Heat is required to maintain the body temperature necessary in order that the processes of life may be carried on.
The work performed may be considered as of two kinds, internal and external. The internal work is that used in maintaining the different functions of the body itself.- The beating of the heart, breathing, the absorption of food, all require the expenditure of energy; this internal work requires a large portion of the available power. As in all machines, energy is lost in the form of radiant heat, but the body is considered an efficient machine because a larger proportion of energy is available for external work than in most engines constructed by man.
The amount of energy required for external work, i. e., muscular work, is a variable factor, and the amount of physical activity is consequently important in determining the amount of food necessary.
So far as present knowledge goes, we may say that the energy of the body is derived from the slow oxidation (or combustion) of food that takes place in the tissues all over the body. The process is undoubtedly a complex one, far from the simple chemical union of the food materials with the air we breathe. It is easy to understand that this oxidation gives heat, but how it produces the power of muscular contraction, nervous energy, and all the activities of life is not known.
All combustible substances have what is known as potential energy. This might be defined as stored-up energy. It implies that energy from some exterior source has been used in producing the substance in its present form. For instance, heat from the sun has been utilized in the formation of the starch or proteid in the plant, and this energy is again set free in the oxidation or the decomposition of the substance.
Potential energy may perhaps be most easily understood by thinking of one form of it, energy of position. A weight lifted to a height has by virtue of its place a certain amount of potential energy. The fall of the weight from its position will convert its potential energy into active or kinetic energy by which work is accomplished.
The waste materials of the body have little or no potential energy, and the outgo of the body differs in this important respect from its income. If the food taken in is only partially oxidized, the waste material still contains some energy, and this potential energy must be substracted from that of the income in order to find the amount available for the use of the body.
The value of a food to produce heat and mechanical energy is measured by the amount of heat that may be produced by it, and the unit of measure is the calorie. A calorie is the amount of heat required to raise about one pound of water f6ur degrees Fahrenheit, or, accurately, the amount of heat required to raise one kilogram of/water one degree centigrade. This is the large calorie, and it is sometimes written with a capital C to distinguish it from the small calorie. The small calorie has a value one-thousandth as great. The term used in this paper means the large calorie.