The early studies relating to energy metabolism led to the discovery that the protein of the food differs in one marked respect from either carbohydrate or fat, by its great stimulating effect upon metabolic processes as measured by the rate of oxidation. This is illustrated by the following type of experiment: If a dog is observed when it has been for some hours without food, and the rate of energy metabolism carefully measured, it will be found that this remains constant within narrow limits so long as the animal remains at rest. If now it is fed liberally with either fat or carbohydrate and the observations on its energy metabolism are continued, there is observed a slight increase in metabolism which is the direct result of the utilization of these foods. If on the other hand, a liberal portion of protein rich food, such as meat is fed to the animal instead of the non-nitrogenous nutrients, there is a surprising acceleration of the metabolic processes, attended with a marked increase in energy set free within the tissues. This was first shown by Bidder and Schmidt in 1852. Their experimental animal was a cat. During a certain interval of time the animal used 50.18 grams of oxygen and eliminated from its lungs 53.52 grams of carbon dioxid. The animal was then allowed to eat all the meat it would consume and the observations continued. During a similar interval the cat used 103.84 grams of oxygen and respired 113.52 grams of carbon dioxid (3).

* For a detailed history of this important phase of nutrition, the reader is referred to "The Science of Nutrition," Graham Lusk, 3d Ed. Philadelphia, 1917.