While there is a dominating substance in all foods, yet they usually contain many compounds which render them, from a chemical standpoint, very difficult to classify accurately. For example, the principal nutrients in wheat are carbohydrates (starch and sugar), yet wheat contains mineral salts, fat, and protein, the latter being a compound consisting of carbon, hydrogen, oxygen, nitrogen, and sulfur. Wheat would, therefore, be placed in the carbohydrate class, but it would overlap into several other classes. What is true of wheat, is true of nearly all other articles of food. Furthermore, foods do not chemically reproduce themselves when taken into the body, but in the process of metabolism they are converted either into other elements or into other compounds. From this it will be understood that the articles listed under the following headings are classified according to the nutritive substance which predominates in them, and are given for the purpose of guiding the practitioner in the selection of such foods as will supply the various chemical constituents of the body.

Foods which contain two or more substances in generous proportions may appear under two or more of the following headings, as in the case of peanuts. This humble article of food contains 19 per cent carbohydrates, 20 per cent protein, and 29 per cent fat, hence it is listed under the three headings - carbohydrates, proteids, and fats.

The tables comprise the best selections of food available in all countries and at all seasons of the year. They contain everything the body needs under the varying conditions of age, climate, and activity, except, perhaps, in some parts of the frigid zone.

In compiling these tables I have selected only such articles of food as experience has proved most useful.

Simple Classification Of Foods Based On Principal Nutritive Substances

Purposes Which The Different Classes Of Food Serve In The Human Body

While all the articles of food in the four above-named classifications contain other elements than the one under which heading they appear, yet the body uses or appropriates them for the following purposes:

Purpose Of Carbohydrates

The carbohydrate substance in food is used by the body chiefly for the purpose of keeping up body-weight; that is, for the purpose of supplying the various fluids which fill the cell-structure. If one is suffering from emaciation, the carbohydrate element in food should predominate. While some of the more soluble proteids, especially milk and eggs, will give a rapid gain in weight, the weight will not be permanent unless sufficient carbohydrates are taken to supply the blood with all the required elements of nutrition, or, in other words, to level or to balance the body requirements.

Purpose Of Fats

Fats are used by the animal body primarily for the purpose of producing heat. Food is burned or oxidized in the blood, undergoing very much the same action as does the combustion of coal in a grate. The heat thus generated is delegated to the blood, and the blood, by its circulation, distributes this heat throughout the body. The carbon di-oxid or waste matter formed during the circulation, is carried to the lungs, where it reunites with the oxygen which we breathe, and thereby again passes back into the atmosphere.

Purpose Of Proteids

Proteid is a compound containing chiefly nitrogen, oxygen, and carbon.

Its purpose is to form the muscular and the tissue structure of the body. To use a homely illustration, proteid may be compared to the material which makes the honeycomb, while the carbohydrate substance may be compared to the honey; that is, to the fluids which fill the cells.

Those performing heavy or active muscular labor should eat liberally of the proteid class of foods.

Under normal conditions, natural hunger will call for the quantity of proteid needed. The tendency, however, should be toward the minimum; that is, one should take the lowest quantity of proteid that the body requires to keep up the cell-structure. (See Lesson VI (Chemistry Of Metabolism), p. 216.) Modern investigations have shown that, in many cases of extreme athletic tests, a low proteid diet has given the greatest endurance. This is accounted for by the fact that nearly all carbohydrates, especially of the grain family, contain from 8 to 12 per cent of proteids, which is quite sufficient, in many instances, to supply the body with all the tissue-building material necessary. Inasmuch as the several nutritive elements found in a single article of food are better proportioned by Nature, than man can usually proportion them, the relation of one substance to another will be better divided if the entire meal be made to consist of only one kind of food, and both digestion and assimilation will therefore be more perfect. Under these conditions the blood will be laden with very little waste matter, which is the thing that reduces our powers of endurance. Therefore, when it is possible to secure the carbohydrate, the proteid, and the fatty substances from a single article of food which will give to the body greater strength and endurance than when we secure these substances from several sources, we should confine our menus to single articles of well-proportioned food. This thought, carried to its logical end, leads one more and more, as experience progresses, toward the mono-diet system.

Purpose Of Mineral Salts

Mineral salts serve two distinct purposes in the body:

1 They assist in building up the cartilage and the body-structure

2 They assist in the digestion, and in the dissolution of other foods, especially of the carbohydrate group, and more especially of the grain family

Grains are very difficult to subdivide into their constituent elements; that is, to reduce to a solution so fine that assimilation will be perfect. A liberal use of the foods containing mineral salts aids very materially in this process of solution.