As a matter of fact, each class of foodstuff has a very well-defined function. The carbohydrates are primarily energy-formers. Our muscular energy is mainly derived from the carbohydrates we eat.

It is in the course of such muscular activity that heat is produced.

These carbohydrates, and to a certain extent the fats, have still another important function. We say they "protect" the proteins. This "protection" consists in allowing the protein to attend to its particular business of tissue up-building and repair, without having to engage to any large extent in the side line of supplying energy. Within certain limits, the protein can take the place of carbohydrate as a source of energy, but the carbohydrate cannot take the place of protein in building up tissue. We wish then to hamper as little as possible a task which the protein alone can do.

Some physiologists are now of the opinion that the proteins' all-important function of tissue upbuilding is only possible in the presence of carbohydrate, and the constant production of sugar (a typical carbohydrate) in diabetes, even though no sugar is supplied to the body, is pointed to in support of the theory. This suggests another extremely important function of carbohydrates.

What has been said of carbohydrates is very largely true of fats, with this difference: that on the whole the carbohydrates are more readily "oxidized" in the body; and - and this is the important point - the fats act as a reserve supply for the fuel needs of the body. The humps of camels and the fattening of hibernating animals before winter illustrate this property of fats. Your fat is not used until your carbohydrate supply becomes exhausted; and if you eat more carbohydrate than the immediate needs of the body require, a considerable portion of the excess will be converted into fat and stored as such. That explains why fat people avoid not only foods rich in fat but also those rich in carbohydrates.

The proteins are by far the most important of the three classes of foods, and reasons have already been given for this view. Let us give still another. When we analyze living matter - which, unfortunately, can be done only after all life has left the living matter - we find that the elements always present are carbon, oxygen, hydrogen and nitrogen. There are also other elements in much smaller proportions, but they need not concern us just yet. When we analyze the food we eat we find that the carbohydrates and fats consist of carbon, hydrogen and oxygen only. To be sure, the proportions of carbon, hydrogen and oxygen vary with the different fats and carbohydrates; but in so far as the elements out of which the fats and carbohydrates are formed are concerned, they are always found to be carbon, hydrogen and oxygen. It is only when we analyze proteins that we discover the other element so essential to life, nitrogen. Hence the indispensability of proteins.

You might say, but is not there plenty of free nitrogen in the air, and do we not therefore absorb nitrogen every time we breathe? We do take in nitrogen every time we breathe, but we do not assimilate it; we cannot assimilate nitrogen in the uncombined state. If you analyze a sample of air before you inhale it and then analyze it after you have exhaled it, you will find that the percentage of nitrogen remains the same; only the percentage of oxygen has changed.

Now just as the soil's need for nitrogen cannot be obtained from the nitrogen of the air, but from one of its compounds, such as Chili saltpeter, so the body's need for nitrogen is likewise unobtainable from free nitrogen, but must be supplied by some nitrogen compound, preferably protein.

The carbohydrates and fats are the main source of energy of the body. The carbon, hydrogen and oxygen in these substances are changed to carbon dioxide and water, which in their turn consist of carbon, hydrogen and oxygen; for remember in all chemical changes - in all changes - we never create and never destroy, but merely change. If we take sugar into our system (and what is true of sugar is true of fats and carbohydrates in general) the ultimate change that it undergoes may be represented by this equation:

C12H22O11 + 12 02 ->12 C02 + 11 H20

(sugar) (oxygen) (carb. diox.) (water)

(C, carbon; H, hydrogen; O, oxygen)

Without attempting to go into the chemistry of such a reaction, without in fact requiring any knowledge of chemistry, you can easily see for yourself that not only are the same elements present on both sides of the equation, but the same amounts of these elements. They are in different combinations, however.

But this equation, like every other chemical equation, fails to show everything about such a reaction. For example, it fails to tell us that a very considerable amount of heat, measured in calories, is evolved when the sugar and the oxygen combine; and this heat is the all-important factor in so far as the body is concerned.

But the equation fails to answer another question, of particular consequence just now to the student and philosopher. Extensive researches have shown that fats and carbohydrates are not immediately broken down into carbon dioxide and water; there are a number of intermediate stages. One or two of these intermediate steps have been located, but much of what goes on in the body factory still remains a mystery.

Since the proteins also contain carbon, hydrogen and oxygen, besides nitrogen, the first three elements in the protein molecule can be oxidized to carbon dioxide and water in much the same way as the fats and carbohydrates, with the consequent liberation of energy in the form of heat. But proteins, as we have already discussed, serve another purpose; and besides, they are more expensive foods than the fats and carbohydrates. And even if, theoretically, there is no reason why a person cannot live on protein alone, provided he takes enough of it, and does not mind the extra expense, experience teaches us that to live on protein alone is not advisable. The tax on the body in having to handle such large quantities of protein is such that, in time, the vitality of the organism is appro.-ciably diminished.