Popular Science Monthly/Volume 3/August 1873/The Nature and Influence of Foods
By EDWARD SMITH, M. D., F. R. S.
BEFORE proceeding to consider the numerous foods which will come under review in the course of this work, it seems desirable to offer a few remarks of a general character on their nature and qualities, and the necessity for them.
As a general definition, it may be stated that a food is a substance which, when introduced into the body, supplies material which renews some structure or maintains some vital process; and it is distinguished from a medicine in that the latter modifies some vital action, but does not supply the material which sustains such action.
This is certainly correct so far as relates to the substances which supply nearly all our nourishment, and which the Germans class under the term Nahrungsmittel, but there are certain so-called foods known as Genussmittel, which seem to form a connecting link, in that they increase vital actions in a degree far beyond the amount of nutritive material which they supply. They thus resemble certain medicines in their action, but, as they supply a proportion of nutritive material, they should be ranked as foods.
It is essential to the idea of a food that it should support or increase vital actions; while medicines usually lessen, but may increase, some of them.
It is not necessary that a food should yield every kind of material which the body requires, for then one might suffice for the wants of man, but that it fulfils one or more of such requirements, so that by a combination of foods the whole wants of the body may be supplied. Neither is it essential that every food should be decomposed or broken up, and its elements caused to enter into new combinations when forming or maintaining the structures of the body, since there are some which in their nature are identical with parts of the body, and, being introduced, may be incorporated with little or no change.
But there are foods which are more valuable to the body than others, in that they supply a greater number of the substances which it requires, and such are known as compound foods, while others, which supply but one element, or which are incorporated without change, may be termed simple foods. Other foods are more valuable because they are more readily changed into the substance of the body, or act more readily and quickly in sustaining vital actions, and these may be called easily-digested or easily-assimilated foods. Others are preferred because they supply a greater quantity of useful nutriment at a less proportionate cost, and are known as economical foods; and foods varying in flavor are classed as more or less agreeable foods.
Some foods are classed according to the source whence they are derived, as animal and vegetable foods; and others according to the density of their substance, as fluid and solid foods.
There are foods which nourish one part of the body only, and others which sustain one chief vital action, and are called flesh-forming or heat-forming foods, while others combine both qualities.
Besides these larger divisions, there are qualities in foods which permit of further classification, such as those which render them particularly fit for different ages, climates, and seasons, and others which possess a special character, as sweetness, acidity, or bitterness.
There are also effects produced by foods apart from or in addition to those of nutrition, which are not common to all; so that some foods more than others influence the action of the heart, lungs, skin, brain, bowels, or other vital organ, while others have antagonistic qualities, so that one may destroy certain effects of another.
Foods are derived from all the great divisions of Nature and natural products, as earth, water, and air, solids, liquids, and gases; and from substances which are living and organic, or inanimate and inorganic. The popular notion of food as a solid substance derived from animals and vegetables, while comprehensive, is too exclusive, since the water which we drink, the air which we breathe, and certain minerals found in the substance of the earth, are of no less importance as foods.
It is, however, of great interest to note how frequently all these are combined in one food, and how closely united are substances which seem to be widely separated. Thus water and minerals are found in both flesh and vegetables, while one or both of the component parts of the air, viz., oxygen and nitrogen, are distributed through every kind of food. Hence, not only may we add food to food to supply the wants of the body, but we may within certain limits substitute one for another as our appetites or wants demand. The necessity for this in the economy of Nature is evident, for, although a good Providence has given to man an almost infinite number of foods, all are not found everywhere, neither can any man obtain all foods found around him.
Further, there seems to be an indissoluble bond existing between all the sources of food. There are the same classes of elements in flesh as in flour, and the same in animals as in vegetables. The vegetable draws water and minerals from the soil, while it absorbs and incorporates the air in its own growth, and is then eaten to sustain the life of animals, so that animals gain the substances which the vegetable first acquired. But, in completing the circle, the vegetable receives from the animal the air which was thrown out in respiration, and lives and grows upon it, and at length the animal itself, in whole or in part, and the refuse which it daily throws off, become the food of the vegetable. Even the very bones of an animal are by the aid of Nature or man made to increase the growth of vegetables, and really to enter into their structure; and, being again eaten, animals may be said to eat their own bones and live on their own flesh. Hence there is not only an unbroken circle in the production of food from different sources, but even the same food may be shown to be produced from itself. Surely this is an illustration of the fable of the young phoenix arising from the ashes of its parent!
Food is required by the body for two chief purposes, viz., to generate heat and to produce and maintain the structures under the influence of life and exertion. The importance of the latter is the more apparent, since wasting of the body is familiarly associated with decay of life; but the former is so much the more urgent, that, whereas the body may waste for a lengthened period and yet live, it rapidly dies when the source of heat is removed or even greatly lessened.
The production of heat in the body, so wonderful in the process and amount, results only from the chemical combination of the elements of food, whether on the minute scale of the atoms of the several tissues, or on the larger one connected with respiration, and is thence called the combustion of food. As familiar illustrations of the production of heat from chemical change, we may mention that, when cold oil of vitriol and cold water are added together, the mixture becomes so hot that the hand cannot bear it, and the heating of hay-stacks, and also of barley in the process of malting, is well known. This action in the body is not restricted to changes in one element alone, but proceeds with all; yet it is chiefly due to a combination of three elements, viz., oxygen, hydrogen, and carbon, and requires for its support fat, starch, or sugar, or other digestible food composed of those substances, precisely as coal and wood supply fuel for lire without the body.
This effect is made extremely striking, by Prof. Frankland, in the following table, which shows the amount of heat generated from so small a quantity as ten grains of certain foods during their complete combustion within the body, and the force which scientific calculations have shown to be equivalent to that amount of heat. The original quantity used by Prof. Frankland has been reduced by Dr. Letheby to ten grains, for the convenience of English readers:
Thus we prove that an ounce of fresh lean meat, if entirely burned in the body, would produce heat sufficient to raise about 70 lbs. of water 1° Fahr., or a gallon of water about 7° Fahr. In like manner, one ounce of fresh butter would produce ten times that amount of heat; but it must be added that, as the combustion which is effected within the body is not always complete, the actual effect is less than that now indicated.
It may thus be shown that the division of foods into the two great classes of flesh-formers and heat-generators is not to be taken too incisively, for while a food is renewing flesh it also produces heat, and while the heat-generating food is acting it may also produce a part of flesh in the form of fat; but, although they are so closely associated in their vital work, the leading characteristic of each kind is so marked as to warrant the classification which Liebig has formulated.
It is understood that the structures of the body are in a state of continual change, so that atoms which are present at one hour may be gone the next, and, when gone, the structures will be so far wasted, unless the process of waste be accompanied by renewal. But the renewing substance must be of the same nature as that wasted, so that bone shall be renewed by bone and flesh by flesh; and hence, while the body is always changing, it is always the same. This is the duty assigned to food—to supply to each part of the body the very same kind of material that is lost by waste.
As foods must have the same composition as the body, or supply such other materials as by vital action may be transformed into the substances of the body, it is desirable to gain a general idea of what these substances are.
The following is a summary statement of the principal materials of which the body is composed:
Flesh in its fresh state contains water, fat, fibrine, albumen, and gelatine, besides compounds of lime, phosphorus, soda, potash, magnesia, silica, and iron, and certain extractives.
Blood has a composition similar in elements to that of flesh.
Bone is composed of cartilage, gelatine, fat, and salts of lime, magnesia, soda, and potash, combined with phosphoric and other acids.
Cartilage consists of chondrine, which is like gelatine in composition, with salts of soda, potash, lime, phosphorus, magnesia, sulphur, and iron.
The brain is composed of water, albumen, fat, phosphoric acid, osmazome, and salts.
The liver consists of water, fat, and albumen, with phosphoric and other acids in conjunction with soda, lime, potash, and iron.
The lungs are formed of a substance resembling gelatine, albumen, a substance analogous to caseine, fibrine, various fatty and organic acids, cholesterine, with salts of soda, and iron and water.
Bile consists of water, fat, resin, sugar, fatty and organic acids, cholesterine, and salts of potash, soda, and iron.
Hence it is requisite that the body should be provided with salts of potash, soda, lime, magnesia, sulphur, iron, and manganese, as well as sulphuric, hydrochloric, phosphoric, and fluoric acids and water; also nearly all the fat which it consumes daily, and probably all the nitrogenous substances which it requires, and which are closely allied in composition, as albumen, fibrine, gelatine, and chondrine. It can produce sugar rapidly and largely, and fat slowly and sparely, from other substances; also lactic, acetic, and various organic acids, and peculiar extractive matters.
So great an array of mysterious substances might well prevent us from feeding ourselves or others if the selection of food depended solely upon our knowledge and judgment; but it is not so, for, independently of the aid derived from our appetites, there is the great advantage of having foods which contain a proportion of nearly all these elements; and combinations of foods have been effected by experience which protect even the most ignorant from evil consequences.
Thus flesh, or the muscular tissue of animals, contains precisely the elements which are required in our flesh-formers, and, only limited by quantity, our heat-generators also; and life may be maintained for very lengthened periods upon that food and water when eaten in large quantities. Seeing, moreover, that the source of flesh in animals which are used as food is vegetables, it follows that vegetables should have the same elements as flesh, and it is a fact of great interest that in vegetables we have foods closely analogous to the flesh of animals. Thus, in addition to water and salts, common to both, there is vegetable jelly, vegetable albumen, vegetable fibrine, and vegetable caseine, all having a composition almost identical with animal albumen, gelatine, chondrine, and caseine.
Hence our appetites and the bountiful provision made for us extend our choice to both the vegetable and animal kingdoms, and it is possible to find vegetable foods on which man could live as long as upon animal food alone. Bread is in vegetable foods that which flesh is in animal foods, and each within itself contains nearly all the elements required for nutrition.
When, however, we bring knowledge of a special kind to the aid of our appetites, we are able to discover both the deficiencies in any given food and the kind of food which would meet them. Thus a knowledge of the requirements of the system and of the available uses of food leads to the proper combinations of food, or to the construction of dietaries.
We have thus placed face to face the requirements of the body and the qualities of the foods to be used to supply them, but it is of very common observation that the effect of the supply is but temporary, and needs renewal at definite periods. Hence we show that the needs of the body are tolerably uniform, while the effect of the supply is temporary, or that both the need and the supply are intermittent. This may be readily represented by showing the line of change in the degree of vital action on the body during the twenty-four hours, as produced by my own investigations, and delineated in the graphic diagrams of the present work.
It is there illustrated that, during the repose of the night, the amount of vital action, as shown by the respiration and pulsation, is low and tolerably uniform, while under the influence of food it is high, and varies during the day extremely, but the general course is such that a large increase takes place after a meal, and a considerable decrease before the following meal. This increase, followed by decrease, being due to the action of food, proves that the influence is temporary, and that after a sufficient interval another supply of food is required. At the same time it must be allowed that the body is not entirely a passive agent subject to the controlling action of food, for no supply could prevent the vital actions subsiding at night, or make them equal both by night and day.
There is a power inherent in the body which accepts or rejects food as to amount, as well as to quality, and which might at length act through the appetite, and refuse the kind supplied. Moreover, the wants of the body vary from many other well-known influences, and cause an increase or decrease in the vital actions which proceeds pari passu with the consumption of the transformed or stored-up food in a degree proportionate to the cause, but such effects are often more rapid and transitory than that of food.
The variations in the requirement for food are induced by age, climate, season, and degree of exertion, and will be more fully discussed in the work on Dietaries; but it may now be desirable to give a glance at some of them.
In reference to age, there can be no doubt that all vital processes, including the action of foods, are greater and more rapid in early, and less and slower in later, than in mature life, and in both the former a more frequent administration of food is necessary. In early life, more-over, there is the important function of growth, which demands a large and more frequent supply of food, not only for daily wants, but to promote a due increase in the bulk of the structures of the body. I have also shown that the season of the year has also a decided influence over the vital actions, so that they are the greatest in the spring and the least at the end of summer.
The action of climate is similar to that of season, and shows that the vital actions are greater in cold than in hot climates, and in the uplands than in close valleys.
The influence of exertion over vital changes is immediate and proportionate, while the subsidence with the rest is less rapid than the increase. The following table of experiments upon myself shows the proportionate effect of exertion of varying degrees on the basis of the increased volume of air inspired:
Another table, from the same series of experiments, illustrates the same effect on the basis of the amount of carbonic acid evolved by respiration per minute:
Thus it is possible that the amount of vital change proceeding in the body may be ten times greater in one state than in another, and it follows that a proportionate quantity of food will be required to sustain it.