The modern era of the science of nutrition may be said to have been introduced by the famous French scientist, Lavoisier, in 1780. He was the first to apply the thermometer and the balance to the study of the chemical changes taking place in the living body. He established the fundamental fact that the production of heat involved the combination of the oxygen of the air with the substances taken into the body as food, and was in fact a process of combustion. He discovered that the temperature of the air had a pronounced influence on the rate at which substances in the body were oxidized. It is slow at temperatures which we find comfortable with light clothing, and is greatly accelerated by cold. Exercise, he found to stimulate combustion in the body, and he also observed that during work it might rise to several times the resting metabolism. These fundamental facts established by Lavoisier have formed the subjects of numerous investigations, which greatly increased our knowledge of detail in this important field.

About 1842 Joule described experimental data establishing the mechanical equivalent of heat. Heat is measured in terms of the calorie, which is the amount of heat necessary to raise the temperature of 1 cubic centimeter of water 1 degree centigrade, or one pound of water 4 degrees Fahrenheit. Another unit, the large Calorie, is 1,000 times the small calorie. The rate at which energy metabolism varies with the activity of the individual, and in the absence of suitable clothing, with the temperature, is the most spectacular phase of the metabolic processes, and is easily observed. A man at rest may have need of only about 1,600 calories a day in the form of suitable food, in order to cover his energy requirements. The same man at very severe labor may require 10,000 calories a day. It is not surprising that this obviously important phase of metabolism was early studied in great detail.

In 1866, Pettenkofer and Voit in the University of Munich, began the publication of their elaborate studies on energy metabolism carried out with the aid of a respiration apparatus by means of which they could measure accurately the amount of oxygen absorbed, the amount of carbon dioxid and of water in the products of respiration and the heat given off by the body under various conditions (2). They introduced into their experimental work the idea suggested by Liebig in 1842, that the nitrogen eliminated in the urine could be made a measure of the amount of protein destroyed in metabolism, since protein is the one prominent food-stuff which contains this element. Pettenkofer and Voit studied the metabolism of fasting men and dogs, and of the same subjects when taking different kinds and amounts of food. From the magnitude of the "respiratory quotient," which is the figure obtained by dividing the volume of carbon dioxid eliminated by the volume of oxygen consumed in respiration it was possible to decide what kind of food was being oxidized as a source of energy in the body. When carbohydrate is burned the quotient is 1. When fat is burned it is 0.7, and when protein is burned the figure is approximately 0.8. They found that a dog could be maintained in nitrogen and energy equilibrium during a period of a few days on a diet of protein alone (muscle tissue).*