PART III. HUMAN PHYSIOLOGY

XXVII. FOODS

Why we need Food. We have already defined food as anything that forms material for the growth or repair of the body of a plant or animal, or that furnishes energy for it. Food, then, not only furnishes our body with material to grow, but also gives us the energy we expend in the acts of walking, running, breathing, and even in thinking.

Nutrients.-Certain nutrient materials form the basis of food of both plants and animals. These have been stated to be carbohydrates (starches, sugars, gums, etc.), hydrocarbons (fats and oils, both animal and vegetable), proteids (such as lean meat, eggs, the gluten of bread), and mineral matter and water. Oxygen, although not a nutrient, ought to be considered as food because it enters into the composition of the body, and without it no energy could be released. Let us apply this general knowledge with reference to the human body in order to determine the uses made of food taken into the body; for parts of the human body, be they muscle, blood, nerve, bone, or gristle, are built up from the nutrients in our food.

The Body a Machine. - The body has been likened to a machine in that it turns over the latent or potential energy stored up in food into kinetic energy (mechanical work and heat) which is manifested when we perform work. One great difference exists between an engine and the human body. The engine uses fuel unlike the substance out of which it is made. The human body, on the other hand, uses for fuel the same substances out of which it is formed; it may, indeed, use part of its own substance for food. Let us now consider more in detail the nutrients used by man as food, and determine the use of each to him.

Proteids. As we know, proteids, in some manner unknown to us, are manufactured in the leaves of plants. Proteid sub

stances contain the element nitrogen. Hence such foods are called nitrogenous foods. Although about four fifths of the atmosphere is composed of nitrogen, yet plants are unable to take it from the air, but are forced to absorb it through the roots in the form of nitrates dissolved in the water in the soil. Herbivorous animals eat the plants, take into their bodies the stored nitrogenous foods, and change this food into protoplasm. Man himself 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.) 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). Because of their chemical composition, proteids are considered to be flesh-forming foods. They are, however, oxidized to release energy if occasion requires it. Organic Fats and Oils. — Fats and oils, both animal and vegetable, are the materials from which the body derives most of its energy. The chemical formula of a fat shows that, compared with other food substances, there is very little oxygen present; hence the greater capacity of this substance for uniting with oxygen. The rapid burning of fat compared with the slower combustion of a piece of meat or a piece of bread illustrates this. A pound of butter releases over twice as much energy to the body as does a pound of sugar or a pound of steak. Fats and oils are stored in the human body as fatty tissue. This material may be drawn on as reserve fuel when the body requires it. The Arctic-living tribes exist almost entirely on the blubber of the walrus or the whale. The blubber not oxidized in the body is stored as fat, thus forming an insulation under the skin, which aids in keeping the body warm.

Carbohydrates. We see that the carbohydrates, like the fats, contain carbon, hydrogen, and oxygen. Here, however, the oxygen and hydrogen are united in the molecule in the same proportion as are hydrogen and oxygen in water. Carbohydrates are essentially energy-producing foods. They are, however, believed to be of some use in building up or repairing tissue. Some experiments seem to indicate that carbohydrates may be formed directly into tissue. It is certainly true that in both plants and

animals, such foods pass directly, together with foods containing nitrogen, to repair waste in tissues, thus giving the needed proportion of carbon, oxygen, and hydrogen which is to unite with the nitrogen in forming the protoplasm of the body.

Inorganic Foods. Water forms a large part of almost every food substance. The human body, by weight, is composed of about sixty per cent water. When we drink water, we take with it most of the inorganic salts used by the body in the making of bone and in the formation of protoplasm. Sodium chloride (table salt), an important part of the blood, is taken in as a flavoring upon our meats and vegetables. So important is this food that life is often given in exchange for it by herbivorous animals, as the deer. Man will also endure great hardships to get salt.1 Phosphates of lime and potash taken in water are important factors in the formation of bone.

Uses of Nutrients. The following table sums up the uses of nutrients to man:

Proteid..

2

1 Animals will travel long distances to obtain salt. Men will barter gold for it; indeed, among the Gallas and on the coast of Sierra Leone, brothers will sell their sisters, husbands their wives, and parents their children for salt. In the district of Accra, on the gold coast of Africa, a handful of salt is the most valuable thing upon earth after gold, and will purchase a slave or two. Mungo Park tells us that with the Mandingoes and Bambaras the use of salt is such a luxury that to say of a man, 'he flavors his food with salt,' is to imply that he is rich; and children will suck a piece of rock salt as if it were sugar. No stronger mark of respect or affection can be shown in Muscovy than the sending of salt from the tables of the rich to their poorer friends. In the book of Leviticus it is expressly commanded as one of the ordinances of Moses, that every oblation of meat upon the altar shall be seasoned with salt, without lacking; and hence it is called the Salt of the Covenant of God. The Greeks and Romans also used salt in their sacrificial cakes; and it is still used in the services of the Latin church the 'parva mica,' or pinch of salt, being, in the ceremony of baptism, put into the child's mouth, while the priest says, 'Receive the salt of wisdom, and may it be a propitiation to thee for eternal life.'-- LETHEBY, On Food.

2 W. O. Atwater, Principles of Nutrition and Nutritive Value of Food. U.S. Dept. of Agriculture, 1902.

HOW THE NUTRITIVE VALUE OF FOOD HAS BEEN DISCOVERED. For a number of years, experiments have been in progress in different parts of the civilized world which have led to the beliefs regarding food that have just been quoted. One of the most accurate and important series of experiments was made a few years ago by the late Professor W. O. Atwater of Wesleyan University, in coöperation with the United States Department of Agriculture. By means of a machine called the respiration calorimeter (Lat. calor=heat + metrum=measure) which measures both the products of respiration and the heat given off by the body, it has been possible to determine accurately the value of different kinds of food, both as fuel and as tissue builders. This respiration calorimeter is described by Professor Atwater as follows:

"Its main feature is a copper-walled chamber 7 feet long, 4 feet wide, and 6 feet 4 inches high. This is fitted with devices for maintaining and measuring a ventilating current of air, for sampling and analyzing this air, for removing and measuring the heat given off within the chamber, and for passing food and other articles in and out. It is furnished with a folding bed, chair, and table, with scales and appliances for muscular work, and has telephone connection with the outside. Here the subject stays for a period of from three to twelve days, during which time, careful analyses and measurements are made of all material which enters the body in the food, and of that which leaves it in the breath and excreta. Record is also kept of the energy given off from the body as heat and muscular work. The difference between the material taken into and that given off from the body is called the balance of matter, and shows whether the body is gaining or losing material. The difference between the energy of the food taken and that of the excreta and the energy given off by the body as heat and muscular work, is the balance of energy, and, if correctly measured, should equal the energy of the body material gained or lost. With such apparatus it is possible to learn what effect different conditions of nourishment will have on the human body. In one experiment, for instance, the subject might be kept quite at rest, and in the next do a certain amount of muscular or mental work with the same diet as before, then by comparing the results of the two, the use which the body makes of its food under the different conditions could be determined; or the diet may be slightly changed in the one experiment, and the effect of this on the balance of matter or energy, observed. Such methods and apparatus are very costly in time and money, but the results are proportionately more valuable than those from simpler experiments."

FUEL VALUES OF NUTRIENTS. - In experiments performed by Professor Atwater and others, and in the appended tables, the value of food as a source of energy is stated in heat units called calories. A calorie is the amount of heat required to raise the temperature of one kilogram of water from zero to one degree centigrade. This is about equivalent to

raising one pound four degrees Fahrenheit.

foods may be computed in a definite manner.

The fuel value of different
This is done by burning

a given portion of a food (say one pound) in the apparatus known as the calorimeter. By this means may be determined the number of degrees the temperature of a given amount of water is raised during the process of burning.

The Best Dietary.—Inasmuch as all living substance contains nitrogen, it is evident that proteid food must form a part of the dietary; but proteid alone is not usable. 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. We must have a mixed diet which may contain several different food materials.

A growing person must take in a little more food each day than is used up. In an ordinary day's work, a man uses up about two hundred and sixty grams of carbon and nineteen grams of nitrogen. This must be replaced in the correct proportion. More carbon or more nitrogen than needed, would simply mean that some of the organs of the body would have to work overtime to rid the body of the unused material. It has been found as a result of studies of Atwater and others, that a man who does muscular work requires about one quarter of a pound of proteid, the same amount of fat, and about one pound of carbohydrate to provide for the growth, waste, and repair of the body and the energy used up in one day. In addition to this, an ounce of salt and nearly one hundred ounces of water are used. The amount of food consumed varies with the age and occupation of the individual. A child of from five to six years needs only .5 of the food required by a man doing muscular work. A growing boy of highschool age, contrary to common belief, needs only between .7 and .8 of the food needed by an active man. People who lead a sedentary life need much less food than those doing hard work; the latter, on the other hand, need more food than a person who is only moderately active. By means of the table on the following page (modified from Atwater1), which shows the composition of some food materials, the nutritive and fuel value of the foods may be seen 1 W. O. Atwater, Principles of Nutrition and Nutritive Value of Food. U.S. Dept. of Agriculture, 1902.

HUNTER'S BIOL.- -21