We could analyze an adult man and a new-born infant, and know that the infant, in order to reach maturity, would need to add to its body so many pounds of oxygen, carbon, sulfur, iron, etc. The problem of nutrition, however, is more complex. Not only must we consider the formation of new tissue, but we must also allow for the rebuilding of the old, and for all those processes of vital activity that involve the consumption of food material and the destruction of body-tissue. Nor can this allowance be accurately proportioned from the analysis of the body, because the various elements composing it do not change with equal rapidity. Thus, a man in a harvest field might pass through his blood in one day ten or fifteen pounds of oxygen (in the form of water and carbon dioxid), which would amount to ten per cent of the oxygen contained in his body, but if he should take calcium or fluorin to the extent of ten per cent of that contained in the body, death from poisoning would speedily ensue. We can better understand the use of foods and the process they undergo in building the body by considering separately each class of food material from the time it is absorbed from the alimentary tract until it is excreted from the bowels, or from the lungs and the kidneys, or deposited in the body as bone, fat, or tissue. SECOND--THE GENERATION OF HEAT AND ENERGY The second function, or rather group of functions to be considered in the study of metabolism is the generation of heat and energy. If the reader will recall what was said in Lesson II, regarding the production of heat by the process of oxidation, he can more clearly comprehend the method by which heat is produced in the animal body. However, as heat is only one form or expression of energy, these two subjects--heat and energy--should be considered together. [Sidenote: Heat and energy produced by oxidation] The production of heat and energy in the body occurs almost entirely through the oxidation of food. All three classes of foods, namely proteids, carbohydrates, and fats can be oxidized to produce heat. [Sidenote: Heat, a measure of energy] Energy may be mechanical, chemical, electrical, or thermal. The conservation of energy, which is one of the fundamental laws of science, teaches that no energy can be lost, but can only be changed into other forms. This being true, and because all energy can be changed into heat, we use heat as a measure of energy. [Sidenote: The "calory," a unit of heat] The unit of heat, and consequently of energy, that is used by scientists is the "calory," which is the amount of heat required to raise the temperature of one thousand grams of water one degree on the centigrade thermometer scale. The energy in food is measured in calories, as will be learned from the explanation given in the lesson entitled "Vieno System of Food Measurement." [Sidenote: Liberation of energy through metabolism]