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Human protein requirements: A brief update

Nevin S. Scrimshaw

Abstract

In the 1950s and 1960s the risk of protein deficiency in the diets of most developing country populations, both in absolute terms and relative to calories, was considered to be high. An FAO/WHO expert committee that met in 1971 proposed a protein allowance value 20% lower than that previously recommended. Nutritionists and economists accepted this new value, recalculated the risk from dietary intake data, and concluded that protein deficiency was not a problem. This conclusion was reinforced by the concurrent virtual disappearance of the severe form of protein deficiency, kwashiorkor, as developing country conditions improved. However, metabolic studies coordinated by UNU in 16 countries soon showed that the protein required for sustained normal functioning was one-third more than that proposed in the report of the 1971 committee. This higher value was accepted in the 1985 FAO/WHO/UNU expert consultation report on energy and protein requirements. In addition, infections, highly prevalent under conditions of poverty, increase protein losses and often decrease protein absorption. Although it is true that when people can consume enough of their traditional diet to meet energy needs, protein needs are also usually met, problems arise when, because of poverty or illness, people cannot consume their traditional diets. New data on protein digestibility and on protein quality are recognized in a 1991 FAO/WHO expert consultation and a 1994 International Dietary Energy Consultative Croup (IDECG) workshop. Protein requirements are re-evaluated in this paper on the basis of these considerations.

The first internationally recommended allowance for the protein needs of human adults as proposed by the League of Nations in 1936 was not less than 1 g of protein per kilogram of body weight per day [1]. As table 1 illustrates, subsequent estimates made by a series of international expert groups have been lower [2]. The 1973 FAO/WHO Expert Committee was the first with access to sufficient data to permit a factorial calculation of protein requirements, and it arrived at a figure 20% lower than that previously recommended [3].

TABLE 1. Successive recommendations by international groups for protein intake to ensure nitrogen balance in adult man

Sources: ref. 2.

Almost immediately following publication of the 1973 FAO/WHO report [3], three successive long-term nitrogen balance studies at the Massachusetts Institute of Technology (MIT) [4-6] showed that this level of intake resulted in negative nitrogen balance, loss of lean body mass, and deteriorating values for serum protein and serum transferases. These were alarming findings for a recommendation intended to be sufficient for nearly all the world's adult population. However, it was not until an FAO/WHO/UNU expert consultation could be convened in Rome in 1981 that the error could be corrected and a new estimate of 0.75 g/kg/day could be published for a good-quality source of dietary protein that is highly digestible [7]. Ironically, when differences in digestibility of the protein in various diets are taken into account, the most recent recommendations are close to those of the League of Nations in 1936.

In theory, protein requirements must be those needed to replace the nitrogen lost in the urine and faeces and the integument (skin, hair, nails), as well as in semen and menstrual fluid. To obtain the data for the factorial approach, these losses are measured on individuals who are adapted to a protein-free diet. As shown in figure 1 (see FIG. 1. Daily urinary nitrogen excretion of 83 young men given a minimal protein diet for 14 days. Values for the first 3 days are based on 19 subjects and subsequent values are based on 83 subjects [8]), urinary nitrogen drops rapidly under these conditions and reaches a low plateau in a few days [8]. It was recognized subsequently that the 1973 FAO/WHO estimate had been based on too low an allowance for integumental losses and a gross overestimate of the efficiency of utilization of nitrogen intake at requirement levels.

For these reasons a better approach was needed to establish protein requirements from nitrogen balance measurements, and data also were required from different populations and their usual diets throughout the world. In 1978 UNU convened a workshop in Costa Rica that recommended the multilevel, short-term nitrogen balance approach [9]. Measurements were made over five days and the order of intake levels was randomized. Each balance period was preceded by an adjustment period on the same diet. The result in a single study was a series of zero nitrogen balance intercepts from the mean, from which confidence limits could be calculated (see FIG. 2. Four methods of estimating protein allowance from nitrogen balance data. PRm = mean protein requirement; PRsd = standard deviation of protein requirement; PR. 975 = 97.5% protein allowance) [10].

The studies were carried out on a total of 132 young men in 10 countries, with the results shown in table 2. The average protein requirement was 0.72 g/kg/day, with a pooled coefficient of variation of 16.8% [11]. This result indicated a safe practical allowance for protein of 0.96 g/kg/day.

TABLE 2. Nitrogen requirements of men in short-term balance studies in developing countries as predicted by linear regression

Source: ref. 11.

These data served as the basis for the revision of adult protein requirements at the 1981 consultation responsible for the 1985 FAO/WHO/UNU report on energy and protein requirements [7]. However, instead of accepting the average variance, as observed in the UNU-sponsored studies, the 1981 FAO/WHO/UNU consultation made two assumptions that lowered the coefficient of variation applied. First they added data from studies with single protein sources that lower the estimated coefficient of variation from 16.8% to 16.2%. They then proposed that the observed variance should be equally partitioned between inter- and intra-observer variation (where the within-subject variability includes measurement error as well as biological variability). The use of a standard statistical formula yielded 12.5%. This was considered the true coefficient of the protein requirement for adults. Consequently, a value of 25% (2 SD) above the average physiological requirement would be expected to meet the needs of all but 2.5% of individuals in a population. Thus, a mean of 0.63 g/kg/day from the studies in table 2 plus 25% gives 0.79 g/kg/day.

At first the members of the FAO/WHO/UNU consultation agreed to round this out to 0.8 g/kg/day, a level that several long-term studies had shown to be safe, although they did not rule out a lower value. Later a few of the participants were concerned that this would be too large an increase and proposed adding into the calculations the results of three long-term studies at MIT [4-6] and one published and one unpublished experiment conducted by investigators at the University of California at Berkeley [12] .

The MIT studies had been designed to test the adequacy of the 1973 recommendation of 0.57 g/kg/day, and as mentioned previously, they found this level to be inadequate for most subjects. In the three successive studies, four of six, five of six, and three of four subjects, respectively, were in negative nitrogen balance. If the figure of 0.57 g/kg/day were a true mean value, then 7 instead of 12 of the 16 subjects might have been expected to be in negative nitrogen balance. In the published California study, using a mean intake of 0.61 g milk protein per kilogram body weight, the changes in weight and haemoglobin were not significant, and the nitrogen balance remained in a marginal range [12]. From these long-term studies it was concluded that the average amount fed, 0.58 g/kg/day, was a reasonable estimate of the mean requirement of healthy young men whose habitual intake is well above this level [7].

Adding the mean of the long-term studies to that of the short-term studies gave an average of 0.605 g/kg/day, which when increased by 25% gave a calculated safe level of intake of 0.76 g/kg/day. This was rounded to the final value proposed of 0.75 g good-quality protein per kilogram per day. The 1985 recommendations for all age groups are summarized in table 3.

TABLE 3. 1985 FAO/WHO/UNU safe protein intakes for selected age groups and physiological states

Source: ref. 7.
a. Values are for protein sources such as eggs. milk, meat, and fish.

In October 1994 the UNU-sponsored International Dietary Energy Consultative Group (IDECG), with FAO and WHO representation, convened a meeting in London to review protein and energy requirements [13]. The meeting concluded that there was no likelihood of or need for extensive additional multilevel nitrogen balance data on young adults in the foreseeable future. However, it was reasonable that a comprehensive analysis of the published data on protein requirements of adults was needed [14]. It would seem desirable, however, that a future international meeting consider a change to the calculation of variance or perhaps round the figure to 0.8 g/kg/day to indicate the level of uncertainty and assure an adequate margin of safety.

The participants in this meeting recognized the need for additional data on the protein needs of the elderly obtained in compliance with strict methodological criteria. Some of the earlier data on both younger and older adults are subject to criticism on methodological grounds. The potential for error and bias has led some to reject nitrogen balance data in their entirety. It is foolish to accept poor data or to fail to recognize the limitations of good data, but it is equally inappropriate to reject reliable data as long as their limitations are recognized.

The 1985 FAO/WHO/UNU report concluded that there was not sufficient evidence to support a higher requirement estimate for the elderly [7]. When nitrogen balance data on the protein needs of the elderly were reviewed for the IDECG [15], the available data, with one flawed exception, suggested that the 1985 FAO/WHO/UNU safe level of intake of protein for adults might not be adequate for elderly people. This could be due to the lower efficiency of protein utilization in the elderly, their higher body density, or both. Although the working paper suggested that the safe level of intake of protein would be 1.0 g/kg/day or more, the IDECG urged additional data before suggesting an increased protein recommendation for the elderly.

Equally serious doubts were raised about the protein and amino acid recommendations of the 1985 FAO/WHO/UNU consultation for the other end of the age range, namely infancy. A working paper [16] re-examined the assumptions and evidence for the estimates of protein requirements based on breastmilk intake from birth to six months and the application of the factorial approach to requirements after the age of six months. From the review (table 4), the 1994 IDECG accepted the conclusion that the protein values for breastfed infants should be 10% to 25% lower than those in the 1985 report.

TABLE 4. Protein requirements for breastfed infants according to 1985 FAO/WHO/UNU report [7] and 1994 IDECG workshop [13]

The principal bases for the 1985 FAO/WHO/UNU recommendations for infants. children. and adolescents are shown in table 5. After a resurvey of the literature, the 1994 IDECG [16] concluded that the maintenance requirement was approximately 90 mg N/kg/day, compared with the value of 120 mg N/kg/day adopted in the 1985 report. Moreover, they recommended that the 50% increase in the protein allowance for growth to cover intra-individual variation, adopted in the 1985 report, should be replaced by an adjustment in the coefficient of variation for growth, ranging from 24% to 48% during the first year depending on age.

TABLE 5. Protein requirements of infants, children, and adolescents: Summary of essentials of the approach

——————————————————————————————————————————————

1. Infants from birth to 6 months: based on intake data
2. Children over 6 months: modified factorial

1. Maintenance: N balance, ~120 mg N/kg/day at 1 year. Assume 12.5% CV
2. Requirement for growth

1. Mean N increment +50% to account for intra-individual variation in growth
2. Assume protein used with 0.7 efficiency
3. Allowance for inter-individual variation in growth taken to be a CV of 37% for growth; therefore

where

CV = coefficient of variation
CV_m = coefficient of variation for maintenance
CV_g = coefficient of variation for growth
M = maintenance
G = growth (based on 3-month intervals)

The factorial model was accepted as appropriate for calculating protein needs during the first year of life. However, the group concluded that the factorial model used to calculate protein requirements for children and adolescents should not use estimates of maintenance requirements that were interpolated from the previous "anchor point" of 120 mg N/kg/day [7] for infants. They considered this to be too high and concluded, on the basis of data from balance studies of children and adolescents [16], that a value of 100 mg N/kg/day at all ages was more appropriate. It was recognized that this figure might be modified by a future meta-analysis of the more extensive data for the maintenance requirement of adults.

Table 6 compares the 1994 IDECG estimates of the safe level of protein intake for infants during the first year with those proposed by FAO/WHO/UNU in 1985 [7]. At one year of age the IDECG value is about 70% of the FAO/WHO/UNU recommendation. It should be noted that differences in the efficiency of utilization of different protein sources must be taken into consideration, including the possibility that formula-fed infants may have higher dietary protein requirements than those who are breastfed. Also protein allowances must be increased for catch-up growth following episodes of malnutrition or infection in children of all ages. An additional intake of at least 30% is required for this purpose [ 16].

TABLE 6. Revised estimates of safe levels of protein intake for infants: 1985 FAO/WHO/UNU report [7] versus 1994 IDECG workshop [13]

a. ; where CV_m = 12.5%
See table 5 for explanation of abbreviations.

What happens next? A meta-analysis of data relating to adult protein and amino acid requirements is in progress. It is expected that there will then be a new FAO/WHO/UNU consultation to examine all of the evidence and insights since the 1981 Rome meeting, taking the recommendations of the 1994 IDECG meeting into account.

References

1. League of Nations. The problem of nutrition. Report on the physiological bases of nutrition. Technical commission of the Health Committee, vol. 11. Geneva: League of Nations Publication Department, 1936.
2. Munro HN. Historical perspective on protein requirements: objectives for the future. In: Blaxter K, Water-low JC, eds. Nutritional adaptation in man. London: John Wiley, 1985:155-68.
3. FAO/WHO. Energy and protein requirements. Report of a joint FAO/WHO ad hoc expert committee. Technical report series 522. Geneva: WHO, 1973.
4. Garza C, Scrimshaw NS, Young VR. Human protein requirements: evaluation of the 1973 FAO/WHO safe level of protein intake for young men at high energy intakes. Br J Nutr 1977;37:403-20.
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6. Garza C, Scrimshaw NS, Young VR. Human protein requirements: interrelationships between energy intake and nitrogen balance in young men consuming the 1973 FAO/WHO safe level of egg protein, with added nonessential amino acids. J Nutr 1978;108:90-6.
7. FAO/WHO/UNU. Energy and protein requirements. Report of a joint FAO/WHO/UNU expert consultation. Technical report series 724. Geneva: WHO, 1985.
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9. UNU. Protein-energy requirements under conditions prevailing in developing countries. Current knowledge and research needs. Tokyo: United Nations University, 1979.
10. Rand WM, Scrimshaw NS, Young VR. Determination of protein allowances in human adults from nitrogen balance data. Am J Clin Nutr 1977;30:1129-34.
11. Rand WM, Uauy R. Scrimshaw NS, eds. Protein energy requirement studies in developing countries. Results of international research. Food Nutr Bull Suppl 20. Tokyo: United Nations University, 1984.
12. Calloway EM, Chenoweth WL. Utilization of nutrients in milk- and wheat-based diets by men with adequate and reduced abilities to absorb lactose. I Energy and nitrogen. Am J Clin Nutr 1973;26:939-51.
13. Scrimshaw NS, Waterlow JC, Schürch B. eds. Energy and protein requirements. Eur J Clin Nutr 1996;50:S1197.
14. Clugston G. Dewey KG, Fjeld C, Millward J. Reeds P. Scrimshaw NS, Tontisirin K, Waterlow JC, Young VR. Report of the working group on protein and amino acid requirements. Eur J Clin Nutr 1996;50:S193-5.
15. Campbell WW, Evans WJ. Protein requirements of elderly people. Eur J Clin Nutr 1996;50:S180-5.
16. Dewey KG, Beaton G. Fjeld C, Lonnerdal B. Reeds P. Protein requirements of infants and children. Eur J Clin Nutr 1996;50:S119-50.

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