Popular Science Monthly/Volume 68/May 1906/The Body's Utilization of Fat
|THE BODY'S UTILIZATION OF FAT|
RECENT 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 of which the animal as distinguished from the vegetable body is capable. It was thought that animal chemistry was all of a sort which would produce more fixed and stable compounds and convert compounds of greater potential energy into those of little or none. On the other hand, the synthesis of organic compounds was believed to be confined to the vegetable kingdom. This distinction in the character of the chemical processes in the two forms of living things was believed to be one of their fundamental differences. It is still true that the end products of animal metabolism are simple oxidized substances and that plants are largely engaged in synthetic chemistry, but the difference in this regard is one of degree only. The number of known synthetic processes occurring in the animal body is constantly increasing, and the formation of the complex fat molecule from the comparatively simple and partly-oxidized sugar molecule is an instance of a complex synthesis. To build up this fat molecule a number of sugar molecules must be disintegrated and a portion of each must be taken to be combined with others into the large molecule of neutral fat. Another, but more simple, synthesis, to be referred to later, is the synthesis of the neutral fat molecule from the fatty acid absorbed from the small intestine. In this process three molecules of fatty acid are used to make one molecule of neutral fat.
Our text-books only a few years old tell how fats are absorbed from the intestine by a process entirely different from that by which the sugars and proteids are absorbed.
The latter substances by an hydrolysis and cleavage are made soluble and diffusible and in this dissolved form are absorbed. We were told a different story of the fats—that while a portion of the fat was really digested, i. e., converted into a fatty acid and glycerine and thus absorbed—that the greater part was simply emulsified and that the finely divided particles of fat were then 'swallowed whole' by the intestinal epithelium in some such way, to look for an illustration, as the amoeba takes its food. The evidence for this seemed fairly convincing; in the first place, the fat could be seen in the emulsified state in the intestine in contact with the epithelial cells lining it. And in the substance of these epithelial cells, as though just devoured from the intestinal contents, were seen similar droplets of fat. This view, however, has given place to the view that all the absorbed fat is first converted into fatty acid and absorbed in this form or perhaps partly also as a soap, then reconverted into neutral fat. The older theory was abandoned for the following reasons: No one saw the fat droplets passing into the cell; none were seen in the border of the cell in contact with the intestinal contents, but only at the base of the cell farthest removed from the source of supply of fat. The same appearances in the epithelial cells were noted if a dog was fed with no fat, but with fatty acid instead, suggesting in this case certainly that the fat globules in the epithelial cells were made from absorbed fatty acid. Further, the observation has been made that the fat particles in the epithelial cells are small at the beginning of intestinal digestion, but that they grow larger when the digestion has been in progress some time, as though the droplets were made at the point where they are seen. Again it has been shown that the fat ferment in the small intestine is abundantly able to convert the fat of an ordinary meal completely into fatty acid in the time usually required for a meal's digestion.
However, it seemed difficult to understand how a drop of fat in contact with an epithelial cell was first converted into fatty acid, then absorbed by the cell and, before leaving the confines of the absorbing cell, reconverted into a droplet of fat. But a similar transformation is believed to occur in the case of proteid absorption. Proteids in digestion are converted into peptone and thus absorbed, but no peptone is found in the body, not even in the intestinal blood vessels, hence the peptones must be at once reconverted into other proteids in the act of absorption.
The work of Kastle and Loewenhart makes it clear, in the case of fat, how the reverse processes are brought about. Before referring to their work let us remark that it is well known that the action of ferments is never complete unless the product of the fermentation is removed. To illustrate—if grape juice is fermenting to become wine, the conversion of sugar to alcohol at first may be quite rapid, but by the time a wine of ten per cent, alcohol is formed the alcohol present inhibits the further action of the ferment. If the alcohol could be removed from the wine, the action of the ferment would continue so long as there was sugar present to ferment.
Further, it has been shown that the action of a ferment may be reversible, i. e., that the same ferment which will convert a solution of carbohydrate A into carbohydrate B will also convert a solution of carbohydrate B into a carbohydrate A. But, as just mentioned, the action of no ferment is complete, hence whether we start with a solution of A or of B, the ferment action brings about a solution of A and B in such proportion that ferment action ceases, i. e., the condition has become one of chemical equilibrium. Kastle and Loewenhart worked with the ferment lipase, whose known action was the conversion of neutral fats into fatty acid and glycerine. They added the ferment to a solution of fatty acid and were able to demonstrate the formation therefrom of the neutral fat. The importance of this observation is very great. First, it adds another to the list of animal synthetic processes. Second, it offers an easily comprehended explanation of the absorption of fat. For in case the small intestine contains neutral fat immediately after a meal, the ferment will soon begin to convert it into fatty acid and glycerine. This action, according to ferment law, would continue till a mixture of fat and fatty acid in chemical equilibrium was produced. But in the intestine, absorption begins and fatty acid is removed as fast as formed, thus allowing the ferment to continue its action as long as any fat remains in the intestine. But if both fatty acid and ferment are absorbed together, then, as soon as they get inside the absorbing cell, the ferment in the presence of fatty acid only will begin its work over again, which then will be the formation of droplets of neutral fat from the fatty acid absorbed.
This view rests on the assumption that fat ferment accompanies the fat from intestine to tissue. The observers mentioned have investigated this subject. They examined a large number of fat-containing tissues and organs, and found in every case that they contained fat ferment about in the proportion that they contained fat, except that the liver contained a very active ferment out of proportion to the amount of fat in that organ.
The fat absorbed from the intestine finds its way into the lymphatics and thence to the thoracic duct, there to be mingled with the blood. Shortly after a meal if the blood serum of an animal be taken and allowed to stand a layer of fat forms on top. The serum taken some hours after a meal, on standing forms no such layer, showing that fat rapidly disappears from the blood. And here arises one of the interesting problems of fat metabolism. What becomes of fat when it disappears from the blood and what is the origin of the fat in the tissues? A very simple explanation would be that the fat of the blood is deposited in the tissue cells. Another theory, and one that has had the sanction of good authority, is that the fat in the tissues is made there from their own proteid substance.
In favor of the transformation of proteid into fat are usually mentioned the following: In the ripening of cheese, fat is increased at the expense of proteid. In certain damp soils corpses have their proteid converted into a fatty substance known as adipocere. Both of these arguments are somewhat less convincing when it is known that bacteria are the active agents of these changes. As the result of various poisons—notably phosphorus—the liver is found to contain large quantities of fat in the form of droplets in the injured cells of the organ. This has been called fatty degeneration and the protoplasm of the degenerating cells in one stage of degeneration was thought to be changed to fat. On the other hand, it is claimed that if an animal is first starved, so that fat disappears from the body, and then poisoned with phosphorous, no fat appears in the liver. There is too other evidence of an experimental nature to show that the fat of fatty degeneration is fat transported from the usual depots of fat and simply deposited in the degenerating cells. The fat of different animals consists of different proportions of the three common fats, olein, palmitin and stearin. But for each animal the proportion in which the three fats enter into its fatty mixture is fairly constant. Recall the difference between beef and mutton tallow and lard. This proportion of fats being fairly constant for an animal species, it does not change with every change of diet. But if an animal be starved for a time and then fed exclusively on a particular fat, such as some vegetable fat never normally found in the animal, the fat used can be demonstrated as present unaltered in the tissues. The question then of the origin of tissue fat is still somewhat uncertain. It seems safe to say that the fat of the food can be deposited unaltered in the tissues, but that all the fat found in the tissues has had its origin in fatty food is certainly not the case; much of it is made from carbohydrate and some of it may be made from proteid.
Turning to the question of the disposal of fat in the body, we may say that it is completely burned in the tissues and has as its end products carbonic acid and water.
A moderate amount of fat in many tissues, especially the subcutaneous connective tissues, the omentum and tissue about the kidney, is normal, and serves as a store of energy, as a protective covering to the body and to retain the body's heat. But there are many persons in whom this amount is excessive, that is, in no way proportioned to their needs. In seeking an explanation of these cases we are at once struck with individual differences. For instance, one sees persons over whose bodies there is a uniform thick layer of fat; they are of florid complexion and many of them active persons. The term corpulent applies to them better than obese, since their bodies exhibit both an increase of fat and of protoplasm and their blood is of normal specific gravity. In the presence of a good digestion and abnormal appetite, they daily consume more food energy than the daily expense of energy requires. The excess is laid away as tissue proteid and fat. A moderate diminution in food taken with some increase in exercise would rectify the condition. But it is almost a waste of words to tell a man to eat less in the presence of an excellent appetite and digestion.
There are other persons in whom the picture is quite different. The fat is not uniformly distributed, but is largely abdominal, the arms and legs being little enlarged; they are pale, and blood examination reveals anemia and diminished specific gravity of blood. Herter calls attention to the rather striking parallel between this condition and diabetes. In diabetes there is an excess of sugar in the body tissues. This has been shown not to be due to any increase in sugar manufacture, but to an inability of the body to oxidize the sugar. In diabetes the excess of sugar then is removed by the kidneys. In obesity the fat is not removed, but accumulates in adipose tissue. The explanation of both conditions would be an inability of the body cells to oxidize these substances. Herter states the points of similarity of the conditions thus:
1. Diminished oxidation of both materials.
2. The two conditions are frequently associated.
3. In each case the defective utilization of nutritive materials is more or less closely associated with substances produced by certain glands. It has been clearly proved that disease of, or removal of, certain groups of cells imbedded in the pancreas, known as the 'islands of Langerhans,' cause the appearance of sugar in the urine. And no doubt remains that many of the cases of diabetes are primarily disease of the pancreas. On the other hand, obesity may depend on the suspension of function of other glands. The removal of the sexual glands is commonly, though by no means invariably, followed by the laying on of fat. This fact is taken advantage of by the poultryman in supplying capon for the market. Members of the human species may be similarly affected.
But the gland showing the most intimate relation between its function and obesity is the thyroid. This gland is situated in the neck, on either side of the trachea. Its enlargement constitutes goitre. Its absence or disease result in cretinism and myxœdema. When portions of the dried gland of a sheep are mixed with the food of the normal or obese individual marked bodily changes result. The results of thyroid feeding in men and animals have been well summarized by Richardson. In the first place, there is a marked increase of oxygen consumption and carbonic acid excretion, that is, some constituent of the gland promotes oxidation. This is accompanied by a loss of weight.The oxidation is not, however, selective of the fats alone, for most observations have shown that there is an increase in the elimination of nitrogen. This would mean that proteid is being consumed as well as fat. If an animal was in nitrogenous equilibrium at the beginning of the experiment, we should expect it to be considerably weakened by its consumption of its own protoplasm. If, however, the nitrogenous food is increased, while thyroid substance is being fed to the animal the condition of nitrogenous equilibrium can again be restored, and the subsequent loss of weight will be due chiefly to the oxidation of fat. The above statement makes it clear that the question of the amount of fat in any individual is a very complex one, depending on such a variety of factors as condition of digestion, appetite, character and quantity of food, amount of exercise and the proper working of a number of body glands.
- Chittenden has investigated our proteid needs very carefully and would probably agree that these figures fairly well represent what the average man does consume, but he finds that such a quantity of proteid is much beyond actual needs. He found men able to do work of all kinds, both mental and physical, and retain good health on one third to one half this quantity of proteid.
- They employed butyric acid and synthetized ethyl butyrate.
- The question of the origin of fat from proteids is ably discussed by Henry A. Christian, M.D., 'Some Newer Aspects of the Pathology of Fat and Fatty Degeneration,' Bull. of Johns Hopkins Hosp., January, 1905.
- 'The Thyroid and Parathyroid Glands,' Hubart Richardson, 1905.