The reader may wander from the heading of this chapter what a cleansing agent and a source for explosives have to do with the foods we eat. We counsel patience.

Do you perhaps recall that story about the British scientists during the early days of the war when they were set to the task of utilizing all waste products? One of these products that received their attention was the discards from the soldiers' table; these discards contained fair quantities of fat. Whereupon our scientists separated the fat from the rest of the "rubbish," mixed the fat with a suitable alkali such as lye, boiled the mixture and eventually obtained soap and glycerin.

The process just outlined, whereby some kind of fat and some lye are heated, is the principle employed in the preparation of soap and glycerin on an industrial scale. But the juices in the digestive tract, which can change proteins into amino-acids and the more complex carbohydrates into a simpler variety, can also change fats into glycerin and soap. Not only can they do so, but they do do so. Our fats are never absorbed as such, just as our proteins and our more complex carbohydrates are never absorbed as such. The fats are only absorbed after they have first been simplified by being changed into soap, glycerin and substances very closely allied, chemically, to soaps - fatty acids.

Glycerin is a very specific substance. When we say glycerin we have in mind one definite substance - just as definite as water. But the words "fatty acids" and "soaps" are of vaguer import; they include a class of substances, just as carbohydrates and proteins do. Among the fatty acids there are but three that interest us here, for they are the three that are physiologically important: stearic, palmitic and oleic acids. One or more of these three are formed (in addition to glycerin) whenever the digestive juices act on the fat of our food. By an extremely simple process - the action of alkali - some of these fatty acids are converted into soaps. The fats, therefore, before being absorbed are simplified into fatty acid, soap and glycerin. These three substances are absorbed, and for some mysterious reason are immediately reunited to form fat, which is then stored in the adipose tissues until the body has need of this source of energy.

Which among the structural units in fats is the most essential to maintenance and growth? Is glycerin more important than palmitic acid?

To form fat in the body both glycerin and fatty acid are necessary. The question that remains to be answered is, which of the three common fatty acids is the most important? This needs further study.

Are Fats Indispensable? In the meantime, another question presents itself. Are fats themselves absolutely indispensable in the diet?

We have pointed out in previous chapters that fats function as the fuel reserve of the body; first the carbohydrates are utilized, and then, when these are all exhausted, the fats are put to work. If we eat more carbohydrate than is needed for immediate use, the extra supply is converted into fat and stored as such.

But now would anything happen if we were to exclude fat from the diet? This experiment has never been carried out very successfully, for the reason that the means available to eliminate all fat particles from the diet remove substances other than fat. It is very much like trying to extract sugar from a mixture of sugar, salt and water. If water is the only suitable solvent you can employ, shaking the mixture with water will cause not only the sugar to go into solution, but also the salt.

Just how necessary the true fats are in the diet yet remains to be determined;* and until this is done we need trouble little about structural units in the fat molecule.

In the meantime, the attempt to extract fat from food by treating the food with a suitable solvent has led to unexpected results (see next chapter).**

* Drs. Osborne and Mendel have begun investigations in this direction.

** Closely associated with fat are a group of substances called lipoids that are found in every living cell, and whose function is still largely a mystery.