Chemically, foods are composed of a limited number of elements similar to those found in the animal tissues, viz., carbon, oxygen, nitrogen and hydrogen, together with some salts. If nothing more were needed by the economy than a supply of these elements and salts in a proportion like that in which they exist in the tissues, such could be easily obtained from inorganic sources; but, as has already been stated, it is necessary that an animal obtain these elements associated in the form of organic materials of complex construction (namely, proteids, etc.). Allowing the necessity of organic food, it might be supposed that since the elements exist in proper proportion in the proteids, an abundant supply of proteids would suffice for all nutritive purposes, and alone form an adequate diet. Theoretically, pro-teid alone ought to be sufficient for nutrition. It, however, has been frequently tested by experiment, and practically decided, that an animal will not thrive upon a free supply of pure proteid food alone; and in the human subject such exclusive diet would induce dangerous abnormal conditions in a short time. Since nitrogen is an important element in nearly all parts of the body, we could hardly expect that a diet composed of non-nitrogenous food stuffs alone could support the animal economy. In short, the results of numerous experiments show that no one group of the food stuffs enumerated can alone sustain the body, but rather prove that a certain proportion of each is absolutely necessary for life.

Diagram showing the proportion of the principal food stuffs in a few typical comestibles.

Fig. 42. Diagram showing the proportion of the principal food stuffs in a few typical comestibles. The numbers indicate percentages. Salts and indigestible materials omitted.

Special Forms Of Food

The articles of diet we make use of are animal or vegetable, according to the source from which they are derived. It will be seen that a varying quantity of all chemical classes of food stuffs is present in most kinds of food, whether animal or vegetable. The diagram on the preceding page shows the proportion of the more important food stuffs in some examples of the materials commonly used as food.

Among animal foods are included milk, the flesh of various animals, and the eggs of birds. These may be more fully described as typical examples.