"Carbohydrates" are the fuel of the body. They are converted at once into heat and energy, or if there is a surplus they are often stored in the body in the form of fat to be used when nourishment is less abundant.
"Fat" is also fuel, a more concentrated form of fuel than the carbohydrates. A certain quantity is stored in the body as a reserve heat supply.
The word "Ash" in food tables stands for the mineral matters which are used in our bodies for building bones and teeth, and for a few other purposes; these minerals are for the most part building materials, but are not so important as protein and are needed in smaller quantities.
Human bodies are constituted to withstand adversities and to bear the experiments and mistakes which we make; therefore it is that though these food substances usually serve the purposes attributed to them above, yet when need arises the body is able, for a time at least, to use one for the other. This is a provision, however, for special and adverse occasions. Ordinarily food should be supplied in the variety and proportion which will enable the body to use each class of nourishment for its own purpose.
Roughly estimated, an average person's diet should be about one-fifth protein, one-fifth fat and three-fifths carbohydrates. That the carbohydrates exceed the others in quantity is easily accounted for. They are not such concentrated fuel as fat, therefore a greater quantity is needed; they are consumed to make heat instead of being built into the body as protein is; therefore, we need more carbohydrates, just as we need to renew the coal supply in a house more frequently than to renew the carpets.
The foods from which we derive protein are chiefly meat, fish, milk, beans, peas, bread and other articles made of wheat, corn, oats, and like grains.
Vegetables, with the exception of beans and peas, furnish chiefly carbohydrates.
Fats are derived for the most part from the animal food which we eat. Butter, for instance, is chiefly fat, and the proportion of fat in bacon is more than half.
But because nearly every kind of food contains other constituents besides the one which is chief, the housekeeper who wishes to make wise menus will need more and more detailed statements of food values as she is able to get and understand them. If she has hitherto thought little about such matters, she will probably not know that the United States Government has very kindly employed people to make years of experiments and to write books and pamphlets for her help, nor will she know that she may have these last merely by asking the Department of Agriculture for them. They are not made into attractive booklets, but they are by no means dull reading. Farmers' Bulletin No. 142, for instance, called "Principles of Nutrition and Nutritive Value of Food," and written by Dr. W. O. Atwater, is brief, helpful and most interesting. The figures in the table given below were taken from this Bulletin. But there are things which may be derived from this and the many other food pamphlets issued by the Government which are quite as important as definite statistics. They are things which give the housewife a feeling of comradeship with many people who are working earnestly with and for her; things which increase her interest in her own small part of the work and which give her a helpful sense of its dignity.