The Fuel

It is important that we study more fully the oxidation processes through which the food passes after it is absorbed by the wall of the digestive tract. This final process of food reduction is included in the subject, metabolism.

Food does not arrive at its ultimate destination until it reaches the individual cell. Here the nitrogenous portion is built up into the cell itself; the carbohydrate and fat afford energy for cell activity. Thus new cells are made, old cells are renewed, this process of cell building and repair being that part of metabolism known as anabolism.

In connection with the activity and life processes of the cell which are made possible by the energy resulting from the oxidation or combustion of the food taken up by the cell, waste material, made up of broken down cells and the products of combustion, is produced which is carried by the blood stream to its proper outlet. This process of tearing down and waste formation is that part of metabolism called katabolism.

In connection with these changes of waste and repair, or metabolism, the body may be compared to a stove. The food is the fuel which is as truly burned in the tissues as is gas, wood, or coal burned in a furnace. This slow combustion is a true oxidation process and oxygen is as necessary for the body fires as it is for the more rapid oxidation which goes on in the ordinary stove. The lungs serve both as drafts and flue and a perfect circulation of the oxygen received is made possible by the circulation of the blood. The kidneys are the grates through which the ash is eliminated.

The oxidation of carbohydrate results in the production of heat, ever transformable into energy, the end products being carbonic acid gas (CO2) and water (H2O). These are eliminated through the lungs, skin and kidneys as simple gas and water. There is no ash, just as there is none from the burning of gas in a gas heater. Starches and sugars are simply fuel foods necessary for heat and energy, but have nothing to do with tissue repair.

Tissue Building

Tissue repair is the additional work of protein. Protein contains nitrogen in addition to its carbon, hydrogen, and oxygen. The nitrogen is used in the work of cell building and cell repair. It is the iron of the stove and, while not needed in such large amounts, is of vital importance; for the body stove if not kept in constant repair would soon enter a state of dissolution. As protein contains the three elements, carbon, hydrogen and oxygen, heat and energy may result from the oxidation of this part of the protein molecule, some carbon dioxid and water being given off, but the distinctive work of this food element has to do with its nitrogenous portion.

Building Stones

The protein molecule is very complex and varies within wide limits. Its nitrogen is always combined with carbon, hydrogen and oxygen, but is combined with these in many different ways to form different kinds of proteins. The arrangement of the nitrogenous combinations in the food proteins is different than in that of the tissue proteins, so after the breaking down process that the food undergoes in connection with digestion the nitrogenous links are put together again in the various necessary combinations to form the many kinds of tissue. A complete protein contains seventeen or eighteen of these nitrogenous units which are called amino-acids and have been likened to building stones. (See chapter VI (An Ample Supply Of Protein Is Important), page 60.) Rearrangement of these amino-acids makes possible the formation of the tissues that go to make up the body. (An amino-acid contains the radical N H2.

The Body Grates

As the result of cellular activity and oxidation a definite solid ash is formed in which form the nitrogen is eliminated through the body grates, or kidneys, in solution in the urine. Urea, uric acid, purins, creatin and other allied bodies make up this solid ash. The most completely reduced form is urea which is the most important end product of protein metabolism, and the form in which the greater part of the nitrogen is eliminated. There is normally, however, a certain small part of the ash not so completely metabolized which must be eliminated as uric acid and purins, but these in excess soon become abnormal.

An Expensive Fuel

Any protein not needed for tissue building may be used by the body to produce heat and energy, the nitrogen being split off and eliminated in the usual way. But this would be a waste of nitrogen, and a needless task imposed on the excretory organs. It could not, therefore, be considered economy if it were possible to obtain this energy from the strictly fuel foods that leave no nitrogen for elimination. This use of nitrogen would be analogous to burning iron in a furnace. While it might be possible it would hardly, under ordinary circumstances, be considered wise or economical.

Clinkers

Thus as the result of oxidation and reduction processes, the food, whether carbohydrate, fat, or protein, is reduced to a simple state which makes complete elimination from the body possible. If for any reason oxidation is incomplete the process of elimination is greatly hindered; the ash is not finely divided but is full of clinkers, the grate becomes clogged and waste products are retained which further clog the body stove and the more hinder normal oxidation processes.

For this reason it is important that the body fuel, while sufficient, be not excessive, that the drafts be kept well open, and that there be a free circulation of oxygen. Then the fuel will be thoroughly burned, the ash finely divided and elimination will be complete; clinkers will not clog up the grate, back up into the stove and the stove will not smoke. The vital fires will burn brightly and all organic functions will be carried on in a normal way that makes for health and strength.

"I catch my breath

As children do In woodland swings

When life is new And all the blood

Is tvarm as wine And tingles with

A tang divine".

- James Whitcomb Riley.