The first requirement of the body is for fuel, because it has a great deal of work to do. Even when one lies perfectly quiet and appears to be resting, the heart is working to keep up the circulation of the blood, the chest and diaphragm muscles are working to maintain the oxygen supply to the lungs, the alimentary tract is moving food material along, working to digest it and get rid of waste, and the skeletal muscles are being held up to "tone" so as to be ready for further action. All this work that we scarcely realize, may be called involuntary. To it we may add all sorts of voluntary movements, from simply speaking a word to turning somersaults or lifting heavy weights. All work involves energy, which we can obtain only from the fuel foods, proteins, fats, and carbohydrates.
Energy takes different forms. Our supply comes from the sun in the forms of heat and light, and plants store it up in the form of chemical energy when they build carbohydrates, fats, and proteins. This may be changed into the forms of work or of heat when we eat the food. Whenever an attempt is made to change chemical energy to work, some of it will change to heat. So in our bodies, the fuel foods, which enable us to do both involuntary and voluntary work, furnish heat at the same time, to keep our bodies warm. When we are too cold, we can shiver or run or jump, and thus, by doing more work, get more heat too.
In our studies of food materials, we must find out just how much energy, or working power, can be obtained from each kind. We must have a measure of energy or fuel value; and just as the inch is a measure of length, and the pound of weight, so the Calorie serves as a measure of fuel value. This unit1 measures energy as heat, being the amount of heat required to raise 1 kilogram of water 1° C. (or 1 pound of water about 4° F.), but we can express it also as work, being sufficient energy to raise a 1-pound weight 3087 feet into the air (or 1 ton about 1 1/2 feet) if it were possible to convert it into mechanical work without loss. By burning foods in pure oxygen in a vessel placed in water so that all the heat is given off to the water, and then noting the change in temperature of the water, it is possible to find out just how much energy each will yield. Such a device is called a calorimeter. In the body there is usually a small portion of each kind of foodstuff which escapes digestion, and protein is not quite so completely burned as in the calorimeter. When allowance for the probable loss is made, the energy values of the fuel foodstuffs are as follows:
4 Calories per gram or 1814 per pound 9 Calories per gram or 4082 per pound. 4 Calories per gram or 1814 per pound.
Knowing the composition of any food material, it is possible from these figures to calculate the total fuel value, or we can refer to tables in which this has been calculated, and save ourselves labor. For comparison of different foods the Standard, or 100-Calorie, Portion is used, as this corresponds very closely with the amount of food for a single serving in many cases. In the sections treating of different foods the Standard Portion will be stated.
1 This is the "greater calorie" or "kilogram calorie," and is written Calorie to distinguish it from the "lesser calorie" or "gram calorie," largely used in physics and chemistry.