DIET. Man and animals generally require that their food should be of such a nature as to compensate for the wear and tear of the tissues which is perpetually going on, and as at the same time to keep up the animal heat at its proper standard. Various classifi cations of the food of man have been at different times proposed, but those which have been most generally accepted are that of Dr. Prout—in which the different kinds of food are grouped in definite chemical classes—and that of Liebig, which has reference solely to the ultimate destination of the food in the animal economy.
Dr. Prout classifies all kinds of food under these heads: 1. The aqueous; 2. The sac charine; 3. The oily or oleaginous; and 4. The albuminous; to complete which, we ought to add 5. The gelatinous, and 6. The Liebig makes only two classes: 1. Those consisting of uitrogenized matters, which are adapted for the formation of blood, and which he terms the plastic elements (f nutrition; and 2. The non-nitrogenized sub stances, which from their large amount of carbon, serve (as fuel) to keep up the animal heat, and which he names the elements of respiration. Recent investigations, however, throw doubt on Liebig's view, that before food can be made available for the perform ance of work, it must first be turned into muscular tissue and then Oxidized. Now it seems most probable that it is the oxidation of the non-nitrogenous substances, and not muscle, that contributes chiefly to the production of muscular force. Starch, fat, and the other non-nitrogenous substances would therefore have to be regarded as force producers, and not, as formerly, mere heat-givers. We shall therefore adopt Front's classification. It was based on the consideration, that the milk (the only single article of natural food that serves to support the animal body) is made up of substances which may be taken as representatives of his groups; for this, our earliest natural diet, con tains water, sugar (representing his saccharine group), butter (representing his oleagin ous group), caseine (a nitrogenous matter very similar to albumen, and representing his albuminous group), and salts; and recent researches have shown that the yolk of the egg, which serves for the nourishment of the chick or other young animal before birth, similarly contains one or more representatives of the aqueous, saccharine, oleaginous, albuminous, and saline groups.
We shall now briefly consider these individual groups, and the uses to which the substances included in them are applied in the animal economy.
1. The aqueous group includes water and all the fluids which we use as drinks; and we must additionally bear in mind that all the varieties of animal and vegetable food which we term solids, in reality contain it, generally in great abundance; thus, for example, uncooked beef contains from 70 to 80 per cent, and some vegetables even a larger proportion of water. The uses of water are sufficiently obvious from the abun dance in which it occurs in all the most important fluids of the body, as the blood, and the various digestive fluids. See DIGESTION, ORGANS AND PROCESS OF.
2. The saccharine group includes the different varieties of sugar, starch, gum, and cellulose, together with vinegar. This group is chemically characterized by all its members being included in the formula that is to say, they consist of carbon, together with hydrogen and oxygen in the proportions in which these elements form water. Hence (excepting vinegar) these substances have received the name of carbo hydrates.
Of the sugars, grape-sugar or glyeosc is the most important, partly from its frequent occurrence in ordinary articles of food, such as fruits of most kinds, honey, etc., and partly because it is the form of sugar into which, starch (a most abundant ingredient in most kinds of vegetable food) is converted by the saliva, and pancreatic and intes tinal juices, before it is fitted for absorption or any further changes. Since the sugars (which may thus be regarded as including starch) do not, in the normal condition, pass into the excretions, but are oxidized in the blood into carbonic acid and water, as ultimate products, they must contribute materially to the support of the animal heat. But they have other uses which we shall endeavor to explain, and which will, we trust, be intelligible if this article is read in conjunction with that on DIGESTION, to which we have already referred. Before becoming oxidized into their final products, they yndergo various phases of less perfect oxidation, in which lactic, acetic, butyric, and ..dier acids are evolved, of which the most important and abundant is lactic acid, which is found in considerable quantity in the small intestine, where it is doubtless of service in contributing to dissolve any nitrogenous matters which have escaped the action of the gastric-juice. Another use of these acids which are developed from the sugars is, that by acidifying the albuminous intestinal contents, they greatly increase their diffusibility through the intestinal membranes into the lacteals, and probably the capillaries. Here we probably have the clew to the therapeutic use of acids in various disorders of the chylopoietic viscera. Under certain conditions, the sugars are also converted into fat in the body.* The remaining carho-hydrates are of little or no value as food. There has been much difference of opinion as to whether gum can be taken up and applied to any definite uses in the organism; but the great mass of the most trustworthy observations seem to show that it passes through the system unchanged. Independently of experiments on animals by and others, Dr. Hammond (an American chemist and physician) found from experiments upon himself (1) that gum is altogether incapa ble of assimilation, and therefore possesses no calorifacient or nutritive power what ever, but is, on the contrary, a source of irritation to the digestive organs; and (2) that in consequence of the above fact, the solids of the urine during a purely gum-and-water diet are entirely derived from the waste of the tissues of the body, while time carbon exhaled (as carbonic acid) from the lungs is derived from the consumption of the fat.