While all the articles of food in the four above-named classifications contain other elements than the one under which heading they appear, yet the body uses or appropriates them for the following purposes: PURPOSE OF CARBOHYDRATES The carbohydrate substance in food is used by the body chiefly for the purpose of keeping up body-weight; that is, for the purpose of supplying the various fluids which fill the cell-structure. If one is suffering from emaciation, the carbohydrate element in food should predominate. While some of the more soluble proteids, especially milk and eggs, will give a rapid gain in weight, the weight will not be permanent unless sufficient carbohydrates are taken to supply the blood with all the required elements of nutrition, or, in other words, to level or to balance the body requirements. PURPOSE OF FATS Fats are used by the animal body primarily for the purpose of producing heat. Food is burned or oxidized in the blood, undergoing very much the same action as does the combustion of coal in a grate. The heat thus generated is delegated to the blood, and the blood, by its circulation, distributes this heat throughout the body. The carbon dioxid or waste matter formed during the circulation, is carried to the lungs, where it reunites with the oxygen which we breathe, and thereby again passes back into the atmosphere. PURPOSE OF PROTEIDS Proteid is a compound containing chiefly nitrogen, oxygen, and carbon. Its purpose is to form the muscular and the tissue structure of the body. To use a homely illustration, proteid may be compared to the material which makes the honeycomb, while the carbohydrate substance may be compared to the honey; that is, to the fluids which fill the cells. Those performing heavy or active muscular labor should eat liberally of the proteid class of foods. Under normal conditions, natural hunger will call for the quantity of proteid needed. The tendency, however, should be toward the minimum; that is, one should take the lowest quantity of proteid that the body requires to keep up the cell-structure. (See Lesson VI, p. 216.) Modern investigations have shown that, in many cases of extreme athletic tests, a low proteid diet has given the greatest endurance. This is accounted for by the fact that nearly all carbohydrates, especially of the grain family, contain from 8 to 12 per cent of proteids, which is quite sufficient, in many instances, to supply the body with all the tissue-building material necessary.