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Part I. Techniques for protein quality evaluation: background and discussione

1. Human protein and amino acid requirements and their relevance to protein quality evaluation

Protein Requirements
Essential Amino Acid Requirements
References

In this chapter, a brief statement is made concerning the determination of protein and amino acid needs and the estimated requirements for these nutrients in order to provide a basis for establishing the protein quality of food sources and diets. The protein needs of humans have been studied for more than a century, and knowledge of human amino acid requirements dates from the classical studies of Rose and co-workers at the University of Illinois in young men and Leverton and co-workers in young women (1, 2). The amount and proportion of essential amino acids needed at any given age determine the utilization of dietary proteins for maintenance of body protein in adults and for growth in children.

Estimates of the amounts of protein and amino acids needed by humans have been published periodically by the Food and Agriculture Organization (FAO) and the World Health Organization (WHO), and more recently by the National Research Council (NRC) ( 1). The 1973 report Energy and Protein Requirements by an FAO/ WHO Ad Hoc Expert Committee (2) provides a review of knowledge in this field. This report has been extended by a joint FAO/WHO informal gathering of experts in 1975 (3) and by the 1977 first joint FAD/WHO expert consultation on energy intake and protein requirements (4), as well as by the United Nations University publication Protein-Energy Requirements under Conditions Prevailing in Developing Countries (5); so only brief reference to some aspects of human protein and amino acid requirements need be given.

Protein Requirements

The requirement for dietary protein in adults has been based both on studies of the amounts of nitrogen lost by subjects receiving very low protein or protein-free diets and on estimates of the amounts of protein from different sources needed to bring adults into nitrogen equilibrium. The first of these two procedures, the "factorial method," estimates the losses of nitrogen (N) from the body by way of the urine, faeces, skin, plus other minor routes in healthy adults following short-term adaptation to essentially proteinfree diets. These summated losses, the obligatory nitrogen loss, represents the minimum need of the body for dietary protein nitrogen. Extensive studies in the United States and elsewhere are in close agreement about the amounts of nitrogen lost under these experimental conditions, with urinary nitrogen output averaging 37 mg/kg body weight/day, faecal nitrogen about 12 mg/kg, and cutaneous losses about 3 mg N/kg {2). When a series of minor routes of nitrogen excretion amounting to 2 mg N/kg are added to these estimates, a total obligatory nitrogen output of 54 mg/kg for an adult is obtained. If the protein of the diet is utilized with complete efficiency, an amount of dietary protein equivalent to the output of 54 mg N/kg, namely 0.34 9 protein/kg body weight should compensate for this obligatory loss and bring the subject into equilibrium.

This thesis has been tested by giving whole egg protein or egg albumin to human adults in quantities sufficient to bring the subjects into nitrogen equilibrium (6-9). These studies demonstrate that, even with a good quality source of nitrogen such as egg or milk, considerably more dietary protein is needed to achieve body nitrogen equilibrium than the estimated 54 mg N/kg/day. For subjects given experimental diets in which the protein is provided by milk, eggs, casein, or mixed diets containing appreciable amounts of animal protein, the amount of dietary protein needed for nitrogen equilibrium averages about 70 mg N/kg daily - that is, 0.44 9 protein/kg (2).

Taking all information then available into account, the 1971 FAD/WHO Expert Committee (2) selected a value of 30 per cent to correct for the difference in efficiency of utilization of protein at low-protein intakes and at requirement levels. More recent data suggest a somewhat higher figure (10).

When a 30 per cent addition was made to the factorial loss figures to allow for this effect, plus another 30 per cent to allow for variation in requirements among apparently similar individuals, the estimate of an intake of high-quality protein sufficient to cover almost all of the population was, for the adult male, 0.57 g/kg/day. Calculations for the adult female, but not including menstrual losses, gave a similar figure of 0.52 g/kg/day. Thus, separate recommendations were not made for men and women.

These allowances for high-quality protein were based on relatively short experimental periods. A systematic attempt to evaluate whether this intake level of egg protein would be adequate for long-term maintenance in healthy young adult males, given adequate dietary energy, revealed changes in body protein status after two to three months ( 11, 12). These included a loss of lean body mass, as judged by changes in 40K and 24-hour urinary creatinine excretion, a fall in serum albumin and haemoglobin, and an increase in serum aminotransferase activity in some individuals. These changes were not observed with the feeding of beef protein at a level of 0.8 g/kg of body weight for three months, but a lower level than this has not been critically tested for its possible adequacy. Thus, there still is a need to define more precisely minimal allowances that are adequate for longterm maintenance.

It was pointed out in the 1973 FAD/WHO report (2):

Infections affect protein requirements by inducing some degree of depletion of body N during acute episodes . . . the quantitative effects of infections on the protein needs of an individual cannot by stated, since they are likely to vary with the frequency, severity, and nature of the infection and other host factors, including nutritional status.

Conditions prevailing in developing countries may also increase requirements because of reduced absorption of protein as well as the periodic increases in requirements for recovery from infections.

The UNU working group report Protein-Energy Requirements under Conditions Prevailing in Deve/oping Countries (5) also emphasized the quantitative effects of infections, and the variations in protein and energy digestibility among populations consuming various types of diets.

For the growing child, the criterion of requirement is satisfactory growth. Indeed, where long-term feeding studies can be conducted with a diet providing protein at the level of the estimated requirement together with adequate energy and all essential nutrients, the demonstration of normal growth and maintenance of normal indices of protein nutritional status is a convincing validation of the estimated requirement. For infants, these requirements are approximately 2.4 g/kg up to 3 months of age, 1.85 g/kg at 3-6 months, 1.6 g/kg at 6-9 months, and 1.4 g/kg at 9-11 months. Some investigators, for example Fomon (13), would, however, set these requirements at a lower level, ranging from 1.8 g/kg at less than 4 months to 1.4 g/kg/day between 4 and 12 months of age. During later childhood, the estimates of requirements are less securely based because they are derived using interpolations of average daily growth rates.

These protein levels assume daily growth at a rate of 1/365 that of annual growth However, it has recently been emphasized (5) that the variations in normal growth rate are such that the diet should allow for normal growth rates three to four times higher than this figure for some periods, and that for recovery from diarrhoea! disease and acute infectious diseases of childhood, this figure may be as high as eight or nine times greater in ambulatory village children. For children recovering from severe protein-calorie malnutrition under hospital conditions, growth rate can be up to 18 times greater than that for healthy, well-nourished children.

It would, of course, be desirable to eliminate the causes of the high frequency of infection among children in developing countries, but until this can be accomplished, it must be recognized that their normal growth and development will require the consumption of significantly more protein than is assumed to be a safe allowance for healthy children. How much more will depend on the circumstances of the individual child and cannot be generalized.

Thus, although there is still uncertainty surrounding estimates of human protein requirements and the definition of safe practical allowances for various population groups, current evidence is sufficient to confirm that the major sources of food protein differ in their capacity to meet protein needs.

Essential Amino Acid Requirements

Using nitrogen balance and growth as the criteria, and more recently with the aid of studies of blood amino acid levels, the requirements of children and adults for essential amino acids have been estimated. Such estimates show wide variations among

TABLE 1 Protein and Amino Acid Requirements of Human Subjects

Data taken from FAO/WHO (2). The estimates of requirements are calculated for 0.98 of the population at each age and thus express the upper limit of the range of requirements found in a healthy population Histidine is also an essential amino acid for the infant and is required at a low level by the adult. It was omitted from the pattern because it is limiting only under extreme experimental conditions. individuals and between laboratories, but on careful analysis of the published data it is possible to arrive at values that are reasonably concordant and certainly relevant to utilization of dietary proteins of varying amino acid composition (1, 2). Estimates of needs for the essential amino acids are given in table 1. Similar values for amino acid requirements have been proposed for different age groups by Williams et al. (14). In table 2, the pattern of essential amino acids required is shown, based on the estimates summarized in table 1. In addition the FAO/WHO (2) scoring pattern is given, and the use of such a pattern is discussed in chapter 3.

It is important to emphasize, however, that current estimates of amino acid requirements, including those based on plasma amino acid levels, may reflect the capacity of the organism to adapt maximally to low levels of amino acid intake and, thus, may underestimate the minimum need for long-term protein nutritional maintenance. This point may be more important in relation to adult protein nutrition because the estimates of essential amino acid requirements of children appear to be in good agreement with intakes provided by goodquality proteins when consumed at intakes that are sufficient to support the requirements for growth (15).

The available data demonstrate that the requirements for essential amino acids fall with increasing age (table 1), as do requirements for total dietary protein However, the available data suggest that essential amino acid requirements decline more rapidly than do total protein needs. The proportion of protein needs in infants represented by the content and pattern of the eight essential amino acids in a dietary protein source will be much more significant for the infant and growing child than for theadult.

TABLE 2. Patterns of Amino Acid Requirements and Amino Acid Scoring Patterns for Evaluation of Proteins (mg/g N)

Data adapted from FAO/WHO (2).

Similar conditions apply, of course, to the problems of protein quality and formulation of rations in the feeding of commercially important simple-stomached animals and birds.

In any event, it is these current estimates of protein and amino acid requirements that provide the rationale for the determination and assessment of the significance of dietary protein quality in human nutrition.

References

1. Committee on Amino Acids, Food and Nutrition Board, National Research Council, Improvement of Protein Nutriture, ed. A.E. Harper and D.M. Hegsted (National Academy of Sciences, Washington, D.C., 1974).

2. Joint FAD/WHO Ad Hoc Expert Committee, Energy and Protein Requirements, WHO Technical Report Series, no. 522; FAO Nutrition Meetings Report Series, no. 52 (WHO, Geneva; FAO, Rome, 1973).

3. Joint FAD/WHO Informal Gathering of Experts, "Energy and Protein Requirements," Food and Nutrition, 1 ( 2): 1 1 ( 1975) .

4. FAD/WHO, "Report of the First Joint FAD/WHO Expert Consultation on Energy Intake and Protein Requirements" (FAO, Rome, 1978).

5. F. Viteri, R. Whitehead, and V. Young, eds., Protein-Energy Requirements under Conditions Prevailing in Developing Countries: Current Knowledge and Research Needs (United Nations University, Tokyo, 1979).

6. H.N. Munro, "Amino Acid Requirements and Metabolism and Their Relevance to Parenteral Nutrition," in A.W. Wilkinson, ea., Parenteral Nutrition (Churchill-Livingston, London, 1972), pp. 34 67.

7. D.H. Calloway and S. Margen, "Variations in Endogenous Nitrogen Excretion and Dietary Nitrogen Utilization as Determinants of Human Requirements," J. Nutr., 101: 205-216 (1971).

8. V.R. Young, Y.S.M. Taylor, W.M. Rand, and N.S. Scrimshaw, "Protein Requirements of Man: Efficiency of Egg Protein Utilization at Maintenance and Submaintenance Levels in Young Men,"J.Nutr., 103: 1164-1174 (1973).

9. K. Kishi, S. Miyatani, and G.Inoue, "Requirement and Utilization of Egg Protein by Japanese Young Men with Marginal Energy Intake," J. Nutr., 108: 658-669 (1978).

10. C. Garza, N.S. Scrimshaw, and V.R. Young, "Human Protein Requirements: The Effect of Variations in Energy Intake within the Maintenance Range," Am. J. C/in. Nutr., 29: 280-287 ( 1 976).

11. C. Garza, N.S. Scrimshaw, and V.R. Young, "Human Protein Requirements: Evaluation of the 1973 FAD/WHO Safe Level of Protein Intake for Young Men at High Energy Intakes," 8rit. J. Nutr., 37: 403420 (1977).

12. C. Garza, N.S. Scrimshaw, and V.R. Young, "Human Protein Requirements: Interrelationships between Energy Intake and Nitrogen Balance in Young Men Consuming the 1973 FAD/WHO Safe Level of Egg Protein, with Added Non-essential Amino Acids," J. Nutr., 108: 90-96 (1978).

13. S.J. Fomon,lnfant Nutrition, 2nd ed. (W.B. Saunders, Philadelphia, Penn., USA, 1974).

14. H.H. Williams, A.E. Harper, D.M. Hegsted, G. Arroyeve, and L.E.J. Holt, "Nitrogen and Amino Acid Requirements," in Committee on Amino Acids, Food and Nutrition Board, National Research Council, Improvement of Protein Nutriture (National Academy of Sciences, Washington, D.C., 1974), pp. 23-63.

15. A.E. Harper, "Amino Acid Requirements - General," in H.L. Greene, M.A. Holliday, and H.N. Munro, eds., Clinical Nutrition Update: Amino Acids [American Medical Association, Chicago, 111., USA, 1977), pp. 58-65.

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