From these deductions it is evident that carbohydrates and fats perform very similar functions within the body, and can, in a large measure, replace each other as a source of heat and muscular energy. METABOLISM OF PROTEIDS [Sidenote: Importance of proteid or nitrogenous foods] Owing to the fact that the tissues of the normal body are constructed chiefly from proteids, the metabolism of proteids or nitrogenous foods is of very great importance. When we realize the fact that muscle, blood, brain, nerves, cartilage, tendons, the various internal organs and the tougher material of the skeleton are only various forms of proteid material, and must contain their proportions of available or organic nitrogen, we can understand why nitrogenous foods form a distinct class that must be considered by themselves. Only the mineral deposits of the bones and the teeth, and the globules of fat that are deposited as a source of stored energy represent the nitrogen-free class of substances within the animal body. THE USE OF PROTEIDS IN THE BODY [Sidenote: Proteids as tissue-builders] The first use Nature makes of proteids in the body is in the actual adding to or increasing of body-tissue. When an emaciated young man from the city goes to work on a farm and gains twenty pounds, the cells of his muscles have actually increased in size and number. This requires proteids, which can be obtained only from the nitrogenous material in food. The growth during early life is due to an actual increase in the size of all the organs of the body, and is merely an accumulation of proteid substance. [Sidenote: Proteids form the nitrogenous part of the body] The second use of proteids, and the one which, in matured life, is of more importance than those already referred to, is in the formation of the various nitrogenous products which are produced in connection with the different processes of the body and which are destroyed by the function of life. For example, the pepsin of the gastric juice is a nitrogenous substance which can be formed only from proteids. All digestive enzyms and other substances in the muscles, nerves, and in the various organs throughout the body are of a nitrogenous nature, and in their formation and use a certain amount of proteid material is consumed. When the digestive enzyms are formed from proteids, they consume more than their own weight of proteid material. [Sidenote: Proteids replace worn-out cells]