MEAT P80T€IN'S IN HUMAN NUTRITION J. KASTELIC Before I can begin to discuss the many questions which are implied in the title of this discussion I must beg your forbearance. I must clearly specify that I am appearing before you as a student of protein nutrition and not as an authority. This is not meant to be an apology. It is a statement which is not only pmdeut but entire1.y appropriate. And I must clear a bit of underb-msli amy before I give the su'oject before us specific consideration, ;^or I can be certain none of you came here only to listen to me reiterate the resalts of nitrogen balance studies published in the literature or to look at relmoductions of tables of data and of graphs which describe the results of the many biological value determinations of proteins that have been obt,ained frola laboratory animal experimentation with which so many of you are now familiar. It would be equally superfluous to call attention to the tremendous volume of literature that is now being published about the underlying eco- nomic, social, technological and medical problems which are confronting man in so many parts of the world today where food production is only sufficient to mainkin life. !here we were once so provincially concerned with vitamin defi- ciency diseases, we now appear to be rather singularly concerned about pro- tein malnutrition and how m might best cope with protein malnutrition in the human obliged to subsist on diets composed of protein-deficient foods. If there is still some controversy about the relative merits of proteins from plant and animal sources in 'numan nutrition it must stem from vexing economic considerations, meat animal production capabilities and problems associated with processing and storage; not fmm a lack of an appreciation of the nutritional role and functions of the amino acids which are essential to life. Meat proteins along with the proteins present in dairy products and eggs stand alone as superior sources of the amino acids required for the maintenance of health in man. Among a large number of facts about the unique functions served by the amino acids derived from dietary proteins subsequent to their diges- tion is that they are all directly related in one way or another to anabolic functions; i.e., (a) to the formation of new cellular constituents, (b) to the renewal or replacement of cellular constituents that have undergone catabolism. 60. However impressive these facts may be, it must be conceded that the best evidence forthe superior nutritive quality of animal proteins is derived in a mch more general way. We have long appreciated the nutri- tional deficiencies of human diets cornposed solely of plant proteins and the beneficial complemehtary effects animal proteins exert when they are added to all-plant protein diets. Clinical findings fn infants, in children and in the aged subsisting on predominately calorie-rich vegetable diets that are low in protein and perhaps also deficient in one or more of the essential amino acids, being accumulated as a result of nutritional studies being done on man living in underdeveloped countries, provide unequivocal evidence that inadequate pro- tein intakes lead to disasterous consequences to the health of man. The ill effects of anemia, low plasma protein, edema, the develop- ment of fatty livers and cirrhosis and the evidence of muscular wasting that follow the chronic consumption of calorie-rich, protein-deficient diets are very well known. The treatment for these nutritional disorders, in children especially, is remarkably simple. It is to correct a deficiency of animal protein. One may do it with an appropriate combination of vegetable pro- teins, of course. A proper dietary supply of essential amino acids, as we have just mentioned, is a fundamental consideration because of the unique functions that the essential amino acids serve in the growth, development and mainte- nance of the protoplasmic mass of the body as 8 whole. These cardinal metabolic processes obtain throughout life, but they are quantitatively variable depending upon the physiological state of the animal, For example, in so-called adult animals the formstion of new cellular elements is con- fined largely to hempoesis, growth of hair and skin, but in very young animals the fomation of new muscle and osseous tissues is the dominant process The dietary need for proteins, or more specifically for the amino acids, is in turn determined by the ability of the animal tissue systems to utilize them directly or to utilize metabolic products derived from them for the de novo synthesis of other nitrogen-containing compounds. The delineation of the amino acids which cannot be synthesized in the body and those which can be synthesized, provided there is an adequate source of dietary nitrogen available to the organism, was accomplished only about 20 years ago by Rose and his associates (1). The result of these studies has clearly established that animals (the ruminants excepted) cannot thrive unless they receive dietary sources of specific amino acids and sufficient amounts of dietary nitrogen to satisfy the non-essential nitrogen needs of the body tissues. The indispensability of certain amino acids for the various species show general similarities, but it seem apparent that differences in absolute requirements do exist. This phenomenon reflects the changes in requirement that vary with changing physiological needs of an animal as it grows and develops . The requirement for an adequate dietary source of proteins from which the needed amino acids and non-essential nitrogen are derived, from a biochemical point of view, is often related to the need for maintaining a 61. satisfactory status of the so-called "nitrogen-pool" of the body. This pool can be simply regarded as a compartment of non-protein nitrogen, containing a mixture of nitrogenous compounds derived from the dietary protein or from the catabolism of tissue constituents. These substances in aggregate firnish the substrabs for the synthesis of new cellular constituents or for their maintenance or replacement. The ebb and flow of nitrogenous elements from one body nitrogen com- partment to another or, to put it in another way, into and out of cellular constituents present in the total cellular fabrlc of the living organism is subject to a wide variety of influencing factors. Wny interesting facts about the dynamic nature of the factors governing the nitrogen interchanges in the body have been established since the pioneering investigations of Schoenheimer (2) were published a number of years ago. It has been well established that the half-life of many cellular proteins may be as short as a few hours, whereas for some, notably collagen, it may be so long as 1000 days or more. ]Enzymes such as xanthine oxidase, succinic dehydrosenase and serum aldolase appear to be degraded and resynthesized at such a rapid rate that the removal of protein from the diet of an animal will result in a diminution of the activity of these enzymes in the tissues of the animal. long before any other symptoms of protein depletion can be demonstrated. These are the biochemical phenomena that must be considered when- ever we discuss the nutritional value of a protein or whenever we attempt to compare different proteins as sources of essential amino acids. Hence it follows that the ultimate problem in protein nutrition is to determine how one may achieve optimum support of growth, and maintenance of the integrity of the cellular constituents of man throughout his life-span through proper choices of the dietary sources of the amino acids. mantitative dei'initions of the adequacy of a pro-tein to provide the amino acid requirements of animals are invariably based on data derived from nitrogen balance experiments. One fundamental conclusion can be drawn from such studies. The nutritive quality of a protein is fixed by its essential amino acid content and the availability of the amino acids in the protein for absorption after the protein is subjected to the digestive pmcemes. As a result of such studies, whole egg proteins have been found to be an almost perfect source of amino acids for the growing rat. Whole egg proteins contain proportions of the essential amino acids that are similar if not identical with those which obtain for the requirements of these amino acids. Since we now have available a rather large body of quantitative data on the distribution of the amino acids that are present in vegetable and animal proteins, it is possible to appreciate rather well why proteins differ in their ability to sustain good health. While the evaluation of the nutritional adequacy of a protein based only on its amino acid content is not entirely satisfactory, some usefil comparisons can be made by examination of the amino acid patterns of a number of high quality proteins. In the following table (Table I) are shown the patterns of 10 amino acids in 6 animal proteins computed from data presented in the U.S.D.A. publication (3) as ratiorj to lysine = 1. These figures reveal some interesting differences and similarities. It is to be 62. emphasized that these ratios will vary depending upon tFe values one selects for the amino acid content expressed in terms of the amino acid content per gram of total nitrogen in the edible portion of food. The choice of lysine as the base line may not be the best one could make, for the requirement of lysine for maintenance is less critical. than for some of the other essential amino acids. On the other hand, comparisons of dietary proteins with the egg reference acid patterns or the devised by the FA0 codttee(4) amino one Protein Require- (Food and Agriculture Organization of the United Nations : ments, FA0 Nutritional Studies No.