In Liebig's time it was recognized that animal heat was generated by carbohydrates and fats, but the physiologists of that day did not realize that these were the nutrients which furnished the body with energy for its activity. They thought that all muscular work was done at the expense of nitrogenous material. It has taken many years for us to realize fully that animal energy in all its forms is derived primarily from carbohydrate material, secondly from fats, and thirdly from proteins only in so far as they yield combustible, non-nitrogenous bodies.

Energy may be defined as the power to do work. This force manifests itself in different ways. It may be latent, or inactive; or it may be active in various forms, such as heat; chemical or electrical energy; or mechanical movement, which we call work. It may be changed from one of these forms into another without loss. The most convenient way to measure energy is by transforming it into heat. A unit has been devised to express different amounts of heat, corresponding to a definite number of work units; this is called the calorie. One calorie represents the amount of heat required to raise the temperature of one kilogram of water one degree Centigrade. This is called the large calorie, which is commonly used in determining the energy value of food. The small calorie is 1/1000 of a large calorie.

Energy cannot be created. We must put into any machine as much force as we expect to get from it. Even in the most efficient machine we cannot recover all of the energy in the form of useful work; some of it is always transformed into heat, owing to friction. This is true even in the body, which is a most efficient machine, but here much of the waste energy in the form of heat is turned to good account in maintaining the body temperature.

The source of energy for the body is food, just as much as the source of energy for the engine is coal. Plants store up the sun's energy, it is transferred to animals which eat the plants, and eventually man gets it by eating the plant and animal foods. The amount of energy in any given food material is measured by the amount of heat which it will produce. In general, this is the same, whether the food be burned outside or inside the body, the total result in any case depending upon the amount of the food-stuff which is utilized by the body.

An instrument devised for the measurement of heat is called a calorimeter. To determine the fuel value of any food material outside of the body, a given amount is placed in a calorimeter, where it is burned in an atmosphere of pure oxygen, in a vessel surrounded by water. The heat generated raises the temperature of the water, and the change is observed with a very delicate thermometer. From this the total heat evolved is calculated. To determine the fuel value of this material within the body, the average amount which is lost in digestion, or which is not completely oxidized before excretion, is deducted from the fuel value outside the body. The result is the physiological fuel value. Many years ago, Rubner determined averages for proteins, fats, and carbohydrates from experiments on dogs. In recent times, further experiments made in this country on human subjects by Prof. Atwater and his associates have modified these factors somewhat. The following are therefore accepted as the average energy values of food in the body to-day:

1 gram of Protein ........ yields 4 Calories.

1 gram of Fat ............ yields 9 Calories.

1 gram of Carbohydrate... yields 4 Calories.