Food and You

Dallas E. Boggs, PhD

CHAPTER V

The Utilization of Protein

PROTEINS are an essential part of the structure of every cell in the body. When an embryo is formed and begins to grow, it requires a very large amount of protein in propor­tion to its size; and, as it approaches maturity, this need does not stop—but it is considerably reduced. In the adult, the con­stant "wear and tear" of all the tissues results in losses of pro­tein and other nutrients that can only be replaced by the tak­ing of food.

      The first method developed for the study of protein utiliza­tion is based on the fact that most proteins contain I6% of ni­trogen. Since none of the other foodstuffs contribute a signifi­cant amount of this element, it is possible, therefore, to express the protein metabolism simply in terms of nitrogen. The method of nitrogen balance involves chemical analysis of the food that is eaten in a certain period as well as the urine and feces that are excreted in the same time interval. A comparison of the amount of nitrogen eaten with the amount excreted will in­dicate the state of protein metabolism in the body.

       If the intake of nitrogen is greater than the output, the body is retaining nitrogen, and this state of affairs is most striking in a rapidly growing individual. The normal baby retains nitrogen and is therefore said to be in positive nitrogen balance. This means, of course, that he is building new protein into his expanding body.

      If, on the other hand, the output of nitrogen in the excreta exceeds the intake in the food, the individual is in negative nitro­gen balance—he is not getting enough protein in his diet to keep his body in repair. This happens in starvation or undernutrition and in certain diseases accompanied by fever.

      However, the method of nitrogen balance only measures the total turnover of protein in the body and does not furnish any information about the dozens of individual reactions that may concern only certain amino acids.

The Protein Minimum

      The starting point in the study of protein utilization is a determination of the minimum amount of protein used daily by the body, and this quantity may vary widely depending upon conditions of the experiment. Of course, a man subjected to complete starvation had to have a zero nitrogen intake; his nitrogen excretion diminished rapidly from about 15 grams to approximately 6 grams per day at the end of a month of starvation. Since protein is I6% nitrogen, the figures given for nitrogen excretion in starvation may be multiplied by 6.25 to give the number of grams of protein that the body uses from its own stores. Thus, he used 93.75 grams of protein (I5 x 6.25) on the first day of the fast and 37.50 grams (6 x 6.25) on the last day. It should be kept in mind that the experimental subject took nothing but water during this time. This means that the his calorie as well as his protein requirement was supplied from the body stores.

      If the necessary calories are supplied as food, the protein minimum is much lower. In man, the nitrogen excretion has been reduced to 2 grams daily when the diet contained an abundance of carbohydrate calories but no protein. Thus, the daily destruction of body protein in this type of experiment was re­duced to 12.50 grams (2 x 6.25). This phenomenon is referred to as the protein‑sparing effect of carbohydrate. (Fat also has this effect—but not to such a marked degree.) It is clear then that the supply of calories has a very great effect upon the extent of protein breakdown in the body—and, hence, upon the protein minimum.

      It seems reasonable that the feeding of protein in amounts equivalent to the protein minimum should result in nitrogen balance; but this is not true. As mentioned above, it has been demonstrated that a man on a diet that contains no protein, but enough carbohydrate to supply his calories, will still break down 12.50 grams of his body protein. If this amount of protein is added to his diet, his nitro­gen excretion will be increased to such an extent that he will still be in negative nitrogen balance. If the food proteins are furnished by bread and other cereal products, it will require about 35 grams of protein to bring a man into nitrogen bal­ance. This is approximately three times his protein minimum. It would be somewhat less than this if eggs, meat and other protein of high biological value were consumed. When the National Research Council assembled its table of daily allowances, it was considered that 35 grams of protein was the smallest amount that would maintain nitrogen balance and that, in order to be safe, the daily diet should contain twice this amount. In areas of the world where protein sources are lim­ited, it is necessary to operate with little or no margin of safety. The agriculturists are de­veloping and raising plants that yield a maximum of high quality protein.

      Some comment is in order concerning the relative merits of proteins from different sources. It is a fact, well established by scientific methods, that the animal proteins—from meat, milk and eggs—are better utilized than the proteins from cereals and vegetables. It is also a fact that all proteins, regardless of origin, are constructed of the same kind of build­ing blocks—namely, the amino acids. The only difference is in the proportions of the twenty amino acids that are built into the protein molecule.

     The body cannot use proteins as such—before utilization, they must be digested until they again arrive at the amino acid stage. It is evident, therefore, that the source of the amino acids is immaterial so long as the body's needs are met. If a person prefers to eat a vegetarian diet, there is no reason why he cannot meet his amino acid requirements, provided that he eats a generous quantity and a considerable variety of vegetable proteins. The person who prefers animal proteins as a source of amino acids can satisfy his needs with less protein, and his meals will taste better; but they will be more expensive.

The Synthesis and Repair of Body Proteins

      Hemoglobin, the red pigment of blood, is the body protein that has been most thoroughly investigated. Hemoglobin con­stitutes I2% to 15% of whole blood and is found only within the red blood cells. If a sample of blood is allowed to stand, the red cells settle to the bottom and can be separated from the fluid portion (plasma). This makes it rela­tively easy to obtain samples of hemoglobin for study—it is only necessary to insert a hypodermic needle into a blood vessel and remove the amount required.

      As the result of many experiments, it is known that the av­erage life of a red blood cell is approximately 120 days. At the end of this time, the cell falls apart and its hemoglobin es­capes. For various reasons, the hemoglobin also tends to disin­tegrate once its protective shell has been destroyed. The body is able to salvage some of the pieces—but there is always some loss, and this must be replaced from the protein taken in as food. New hemoglobin and red cells are produced in the marrow of the flat bones, especially the ribs and breast bone. The conditions necessary for its normal operation are well established.

      Every tissue has a sort of matrix or mold for building its own special type of protein. The amino acids that fit the pattern are used, and all others are rejected. There is no compromise in this matter; and the bone marrow, like all other tissues, is very meticulous about the exact reproduction of the original hemoglobin molecule. The foods that are most efficient in supplying the raw materials for regeneration of hemoglobin are liver, other meats, and hemoglobin itself.

      The story of hemoglobin and how food affects it is one of the fine examples of long, hard, carefully controlled experimental work in the laboratory that leads ultimately to an outstanding contribution to human health and happiness. In these studies the dog served as the experimental subject. This was a wise choice because the anatomy and physiology of this animal re­semble quite closely those of the human. During the experi­mental period, blood is removed from a vein daily in sufficient amount to make the animal anemic. (As anyone that has donated his blood to the Red Cross knows, this is a nearly painless opera­tion.) Anemia produced by loss of blood immediately stimu­lates the bone marrow to compensate for the loss. The rate at which new hemoglobin can be formed is dependent upon the supply of raw materials available from the food. Many foods were com­pared for their efficiency to form new hemoglobin in the anemic dog. By means of such experiments, the superiority of liver as a source for the rapid regeneration of hemoglobin was firmly es­tablished. The same dietary treatment was tried in human anemias—with instant and complete success.

      A similar technique was used in a study of the other proteins that are in solution in the plasma or fluid portion of the blood . The blood sample is removed and given a spin in a centrifuge to hasten the settling out of the red cells, and the plasma is discarded. The red cells are then suspended in a weak salt solution and injected into the vein of the animal from which the blood was taken. (Such an animal retains his normal amount of hemoglobin and red cells but is deprived of some of his plasma proteins.) The body responds in the usual way—it gets busy at once to make good the loss. It is evident that the liver takes the most active part in the fabrication of fibrinogen, the protein that forms the framework of a blood clot. This is the body's first line of defense against bleeding to death. In this work the superiority of animal proteins, and especially the plasma proteins themselves, was proved by direct experimentation.

      The examples given above illustrate the fact that the body responds to damage or loss of its protein stores by immediate mobilization of its repair services and that the rapidity and efficiency of the repair is dependent on the quality as well as the quantity of food.

Effect of Starvation on Protein Metabolism

      When no food is eaten, the body is compelled to live on its own stores, but the amount of amino acids in the blood remains constant for at least as long as a month. Judged by the excretion of nitrogen in starvation, protein breakdown continues—but at a considerably reduced rate. In this situation the body rearranges its scheme of operation so that the tissues least essential for the continuance of life supply most of the amino acids necessary to maintain the vital processes. Thus—in starvation—the heart, lungs and brain tend to retain their protein at the expense of the muscles and glands. The muscles, representing the largest mass of protein in the body, are well able to contribute, in time of need, to the support of the vital organs. The large glands such as the liver and pancreas have less work to do when no food is being eaten; and they are, therefore, able to give up some of their protein for the same purpose. If complete or even par­tial starvation is continues for too long, the body depletes its reserves to the point of death.

The Fuel Value of Protein

      Protein burned in an oxygen calorimeter will yield 5.7 cal­ories; but, in the body, it supplies only 4.0 calories per gram. The difference is due to the fact that nitrogen cannot be burned. The amino acids that are not used for maintenance and repair are eventually taken up by the liver for conversion to fuel. In this process the nitrogen is removed as ammonia, combined with carbon dioxide, and finally excreted by the kidney as urea. The fuel value of urea is, therefore, lost to the body. The re­mainder of the amino acid molecule may be burned as such or converted to either carbohydrate or fat and burned at some later time. The mechanism for removing nitrogen requires energy, and this accounts for the fact that a heavy meal of meat results in increased heat produc­tion in the body.

      The nitrogen removed from an amino acid may not be lost immediately; it may be attached to an organic acid that originated from fat or carbohydrate to give rise to a new amino acid. It may be recalled that there are ten essential amino acids, and they must be supplied pre‑formed in the food; but the re­mainder of the twenty amino acids found in most proteins can be synthesized from other materials that are always available. The body is generally very frugal with its resources. The coupling of nitrogen that could have been wasted with an organic acid that might have been burned—to make a use­ful amino acid—is a good example of thrift in the vital economy of biological systems.

How Much Protein Should be Consumed?

      In the chapter on foodstuff requirements, the daily allowance of protein was given as 70 grams for the average man. There are some writers who have advocated a low protein diet and others who favor the other extreme. The chief objection to a low protein diet is that it is necessary to make a very careful selection of foods to insure an adequate supply of the essential amino acids. Most people are unable or unwilling to do this, and a widespread adoption of such diets would inevitably lead to protein deficiency in some members of the population. The most obvious objection to an extremely high protein diet is its expense. Furthermore, it is a waste of the world's resources to eat more of any foodstuff than is required for good health. We also know that excessively high protein diets can cause excessive loss of calcium in the urine.

      It is possible, however, that present notions about the pro­tein requirement may have to be modified. An interesting study of two African native tribes indicates that a large intake of ani­mal protein is highly beneficial to health. The two tribes that were the subjects of this study live in adjacent areas in central Africa; but one is strictly vegetarian and the other subsists largely on a diet very rich in proteins of animal origin—meat, milk and fresh blood. The meat eaters were four to six inches taller, and had fewer diseases, much greater endurance and muscular strength and much less dental decay than their vege­tarian neighbors. These differences were evidently not due to heredity because there was intermarriage, and the tribe mem­bers acquired by marriage adopted the new mode of life. Many unknown and uncontrolled condi­tions exist in a situation such as this, and it is well to be cautious in drawing conclusions; but the differences are truly amazing and are worthy of more study.  


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