Much the larger proportion of the dry matter of human foods consists of non-nitrogenous material. This is especially true of the cereal grains. While these nitrogen-free compounds are not regarded by many as fundamentally so important as are the proteins, in quantity they unquestionably occupy the first rank. The activities of plant life are largely devoted to their production, and their use by animal life is correspondingly extensive. They may properly be called the main fuel supply of the animal world. Other nutrients aid in maintaining muscular force and animal heat, to be sure, but these compounds are the principal storehouse of that sun-derived energy which furnishes the motive power exhibited in all animal life. They are also important in other ways, for they fill a necessary office in the formation of milk and, in the fattening of animals.

65. Elementary Composition

The compounds of this class contain only three elements, - carbon, hydrogen, and oxygen. They may be derived, therefore, wholly from air and water, and they constitute that portion of human foods which is drawn from never failing and costless sources of supply.

The elementary composition of typical nitrogen-free bodies is iriven in this connection: -

66. Classification

The non-nitrogenous compounds of foods are usually divided into two main classes, viz. carbohydrates and similar bodies and fats and oils. The first class often bears the name nitrogen-free extract, but the carbohydrates are its principal members. The second is known by the chemist as ether-extract, because ether is used to extract the fats or oils from the vegetable substances in which they are contained. The actual fat obtained from vegetable foods is always less, however, than the ether-extract, because the ether takes into solution other compounds than the fats. It should be noted that the last two compounds of the above table, which are fats, are relatively richer in carbon and hydrogen and poorer in oxygen than the other compounds mentioned, which are carbohydrates. This fact has an important relation to nutritive values.

The Carbohydrates

The carbohydrates as a class make up a large proportion of plant substance and constitute a generous share of human food. While the compounds of this class are not structurally important to the animal organism, they fill a large place in the animal economy in maintaining the vital processes.. They are among the longest known and most familiar substances that are now used as food by the human family.

In order to understand the carbohydrates as individual compounds and in their relations to each other and to the processes of nutrition, it is necessary to consider them, in general outlines at least, from the standpoint of the chemist.

The term carbohydrates, like the term protein, is collective, and includes a great variety of compounds. By their common names we know them as celluloses, starches, sugars, gums, vegetable mucilages, and so on. Chemically we distinguish them by their structure and by their relation to one another.