Page:Popular Science Monthly Volume 68.djvu/429

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ECENT physiology has considerably advanced our knowledge of fatty metabolism. Some of the recent work has had an important bearing on metabolism in general, as well as on the special metabolism of fats. This article aims to outline the results of some of the more important experimental work on the subject.

Fat is the form in which the body lays up its greatest supply of potential energy. Plants also store energy, but they do it chiefly in the form of sugar and starch or, to give these substances a single name, carbohydrate. As the animal kingdom is parasitic directly or indirectly upon the vegetable, it results that the animal's food is largely composed of carbohydrate. Thus Voit's figures, expressing the needs of an adult man, are 118 grams of proteid, 50 grams of fat and 500 grams of carbohydrate food in twenty-four hours.

But there are good reasons why carbohydrate, which is our most abundant and cheapest food, would not be an economical store of energy for the animal body. Chief among these is the fact that animals are for the most part motile and hence the advantage of having their store of energy-producing compounds in small compass and of light weight. Fat fulfils the indications admirably, since its atoms, carbon and hydrogen, are light, since a given weight is capable of combining with a large amount of oxygen and since it can be completely oxidized in the body, i. e., the body is able to utilize all its potential energy.

Not all our fat comes from the fat of our food, but is made in the body from other substances. There are two theories—one that it is made from carbohydrate, the other that it is made from proteid.

An animal can be fattened without giving it any fat in its food, in fact, the usual method of fattening animals is by increasing their carbohydrate food. Though there is some question of the origin of fat from proteid, there can be little or none as to the transformation of carbohydrate into fat. This knowledge that fat can be so made upsets one of the notions largely held till recently as to the kind of chemistry