Food and You

Dallas E. Boggs, PhD

Chapter VI

The Utilization of Fat

FATS represent the body's main stockpile of fuel. If dietary fat is absorbed in excess of current needs, it is reserved in various depots. Usually, about half of the fat in the body is stored just under the skin and largely confined to the trunk rather than the extremities. The remainder is deposited as a sort of cushion around some of the internal organs, es­pecially in the abdominal cavity. In a well nourished individual, the kidneys are surrounded by fat; and a great deal is found in the omentum, which is a protective apron that hangs in front of the stomach and intestines. The liver is also capable of storing a fair amount of fat.

      Excess dietary carbohydrate also contributes to the fat stores. It has been emphasized before that the body is very frugal with its resources; its capacity to store carbohydrate as such is rather limited; the major way to conserve this foodstuff is to con­vert it to fat. The intestine cannot exercise any control over the amount of food that it receives; whatever is eaten is auto­matically digested and absorbed. Facilities for the storage of extra protein are also rather limited. If an excess of this foodstuff is eaten, the body indulges in some "high living" and simply wastes, as fuel, most of the amino acids that cannot be used for maintenance or repair of body protein. There is, how­ever, some conversion of protein to fat. With the "fuel food­stuffs", however, the matter is quite different; if any fat or carbohydrate is left over after the daily requirements are satisfied, it is not burned or excreted—but carefully laid away as storage fat, in anticipation of hard times. If the hard times never arrive and the person continues to eat more than is required, he soon takes on the classic contour of a barrel.

      The control of the waistline, in the absence of disease, is strictly a matter of free will. Close attention to the demands of hunger and a judicious curb of the appetite (along with a moderate amount of exercise) will suffice to keep the body fat stores within comfortable working limits.

      Overeating is a sort of self‑perpetuating affair: Large meals distend the stomach and hence large meals must be continued in order to "fill the cavity". Excess fat is deposited under the skin, and this forms additional insulation against heat loss. Diminished heat loss results in less demand for fuel; and, hence, more fat is available for storage.

Transport of Fat

      Fat is easily moved from place to place within the body—by way of the blood stream. This is an interesting problem because the fats are generally quite insoluble in water and still they move freely in the watery medium as found in the blood and tissues of the body. The trick is accomplished by introducing phosphorus and choline into the molecule of fat. If this is done, the resulting compound is a phospholipid that is soluble in water. One of the theories regarding fat absorption from the intestine is that the fatty acids resulting from fat digestion are temporarily combined with phospholipids for the trip across the lining of the small intestine. The framework of the phospholipid can make the cir­cuit repeatedly—each time taking on a new load of fatty acid from one side of the cell and unloading it at the other. After it has passed through the intestinal lining, most of the fat is collected by means of many small lymphatic vessels, which finally join together to form the thoracic duct. This system of vessels by‑passes the liver and joins the blood vessel system directly by entering one of the large veins in the neck. At this point the thoracic duct empties its lymph into the venous blood stream; from here it goes directly to the heart to be distributed to all parts of the body. The fat of the blood, however, is not in true solution. It is largely neutral fat, which is very similar to the fat found in foods, and it is carried as a fine emulsion of small fat droplets of microscopic size. Any tissue that re­quires fat must get it from the blood stream. This means that the fat must escape from the blood capillaries and then pass through the cell walls of the tissues concerned. The phospholipids again act as carriers to get neutral fat across the cell wall barrier.

Fat as Body Fuel

      The burning of fat in the body is normally complete, yielding carbon dioxide and water—just as it would if it were burned in a fire. There are many intermediate steps between the complex molecule of fat and the simple end products of com­bustion. The fatty acids are composed of long straight chains of carbon atoms. A few clues indicate that they are broken down by removing fragments containing two carbon atoms from one end. This fact was established by attaching an "unburnable tag" to one end of the fatty acid. The body cannot burn the benzene ring and hence this was selected as the "unburnable tag". If the fatty acid attached to benzene contained an even number of carbon atoms, the end product that appeared in the urine was a benzene ring with two carbon atoms of the fatty acid still attached. If the original fatty acid contained an odd number of carbon atoms, the end product was a benzene ring with only one carbon atom attached. Thus fatty acids containing 4 or 6 carbon atoms always yielded an excretory product containing 2 carbon atoms plus the benzene ring, whereas the fatty acids containing 3 or 5 carbon atoms always appeared in the urine as a compound of benzene with one carbon of the fatty acid.

      These observations indicate that, (I) the body breaks down fatty acids by removing fragments that always contain two carbon atoms; and, (2) the body is unable to break the bond between the fatty acid and the benzene ring. Either one or two carbon atoms were left attached to the benzene ring depending upon whether the fatty acid contained an odd or an even number of carbon atoms respectively. Since all plant and animal fats con­tain only fatty acids with an even number of carbon atoms, it seems that fatty acids are built up, as well as broken down, two carbon atoms at a time.

      Starvation, diabetes or eating a high fat diet overworks the fat burning mechanism of the body to such an extent that the job is not completed. The fatty acids that contain only four carbon atoms accumulate in the blood so rapidly that they cannot be completely burned, and they escape in the urine. The four-­carbon acids that accumulate in the blood under such condi­tions probably originate in the fat stores. Here the fatty acids contain long chains of 16 or 18 carbon atoms. It is easy to see how shorter, even numbered carbon chains can arise by step­wise removal of two‑carbon fragments from the long chains. Even after several two‑carbon fragments have been removed from these long chains, the resulting products are still insoluble and presumably remain within the tissue cells. When the fatty acid is reduced to four carbon atoms, however, it becomes quite soluble in water and is able to diffuse out of the cells into the blood stream to produce a condition known as acidosis. The presence of some carbohydrate prevents the development of acidosis.

      The complete combustion of fat yields approximately 9 cal­ories per gram whether it is accomplished in the body or in the bomb calorimeter. This is not quite true in the body during acidosis (as can occur in diabetic conditions)  because some of the four‑carbon acids are excreted in the urine—and the energy which they contain is lost. Assuming complete combustion in the body as well as in the bomb calorimeter, the end products are also the same in both instances—namely, carbon dioxide and water. Carbon dioxide forms a weak acid in water, but it is easily eliminated through the lungs as one of the gases in the expired air. It is characteristic of the burning of all foodstuffs that the end products are more or less acid. The body is well equipped to cope with this situa­tion except in diseases like diabetes or extreme conditions such as starvation or a very high fat diet.

Other Functions of Fat

      The nerves generally are covered by a fatty material called the myelin sheath, which serves both as a protection for the nerve trunk and as a store of materials needed for its maintenance and repair. It may also act as insulation in much the same way as the covering on an electric wire. At birth an infant's nerves are not all covered by the myelin sheath, and it may be that his nervous system cannot function perfectly until the nerves have acquired complete insulation. For example, the heat regulating mechanism in a baby is not very effective until myelinization is complete.

      It was mentioned previously that many of the internal organs are surrounded by deposits of fat forming a cush­ion that may be very useful against mechanical shocks. The layer of fat that lies just under the skin is a very efficient insu­lation against heat loss. This fact is strikingly illustrated by the great discomfort experienced by fat people in hot weather. If they live in a cold climate, however, they are fortunate because they are able to conserve body heat and are quite comfortable in surroundings that would cause shiver­ing in thin people.

      Extremes of fatness or leanness are undesirable for various reasons, several of which have already been discussed; but the esthetic effect of a moderate amount of fat should not be overlooked. Either the bean pole or the barrel silhouette is objectionable. A little fat to fill in the hollows, round off the corners and form graceful curves does much to improve the appear­ance of the human body.