The energy needed for the various activities of the body is furnished by the processes of oxidation which take place in every living cell. The oxygen used is obtained from the atmosphere. The chemical substances oxidized are derived from materials taken into the body, which we call food. The constituents of the food are, however, seldom in a suitable condition for absorption into the blood, and before this can be effected they undergo a series of chemical and physical changes known as digestion. The products of digestion are then taken up by cells lining the alimentary canal, and passed into the blood in the process of absorption: this is not a simple transference of a given substance across the alimentary mucous membrane, but itself includes chemical change. Further, the materials absorbed from the intestines into the circulating blood are not even then in a form entirely suitable for oxidation in a cell, but have to suffer later transformations which are comprised in the term assimilation. The molecules finally presented to the living protoplasm may be oxidized rapidly for the sake of the energy thus set free, or may be built up for a longer period of time into the tissues of the cell.
The term metabolism is used to designate all the chemical processes occurring in the tissues of the body; it includes building up, assimilative, or anabolic processes, and breaking down, or katabolic processes, whether they take place in the essential protoplasmic structure or in the tissue fluids which bathe the cells.
Since the body is continually oxidizing its own substance, the materials supplied to the cells must, besides being capable of oxidation, contain the elements necessary to replace the tissues as they are broken down. The degree to which a cell uses its own substance will vary with the needs of the body and the supplies of new material. If nutrition is adequate and the calls made upon the cell not excessive, the cell will be able to furnish energy from the oxidizable substances available, and the demand upon its own tissue will be minimal. In starvation, on the other hand, the body is obliged to fall back upon its own substance, using the less essential tissues first. The food has thus to fulfil two functions, first, to supply the necessary elements for the structure of the body, and secondly, to furnish energy for the activities of the body. We may refer to these by speaking of the metabolism of structure and the metabolism of function.
In the study of dietetics it is of great importance that these two purposes to which food may be put should be clearly borne in mind.
The metabolism of structure includes the building up of the tissues and organs, the replacement of such portions of them as are continually being disintegrated, and the transformations which these portions undergo before they are excreted. Both inorganic and organic substances are required for this purpose; the inorganic, such as mineral salts and water, furnish no energy, but are as essential to the maintenance of the structure of the body as the organic proteins which are built up into living protoplasm.
The metabolism of function is concerned with the furnishing of energy for the performance of the normal functions of the various organs. For this, one or more of the organic food-stuffs, protein, carbo-hydrate, or fat is necessary.
Protein can therefore play two parts : it is essential to the structure of the body because the animal can obtain the nitrogenous materials necessary to form protoplasm from no other source; and, on the other hand, when it is not needed for the growth or repair of tissue it can be oxidized and set energy free.