "The Town Dweller," says that the reason that cod-liver oil is given to delicate children and invalids is, that it is more easily digested than ordinary fats, but it is an inferior form of fat; the next most easily digested is the fat of bacon. When a child can take bread crumbled in a little of this fat, it will not be necessary to give him cod-liver oil. Bacon fat is the much better fat for building tissues. Then comes cream, a natural emulsion, and butter. He further says there is one form of fat not commonly looked at in its proper dietetic value, and that is "toffee." It is made of butter, sugar, and sometimes a portion of molasses. A quantity of this, added to the ordinary meals, will enable a child in winter to keep up the bodily heat. The way in which butter in the form of toffee goes into the stomach is particularly agreeable.

=Carbohydrates.= The principal carbohydrates are starch, dextrine, cane-sugar or common table sugar, grape-sugar, the principal sugar in fruits, and milk-sugar, the natural sugar in milk. They are substances made up, as before stated, of carbon, hydrogen, and oxygen, but no nitrogen. They are important food substances, but are of themselves incapable of sustaining life.

The carbohydrates, both starch and sugar, in the process of digestion are converted into glucose. This is stored in the liver in the form of glycogen, which the liver has the power of manufacturing; it then passes into the circulation, and is distributed to the different parts of the body as it is needed. (The liver also has the power of forming glycogen out of other substances than sugar, and it is pretty conclusively proved that it is from proteids, and not from fats. Carnivorous animals, living upon flesh alone, are found to have glycogen in their bodies.)

It is impossible to assign any especial office to the different food principles; that is, it cannot be said that the carbohydrates perform a certain kind of work in the body and nothing else, or that the proteids or fats do. The human body is a highly complex and intricate organism, and its maintenance is carried on by complex and mysterious processes that cannot be followed, except imperfectly; consequently, we must regard the uses of foods in the body as more or less involved in obscurity. It is, however, generally understood that the proteids, fats, and carbohydrates each do an individual work of their own better than either of the others can do it. They are all necessary in due amount to the nutrition of the body, and doubtless work together as well as in their separate functions. They are, however, sometimes interchangeable, as, for instance, in the absence of the carbohydrates, proteids will do their work. The carbohydrates are eminently heat and energy formers, and they also act as albumen sparers.

The body always has a store of material laid by for future use. If it were not for this a person deprived of food would die immediately, as is the case when he is deprived of oxygen. (Air being ever about us, and obtainable without effort or price, there is no need for the body to lay by an amount of oxygen; consequently only a very little is stored, and that in the blood.)

The great reserve forces of the body are in the form of fatty tissues, and glycogen, or the stored-away carbohydrates of the liver; the latter is given out to the body as it is needed during the intervals of eating to supply material for the heat and energy of daily consumption, and in case of starvation. That they are true reserves is shown by the fact that they disappear during deprivation of food. The glycogen, or liver-supply, disappears first; then the fat (Martin). The heat of the body can be maintained on these substances, and a certain amount of work done, although no food except water be taken.