According to the definition of food that we gave when we were discussing the chemical composition of foods, we said that there were two functions for food in the body. The first of these was the providing of heat and energy, and the other the building of tissue. The body is similar to any machine in that it must have fuel of some sort to keep it going. An automobile is absolutely helpless with an empty gasoline tank, and a locomotive is powerless to pull a heavy train unless steam is supplied by the boiler. The body also, in order to perform work of any kind, must have fuel. The work performed is of two kinds. Whenever we move a muscle, we are performing work; in taking a step the body must lift its own weight. Of course the more violent the work done by the body, the more the fuel that must be provided. The body, however, is doing another type of work all the time, like a clock. The heart is ever in motion, performing enough work in the course of twenty-four hours to be sufficient to raise a man twenty-five hundred feet. At the same time, with every breath we take there is muscular energy expended in the expansion and contraction of the lungs and diaphragm. Also every meal eaten must be digested, and this requires from three to five hours of steady muscular activity.

There is no other way to obtain all of the fuel thus required by the body except from the food we eat. The sun is the source of all energy on the earth. Plants are able to transform this energy received from the sun in the form of light and heat into chemical substances, combining it with elements from the air, soil, and water to form energy-bearing substances which are stored in its tissues. As we have seen, these are carbohydrates, fats, and proteins. They are found in all kinds of vegetable foods in different proportions and can be used directly as human food. Or they may be used as food for animals and the energy contained in them utilized for the life processes of the animal and transformed into animal flesh. Only a small percentage of the energy in the plants is left to be stored in the flesh of the animal, since most of it must be utilized for muscular energy and heat. The animal's flesh, however, which contains fat and protein, is also used for human food, and becomes another source of fuel for the body.

Body need for fuel

When comparing one piece of string with another piece, we use a unit of measure as a basis for comparison; we say one is three feet long and the other five feet, the foot rule being a standard measure for length. In the same way we say one stone weighs two pounds and another one four pounds, the pound being the unit of measure for weight. In the same way it is necessary to have a standard unit of measure in order to compare the amount of heat given by one food as compared with that given by another. For this purpose the calorie is used. Scientifically the calorie is very carefully defined with allowances made for air pressure and other technical qualifications. For our purpose, however, it is necessary to know only that it is the term for the quantity of heat required to raise one pint of water 40 F. For instance, if two cupfuls of water are drawn from the faucet and placed in a saucepan, the temperature of the water will probably be about 500 F. If this quantity of water is placed over a gas flame and the temperature watched with a thermometer, by the time the water has been raised from 500 F. to 540 F. we have burned up one calorie of heat in the form of gas. It is to be remembered that a calorie is always the measure for a quantity of heat. With the calorie used as a standard, the various food substances have been actually burned in order to find how much heat a given weight would produce. This is very carefully done in an instrument known as the bomb calorimeter. A known weight of the food to be examined is burned in pure oxygen in a gas-tight chamber surrounded by a known quantity of water. The change in the temperature of the water will show the amount of heat liberated. In this way all of our American food materials have been examined, and the number of calories produced by a pound of each has been published in a long list by the Bureau of Chemistry of the Department of Agriculture. It has also been found that the food will yield to the body practically the same amount of heat that it does when it is burned in the calorimeter. The pure food substances will give the following quantities of heat:

Fuel foods

Heat unit: calorie

We notice that fat yields two and a quarter times as much heat for a given weight as do carbohydrates and proteins. The two latter yield the same amount.

Foods then which contain a large amount of fat are the best fuel food.

Foods which contain a large amount of water and woody fiber have a proportionately low fuel value.

The figures as published by the Department of Agri-culture are extremely complex and are not in a form that can be readily used by the average housekeeper. In order to overcome this difficulty, Dr. Langworthy of the Department devised the scheme known as One Hundred Calorie Portions; that is, he found that many of our common food materials were approximately one hundred calories in the quantity ordinarily used in the household. These portions are only approximate and therefore discrepancies are found in different lists.1 Some of the most common of these portions are as follows:

Butter or other fats, 1 tablespoon

Bread, I slice (3 in. x 3 1/2 in. x 1 in.)

Uneeda, 4 crackers

Orange, 1 large

Eggs, 1 1/3 medium

Potato, 1 medium

Meat, 2 ounces, cooked, lean

Sugar, 2 tablespoons, granulated

Sweet chocolate, about 3/4 ounces

After it is known how much fuel our foods can give to the body, it is necessary to know how much the body actually needs. The test of a normal diet for an adult is the maintenance of an average weight together with full health and efficiency. For a child it is shown by a steady gain in weight, by

good color, quiet sleep, and general good nature. If an adult is steadily losing in weight and there are no pathological conditions to account for it, it is pretty sure that something is wrong with either the kind or the quantity of the food eaten. If on the other hand, a person is steadily gaining in weight, he is not exercising sufficiently for the amount of food eaten; or, to put the same idea in another way, he is eating more than his daily needs require. It is quite possible to find a point at which the body weight can be maintained where the individual is at his best and does not feel dragged out at four o'clock in the afternoon. When this point is once found, it can and should be maintained with care and thought.