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Home , Essential amino acid, Protein (nutrient)

P80T€IN'S IN

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 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 determinations of that have been obt,ained frola laboratory 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 production is only sufficient to mainkin life.

!here we were once so provincially concerned with defi- ciency diseases, we now appear to be rather singularly concerned about pro- tein and how m might best cope with protein malnutrition in the human obliged to subsist on diets composed of protein-deficient . 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 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, , 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 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 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 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 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 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 = 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. 16, Rome, 1957) indicate that the limiting amino acids generally are lysine, and .

Table I

Amino Acid Patterns of Proteins Calculated as Ratios to Wsine = 1

cow Ihunan Whole Lamb Require- Milk ug Cuts Cuts Cuts menta Ips ine 1.0 1.0 1 .e 1.0 1.0 1.0 1.0 Tryptophan .18 .25 .26 .13 -16 .16 -31 Pheqylanlanine e62 66 .90 .47 50 .48 1,37*

Leucine 1.26 1.3’7 1.37 .94 .96 .90 1.37 .a2 a83 1.04 60 64 .62 88 .59 69 .78 .50 .56 .56 .63 .88 095 1.16 .64 .61 .63 1.00

Methionine 31 .31 *49 .28 .30 .30 1.37w .34 .33 .37 .IO .34 e42 75 .44 .61 1.02 .74 80 . -

Calculated *Amino acid ratios based on daily requirements of young men. from data presented by William C. Rose, Nutr. Abs. Rev, -27, 631, 1957 (5). HDiet devoid of .

=Diet devoid of cystine.

(Basic data taken from Amino Acid Content of Food, Home Economics Res. Report No. 4., by M. L. Orr and B. K. Watt, Household Economics Research Division, A.R.S., December, 1957.) 63

The figures that are presented in this table clearly demonstrate that the amino acid patterns of beef, lamb and pork proteins are remarkably similar. Moreover, they compare very well, with only minor exceptions, with those presented for cow and human milk proteins. The ratios shown for amino acids in whole egQ proteins vary from those shown for the meat pro- teins, but these differences are much smsller than those which would be shown if we had made these comparisons with corn protein, a protein which is not a well-balanced protein.

If we comare these ratios with the amino acid ratios based on the amino acid requirements of young men (Rose, 5), we can see that they conform more closely to the amino acid patterns of egg protein than to any other protein shown in this table. Since there is a wealth of data con- firming the exceptional nutritive quality of whole egg proteins, we are not surprised to find this close corix?lation. Whereas the nutritional value of whole egg proteins and of have been widely and intensively in- vestigated, specific deficiencies of the meat proteins have not been so well evaluated. Any inherent deficiencies in their content can be corrected by increasing the total. intake of meat protein.

In our laboratories Johnson and his associates (6) have fed beef muscle to rate and observed that excellent growth performance can be obtained when it is fed at a level of 14$ protein equivalent. Only about 12$ beef muscle protein is required to susta3.n good growth if a suppleaent of methionine is provided.

1 was ~?3SUCCeS6fulin my attempts to find reports of hunaan nutri- tion experiments whose primary objective was to determine the minimum intake of beef, veal or pork muscle protein required for growth and maintenance in man and to establish whether supplementation of such proteins with one or more of the essential amino acids muld significantly affect efficiency of nitrogen retention. Data that are available for experimental animals (Mitchell, 7) indicate that muscle protein may be slightJy deficient in the amino acids for the rat and chick. The data shown are comparisons with lysine :100. 64 .

Table I1 Relative Proportions Among the Essential Amino Acids in Animal Tissues and in Whole Egg as Compared With Estimated Requirements. Iiysine Is Given a Value of 1.

Estimated Visceral Whole requirements Amino acid fiscle organs egg Rat Chick

usine 1. 1. 1. 1. 1.

Histidine .36 a47 .29 .40 .17 Arginine .88 1.05 a 90 .20 1.33 + tyrosine 1.14 1.66 1.51 .90 l.U

Tryptophan .16 .26 .21 .20 a 20 Methionine + cystine .55 .74 90 .60 .89 Threonine .s7 .82 .68 .50 50 Leu cine .99 1.27 1.28 .80 1.56 Isoleucine .73 .87 1.U .50 67 Valine .74 87 1.01 .70 .84

R. H. Mitchell, Some Species and Age Differences in Amino Acid Require- ments, In Protein and Amino Acid Mtrition, page 26, edited by A. A. Albanese, Academic Press, N. Y. One can draw the obvious conclusion from these data that muscle proteins are excellent sources of essential amino acids for the rat and chick. Flaving satisfied ourselves that the animal proteins are good sources of the essential amino acids, it is now necessary that I point out that it is also possible to formulate good diets containing only plant pro- teins that could rival many anlmal protein-containing diets. And in some circumstances this practice may be the only alternative that is feasible in human nutrition. The many food taboos, poor aanitation practices, the need to maximize the amount of food that can be produced per unit of land and/or labor, and so forth, may dictate in some countries for many years to come that reliance for protein must be placed on the use of plant foods produced directly from the soil (8).

Granting that the formulation of an adequate diet composed exclu- sively of vegetable proteins is possible, it must also be conceded that formulating a balanced diet with respect to all the amino acids to conform to the known requirements of man is not the easiest thing to accomplish. 65

There are two other considerations that are also involved when one must depend on vegetable proteins as sources of essential amino acids, The amino acid composition of plant proteins is quite variable, and we cannot always be certain about the availability of the amino acids in such proteins. In contrast, meat proteins are highly digestible,

I would like to show you agr last slide. I thought it might be of interest to compute the amino acid intake of a human who was served prepared under the direction of' a trained dietitian. For this purpose I computed the amino acid content of food similar to ones actually served to hospital patients at breawast, lunch and dinner (Hunter et al., 9) . This particular diet, one of the five we analyzed, which provided a daily total protein intake of 8G grams of protein and 1900 calories of energy, was primarily designed to provide an adequate intake of and minerals. The slide shows the data for the essential amino acids plus histidine and arginine present in the beef, eggs, milk and potatoes eaten during one day, Foods not inuluded are bread, soup, salads, juice, dessert or breakfast . Table XI1

Amino Acid Content of Foods Consumed Over Period of One Day Beef Eggs Milk Amino Acid (64) (96) (361) (187) Total recorder$o (e; 1 k) (I31 (GI (8 1 (d Igs ine .97 79 98 017 2.91 .40-. 80 Tryptophan 13 0 20 18 03 .54 b15- 25 Phenylalanine *46 c 71 61 0 14 1.92 80-1. 10' 91 1bo8 1.24 3 039 .50-1 .lo Isoleucine .58 *82 .Bl c 14 2.35 65- .70 Threonine 049 e61 9 58 1.81 .30- .SO Valine -62 .91 86 2.56 .40- .80 Cystine + methionine e42 .67 42 07 1.58 .80-1 .lo3 Histidine 38 29 33 bo4 1.04

Arginine 72 81 -46 016 2 S5

'Range of requirements record. Definitely saf'e level 2x highest values recorded. 'Diet devoid of tyrosine. 'Diet devoid of cystine. 66

A comparison of the data for any of the essential amino acids in the proteins present in the beef, eggs, milk and potatoes comprising the day's food intake demonstrates very nicely the contributions that animal proteins make towards meeting the daily requirement of essential amino acids. Please note that the total animal protein is contained in a rather small serving of beef, 2 eggs and less than 3/4 of a pint of milk. It is obvious that the contribution to the total amino acid intake made by the potatoes is small. What we may sometimes overlook is that many foods eaten in large quantities by the human in certain parts of the world are not better sources of amino acids than the potato; i.e., , , yams and plantain are foods whicb also contain less than 2$ protein.

I would like to conclude these general remarks by stating that there is no scientific basis for assuming that there is an absolute require- ment for animal proteins in the diet of man. Nevertheless, meat proteins do have a very inportant place in the human diet. Animal proteins excel as sources of the amino acids because they are readily digested and are well tolerated by the very young as well as the aged. When eaten in amounts which will meet the total nitrogen needs of' the body the intakes of essential amino acids that are necessary for good health will be substantially met. Data currently available suggest that a large proportion of the population in countries with well developed and extensive animal agriculture have diets that are adequate in .

B IBLIOGRAPRY 1. k7illiam C, Rose. Federation Proc, 8, 546-552, 1949; Richard H. McCoy Curtis E. Meyer, and William Cy Rose. J. Biol. Chem., -112, 283-302, 1935-36

2. Rudolph Schoenheimer, S. Ratner and R, Rittenberg. J. Biol. Chem., -130, 703-732, 1939.

3. Wrtha L. Orr and Bernice K. Watt. A.R.S., U. S. Department of Agri- culture, Home Economics Research Report No. 4, Washington, 1957, $2 PP. 4. Food and Agriculture Organization of the United Nations. Protein requirements. FA0 Nutritional Studies No. 16, Rome, 1957. 5. William C. Rose, Nutr. Abst. and Rev. -27, 631-647, 1957.

6. B, Connor Johnson. Personal communication. 7. Harold H. illitchell. Some species and age differences in amino acid re- quirements, in Protein and Amino Acid Nutrition. Ed., Anthony A. Albanese Academic Press, N.--Y. , 1959.

80 mceedings of the Nutrition Society. The comparative merits of animal

and vegetable foods in nutrition. British J. Nutr., I5, 243-274, 1951. 67.

9. G. Hunter, J. Khste1i.c and M. Bell. Can, J. Research, -26, 347-366, 1948.

MR. DOTY: The Chairman indicated that we could run through until12:15, and it is already past that time, but I am going to exercise xiy prerogative and ask if there are some questions or comments that anyone wishes to make from tha floor at this time concerning any of the three discussions which we have had?

On behalf of the committee, I Vish to thank very much the speakers who participated in the discussions. I think that all of us have from these presentations a better picture of the nutritional problems with which all of us ouGht to be familiar in this area of .

Mr. Chairman, I turn the meeting back over to you.

CHAIRMAN KEW: Thank you, Dr. Doty. I have here two brief announcements: If any of you people are interested future employment (1) in within the next nine months or so, please fill out a card at the desk. There are some people looking for others here, so you might be interested; and (2) If anyone brought any visual aids to diaplay, please set them up in the room here to our right (indicating).

We will adjourn now for lunch, and please be back here at 1:15 Sharp.