If the digestive forces are not working perfectly and if the food is not properly cooked, some of the food is not made perfectly soluble for absorption. But in normal conditions, if the food is supplied in proportion to the energy required, the heat and energy given off should exactly equal the latent heat and energy consumed in food.
It is to be noted, also, that no force within the body is lost. In the very process of the removal of waste, heat and energy are created, so that the parts no longer needed are utilized by the system, while they are being removed from it. Here is a lesson in economy of force.
As mentioned before, the fuel for the body consists of fats, starches, and sugar, which, in combination with oxygen, create force. The combination of oxygen with other elements in the body is known as oxidation. This oxidation liberates heat and at the same instant produces energy, either in muscle, gland or nerve. The muscular energy expresses itself in muscular motion, the glandular in chemical action, and the nervous in nervous energy. The nervous energy is closely allied to electrical force.
The starches come largely from cereals and root vegetables; the sugars largely from cane, from certain trees, and from vegetables, fruit, and milk; the fats come from vegetable oils, from animal fat, as fat, and some from milk and butter. Some fats are also formed from proteins.
From the above, it follows that the fuel value of food depends upon the amount of fats, starches, and sugars contained.
The exact process of the conversion of the potential energy latent in food into heat and energy is not known. It is partly released during the digestive process, as the elements of the food come into contact with the oxygen swallowed and with the digestive juices. This combustion gives to the digestive organs the necessary warmth for their effective work. Digestive juices will not flow freely when the body is cold. The heat liberated during the digestive process is necessary, also, to put the elements of the food into condition for absorption, a certain amount of heat being required for the chemical changes. This liberated energy is expressed, not alone in the chemical formation of the compounds, but in the peristaltic movements of the digestive organs.
A small portion of the heat of the body is gained from the sun or from artificial heat, but by far the greater part is generated within the body. If one is cold, the quickest way to get warm is to generate more heat within by “turning on the draught”, or, in other words, by breathing in more oxygen. So many people cover up the body with more clothing to reserve the body heat and forget to generate more heat by arousing the fires within. This is like covering up a dying fire to reserve the heat, instead of turning on the draught to create more combustion.
Nature provides for a reserve of heat and energy, above the immediate needs, by storing up a supply which is called into use whenever the daily supply is inadequate. Many hibernating animals store up sufficient fat in summer to provide heat for the entire winter. This fat would not last the winter, however, were the animal active. Many individuals store up excess of fat sufficient to last them for months, even though all fat building elements be omitted from the diet.
It must be remembered that anything which creates a greater activity of the tissues, such as muscular exercise, liberates a greater amount of heat. The reverse is also true;—a decrease in the amount of muscular movement means a decrease of heat liberated. During exercise, a large amount of fat, protein, and dextrose (sugar) are released by the movements and oxidized; the liberated heat is carried to all parts of the system and the temperature is raised. Mental work, for the same reason, tends to raise the body temperature, though to a much less degree. Food in the alimentary canal causes an activity in the glands of the digestive organs and also increases the temperature.
Of course, while digestion and mental and muscular activity are at their height, the body temperature is highest. These activities usually reach a maximum in the afternoon and the temperature is then highest, while, as a rule, it decreases from about six at night until four or five in the morning, when it is usually at its lowest ebb. This is a point of importance to physicians. Even five degrees above the average human temperature, if recorded about six at night, is not considered abnormal.