The following is from Bulletin No. 21, U. S. Dept. Agr., "Chemistry and Economy of Foods."
"The following familiar examples of compounds, commonly grouped with each of the four principal classes of nutrients, will serve to define the terms as here used, and may perhaps help to avoid the confusion which unfortunately results from the variations in usage by different writers.
Proteids: Albuminoids, e. g., albumen of eggs; myosin, the basis of muscle (lean meat) ; the albuminoids which make up the gluten of wheat. Gelantin-oids, e. g., constituents of connective tissue which yield gelatine and allied substances, e. g., collagen of tendon, ossein of bone.
"Amids: This term is frequently applied to the nitrogenous non-albuminoid compounds of vegetable foods and feeding stuffs, among which are amido acids, such as aspartic acid and asparagin. Some of them are more or less allied in chemical constitution to the nitrogenous extractives of flesh.
"Fats: Fat of meat, fat of milk, oil of corn, wheat, etc., the ingredients of the "ether extract" of animal or vegetable foods and feeding stuffs, which it is customary to group together roughly as 'fats,' include, with the true fats, various other substances, lecithens and chlorophylls.
"Mineral matters: Potassium, sodium, calcium, and magnesium chlorides, sulphates, and phosphates.
The two chief uses of the food of animals are, first, to form the material of the body, and to repair its waste; and, second, to yield energy in the form of (1) heat to keep the body warm, and (2) muscular and other power for the work it has to do.
"In forming the tissues and fluids of the body, the food serves for building and repairing. In yielding energy, it serves as fuel for yielding heat and power.
"The different nutrients of food act in different ways in fulfilling these purposes. The principal tissue formers are the albuminoids. These form the frame work of the body. They build and repair the nitrogenous materials, as those of muscle, tendon, and bone, and supply the albuminoids of blood, milk, and other fluids.
"The chief fuel ingredients of the food are the carbo-hydrates and fats. These are either consumed in the body, or are stored as fat, to be used as occasion demands.
"The albuminoids are the building material for the body. The bodily machine is made from them, but in the making of the machine the albuminoids remain partly albuminoids, and are partly changed to gelatinoids, so that the machine, as built, consists of both albuminoids and gela-tinoids. The gelatinoids cannot, according to the best evidence now at hand, be transformed into albuminoids, but they do serve to protect the albuminoids from being consumed. Both albuminoids and gelatinoids, after they have served as building material, can be broken up and oxidized within the body. In this cleavage and oxidation, they serve as fuel.
"The nitrogenous extractives can neither build tissue nor serve as fuel, but they are useful otherwise. Just how they are useful is not yet fully explained, but they appear to exert some influence upon the nervous system to act as stimulants, and thus to help the body to make use of other materials in its nourishment."