included in the estimated percentage of protein, of fat, or of carbohydrate, whichever they most resemble. Such an analysis will include with the proteins any other substances containing nitrogen which may be present in the food; with the fats will be included any other substances which are dissolved from the dried food by ether, such as the resinous or waxy material which serves to waterproof the surfaces of many fruits and vegetables, and many of the natural coloring matters which occur in small quantities in such foods as green vegetables and tomatoes; and with the carbohydrates will be included the fruit acids and any other undetermined organic substances which are not counted with the proteins or the fats. Such a partial analysis of a food can easily be criticized; yet it often serves very important purposes. Thus with a knowledge of the digestibility of the food it enables us to compute its value as a source of energy in nutrition, this energy value or fuel value being usually expressed in terms of Calories per gram or of Calories per pound of the food. Many foods also furnish us one or more of a group of very important organic substances now commonly called the vitamins. These are present in such small quantities as not to be shown by chemical analysis, but at least three of them (generally known as vitamins A, B, and C) are absolutely essential to normal human nutrition and are therefore very important factors in food values. By means of feeding experiments with laboratory animals the relative values of different foods as sources of a particular vitamin can be compared, and we shall take account of such comparisons of vitamin values in our consideration of the different types of food products in subsequent chapters of this book. Laboratory feeding experiments often serve also to demonstrate other differences in food value such as the differing efficiencies of certain proteins in nutrition; and they show us more conclusively than we can expect to find in any other way how different articles and combinations of food compare in all-round adequacy for the support of nutrition. This is usually tested by means of experiments which include a comparison of the effects of the foods upon the growth of young animals, as in the case illustrated in the accompanying photograph (Fig. 1). The weight curves of the two rats photographed in Fig. 1, and of three others of the same litter which were differently fed, are shown in Fig. 2. The results of such feeding experiments, most often presented in the form of weight charts, have played FIG. 1. Contrasting effects of equally simplified food supplies. These two rats were twin sisters and at weaning time were, of equal size and equally healthy and vigorous. One was then fed with bread and apple, the other with bread and milk. The latter had grown to five times the weight of the former by the time this photograph was taken. (By permission of the Journal of Biological Chemistry.) a large part in the development of the newer knowledge of food values. Figures 1 and 2 show the high food value of a diet of bread and milk as compared with bread alone, bread and meat, or bread and apple. The bread consumed by all the rats was identical; the meat fed No. 45 and the apple fed No. 53 were of as good quality as was the milk fed No. 54. Each of the foods was pure" in the usual sense and good of its kind. Rat No. 53, although retarded in growth and as can be judged from Fig. 1 somewhat enfeebled by malnutrition, cannot be considered as having been injured by the apple of her diet, for at the 66 The time the photograph was taken she had already outlived her litter mates who received bread alone or bread and meat. apple had supplemented the bread to the extent of prolonging Rats on Bread Alone or With One Other Food FIG. 2. Growth curves of rats of the same litter placed at weaning time upon diets consisting of bread alone or with one other food. Rat No. 43 fed bread alone did not grow and lived only six weeks. Rat No. 45 fed bread and meat grew for a time but then failed and lived only a little longer than the one on bread alone. Rat No. 53 fed bread and apple did not grow but lived much longer. Rat No. 54 fed bread and milk grew at a fully normal rate. Nos. 53 and 54, photographed together after two months on their respective diets, are the two rats shown in Fig. 1. (By permission of the Journal of Biological Chemistry.) life but not of supporting growth. The difference in size, development, and vigor of the twin sisters shown in Fig. 1 is attributable entirely to the superior food value of the milk. In the case of the stunted rat here shown, as in most cases of human malnutrition, the fault of the diet was partly in its vitamin content and partly in other respects. Both the older and the newer knowledge of food values must be taken into account if the varied supply of food products which the modern market affords is to be used to full advantage. We must know the composition of our foods in terms of the familiar chemical elements and compounds, their fuel values in terms of Calories, and also their values as sources of the vitamins. The Composition of Food Materials If we consider the composition of food materials first in terms of the chemical elements and then in terms of compounds, we find that the plant and animal tissues which we use as food are composed mainly of the same twelve chemical elements which chiefly compose the tissues of the body; namely, carbon, hydrogen, oxygen, nitrogen, sulphur, phosphorus, chlorine, sodium, potassium, calcium, magnesium, and iron. Iodine, and probably fluorine, silicon, and manganese, are also essential to the body and so must be supplied by the food (or by food and drinking water together); but the amounts of these latter elements are so small that they are usually scarcely measurable by the ordinary methods of food analysis. While the ash of foods is composed of relatively simple inorganic (mineral) compounds such as the chlorides, phosphates, sulphates, and carbonates of sodium, potassium, calcium, magnesium, and iron, it does not follow that these elements exist in the form of the same inorganic compounds in the food. In many cases the inorganic compounds found in the ash are to a large extent formed during the burning of the food, the baseforming elements having existed in combination with organic acids or with proteins, while the acid radicles may also have existed in organic combination or may have been formed by the oxidation of the sulphur, phosphorus, or carbon of the organic matter. The principal chemical elements of foods and the most im portant kinds of compounds in which they are found may therefore be summarized as follows: Chlorine Sodium forming Ash Constituents which exist partly as mineral salts and partly in combination with carbohydrates, fats, proteins, and other organic compounds. The ultimate composition of a food is its composition as expressed in terms of the chemical elements into which it might ultimately be resolved, carbon, hydrogen, oxygen, nitrogen, sulphur, etc. The proximate composition is the composition in terms of the compounds actually present-proteins, fats, carbohydrates, mineral salts, water. These five groups of compounds have sometimes been called the "proximate principles" of food, or the "five food principles." As a precaution against ambiguity |