Food. The term "food" is usually applied to the various substances taken into the body through the mouth, which serve to counterbalance the waste of the tissues. In a general survey of the substances which enter the body, air would necessarily be included, and the oxygen of the air has every claim to be considered a food. In the present article, however, the term "food" will be limited to the solid and liquid substances which enter the body through the alimentary canal. On the supposition that the weight of the body remains constant, it is only necessary to know accurately the amount of the several chemical elements contained in the excreta, to estimate the quantities of these elements which must be taken into the body to replenish waste. It must, of course, be borne in mind that any given element may be eliminated in various ways, and in striking a balance between the substances taken in and the-substances eliminated it must not be forgotten to include in the former both what enters by the lungs and by the stomach, and in the latter not only what is discharged from the alimentary canal, in the urine, and from the skin, but also what is exhaled in the expired air. Again, the food is the great source from which the body derives its energy, and the chemical composition of substances excreted differs markedly from that of substances ingested; in fact, in correspondence with the energy imparted by the food to the body, it is found that the excreta are -composed of substances representing but little work producing capacity, while the ingesta are made up of substances possessed of considerable potential energy. The various food stuffs may be divided into two great classes: - Organic and inorganic, the former comprising (1) nitrogenous foods (proteids and their allies), and (2) non-nitrogenous foods (sugars, starches, and fats). The inorganic foods consist of mineral salts and water. (For the composition of proteids, fats, sugars, and starches, see Digestion.) To come now to the principal forms in which the various food stuffs occur. Lean meat contains from 15 to 20 per cent., milk upwards of 4 per cent., white of egg upwards of 20 per cent., peas, beans, etc., upwards of 25 per cent., wheat flour upwards of 10 per cent., and cheese from about 30 to 40 per cent. of nitrogenous substances. Fats occur in the fat of meat, in butter, lard, suet, and the various oils. Starches enter largely into the composition of most vegetables, especially potatoes, and constitute nearly half the weight of bread. Sugars are widely distributed, being especially prominent constituents of fruit and' vegetables, and forming 5 per cent. by weight of milk. Mineral salts enter more or less largely into the constitution of all foods (nearly 2 per cent. in flour, nearly 1 per cent. in milk, 4 or 5 per cent. in flesh). Green vegetables and fruit supply certain salts not readily obtained from other articles of food, and the health of the body seems to suffer if they are entirely excluded from the diet list. A reference to the chemical constitution of the chief food stuffs will show that the elements which enter most largely into their composition are carbon, oxygen, and nitrogen. Oxygen is obtained, of course, in large quantities from the air. It may be well to consider the relative proportions in which the two remaining elements should exist in foods. In order to counterbalance the waste occurring in an ordinary healthy adult, it is necessary to supply daily about 4,500 grains of carbon and nearly 300 grains of nitrogen. Now in proteid foods there are from three to four times as much carbon as nitrogen, while starches, sugars, and fats contain no nitrogen at all. The necessity for a mixed diet thus becomes apparent; for if an individual were restricted to proteid food, he would clearly have to consume a large excess of nitrogen if he is to obtain the necessary amount of carbon, while, on the other hand, it is manifest that no amount of starches, sugars, or fats would supply him with the required amount of nitrogen. The proportions of the various food stuffs which exist in milk are such, as to exactly meet the requirements of the infant in respect of carbon and nitrogen, and for this reason milk is sometimes spoken of as a "perfect food." Much has been written concerning the several parts played in the animal economy by the various food stuffs. It is clear that fats are in the main heat producers; hence the large share they occupy in the diet of the inhabitants of cold countries. The starches and sugars seem to be in part converted in the body into fat. The changes which the proteids undergo are very complex; they appear to be essential to tissue formation. The mineral salts must-enter into the composition of the diet in a certain proportion for the maintenance of health.
The amounts of the several food stuffs, which should be comprised in the diet of a healthy adult taking moderate exercise, are: - Nitrogenous substances, about 5 oz.; fats, about 3 oz.; starches, about 15 oz.; salts, a little more than 1 oz. These figures represent weights of water-free food stuffs; in practice these food stuffs are met with combined with 50 or 60 per cent. of water, for which allowance may be made. In addition to the water which is contained in ordinary solid food, some three pints of liquid will be required per diem.