We all know that combustion is caused by the combination of the oxygen of the air with the freed carbon of the substance which is being consumed. It was taught by Liebig that fat split up in the body, and that the freed carbon combined with the oxygen which is taken into the lungs in respiration, the result being the production of carbonic acid (C02), and that it is by this act of respiratory combustion in the lungs that the body heat is maintained. Fatty foods were hence considered necessary as heat-producers. Recent investigations have, however, proved that the matter is not so simple, and that though fat is split up and combined with oxygen in the production of heat, especially during muscular exercise, the process is effected in the tissues by the action of the cells, and not in the lungs, as formerly taught. The use of fat seems to be threefold. (1) To maintain the body heat. In cold latitudes, where the body is subject to rapid cooling, fatty foods become a necessity, so that the carbon may be easily supplied for combination with oxygen in combustion. Hence the Greenlander consumes large quantities of blubber and oil. (2) To produce force. As muscular force is only produced at the cost of oxidation in the tissues, fat is rapidly burned off during exercise. If fatty food is absent, the tissues themselves would be wasted. This fact, well known to every athlete and mountaineer, brings us to the third use of fat, viz. (3) To prevent the waste of albumen. If albuminous food alone be given, a very large amount must be consumed for the body to obtain the elements necessary for the production of heat and mechanical energy; but if fat be added a much smaller amount of albumen is required. Hence we see that there is no diet so wasteful as a purely albuminous diet. It has been moreover conclusively proved experimentally that a small amount of meat food taken in combination with bread and fat, suffices to maintain the albuminous structures of the body better than an exclusively lean meat diet.

Fat stored in the body as adipose tissue is a bank on which the body may draw for supplies of energy and heat when required. During great muscular exercise fat is rapidly broken up into its component elements and oxidised, and in the case of enforced starvation the body finds in adipose tissue the materials for its own support. It is stated that in the Franco-German War of 1870, the German Emperor, acting on the strongly expressed opinion of Ebstein that muscular fatigue could best be supported on fat, gave orders that each soldier should have served out to him 250 grammes of fat bacon. It is a well-known fact that fat animals bear privation of food better than thin ones. Hybernating animals during their winter sleep live on the fat stored in their tissues, and awake in spring with their bodies almost devoid of fat.

The carbo-hydrates include starch, sugar, and dextrine. Hydro-carbons and carbo-hydrates resemble each other in being entirely free of nitrogen, and in being composed only of the three elements of carbon, hydrogen, and oxygen; but they differ in that in the carbo-hydrates, hydrogen and oxygen always exist in combination with the carbon in such proportions as to form water. The chemical formula of water is H2O; the chemical formula of starch is C6H10O5; from which it will be seen that starch is composed of six atoms of carbon and five molecules of water. The various substances in the group of carbo-hydrates, such as starch, dextrine, and sugar, are easily convertible one into the other by the action of ferments, as will be shown subsequently in describing how in the process of digestion insoluble starch is converted into soluble sugar by the action of ferments in the saliva and pancreatic juice. The principal carbo-hydrates used in the food of man are starch, and the various kinds of sugar. Starch is contained in all farinaceous foods, and in many vegetables, such as peas, beans, and potatoes. Sugar is found in the form of cane-sugar in the sugar-cane and beetroot, as milk-sugar or lactose in milk, and as grape-sugar or glucose in fruits. Considering, therefore, how largely our ordinary food consists of bread, potatoes, milk, and sugar, it will be seen that the carbo-hydrates form very important elements in diet.