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The paper of Campbell and Evans was circulated at the meeting. Neither author attended the workshop, and the topic was introduced by Roberts. Scrimshaw explained that the protein requirements of younger adults were not scheduled for review at the current meeting, because it was judged that there were not sufficient new data available since the Consultation that produced the 1985 FAO/WHO/UNU report.
The 1985 safe allowance for protein was based on a series of nitrogen balance studies with single protein sources at multiple intake levels, mostly at the University of California at Berkeley and at MIT, plus similar studies sponsored by the United Nations University in developing countries with adults consuming their usual diets. In all there were 146 subjects in 17 studies in 19 countries that provided zero N balance intercepts (Rand et al, 1984). The mean of these short term studies was 0.63 g protein per kg/d with a coefficient of variation of 16.2%. This would have given a safe allowance of 0.83 g per kg/d. However, the consultation decided to assume that this variation is approximately equally partitioned between and within subjects (where the within-subject variability includes measurement errors as well as biological variability) to arrive at a 'true' coefficient of variation for the protein requirements of adults of 12.5%. No further justification of this was given.
Applying the criterion of the mean plus two standard deviations to the above would have given an estimated 'safe allowance' of 0.79 g protein per kg/d. Several long term studies confirmed the safety of this level but did not exclude the possibility that a lower level of protein intake would have been adequate (Rand et al, 1984).
However, some were concerned that moving from 0.57 to 0.8 g protein per kg/d was too great an increase and proposed including in the average data four long term studies. Three by Garza et al (1976; 1977a,b) at MIT had been designed to test the adequacy of the recommendation of the 1971 FAO/WHO Committee at the single level of 0.57 g protein per kg/d. In these three month studies, four of six, five of six and three of four subjects respectively (i.e. 12 out of the 14 subjects in all) were in negative nitrogen balance. If the 0.57 figure were indeed an acceptable mean requirement value, the number of subjects in positive and negative balance should have been approximately equal. The fourth study, by Durkin et al (1984), was one in which five of six subjects lost weight, but it was accepted because no biochemical changes were observed except for a small decrease in hemoglobin. The study had a mean intake of 0.36 g protein per kg/d and was averaged in with the other three.
The mean values of 0.63 g/kg/d derived from the short-term balance studies and 0.58 g/kg/d from the long-term balance studies gave an average of 0.605. Adding 25% for two standard deviations gave a calculated 'safe allowance' of 0.756. This was then 'rounded' to 0.75. A stronger argument might have been made for rounding to 0.8 g of protein per kg/d on the basis of the short-term studies alone, although the difference in the final result is small.
Millward questioned the omission in the 1985 report of a short-term MIT balance study with egg protein which gave a mean intercept value that would have lowered the average of the studies cited. Scrimshaw replied that this study was done before it was realized that in multilevel trials one intake must be close to that required for nitrogen balance. This is necessary because the more negative the N balance, the higher the protein utilization. Therefore, extrapolating to zero N balance from levels well below those required can significantly underestimate the amount of N required to meet protein needs. The egg studies referred to were excluded for this reason, but later ones from MIT with egg protein, designed in accord with the standard UNU criteria, were included. Criteria for valid nitrogen balance measurements of protein requirements are provided in an Appendix to these proceedings.
It was concluded that it would be appropriate to reexamine the issue of the mean protein requirements of young adults, including any new data. This was considered essential to deciding whether progressive aging per se changes protein needs in healthy adults. A meta-analysis was proposed with weight maintenance and the lack of any adverse changes in cumulative nitrogen balance as criteria for inclusion.
The sources of error in the nitrogen balance technique affecting the validity of estimates of protein requirements dependent on this methodology were strongly emphasized by Roberts who pointed out:
(1) Propagation of errors. Since N balance is determined as the difference between dietary N intake and N losses, where N losses are the sum of N excretion in urine and faeces and N losses from other 'miscellaneous routes' (skin, hair, nails, breath ammonia, sweat and so forth), the errors inherent in each of these measurements are of serious concern.
(2) Effects of energy balance on nitrogen balance. Negative energy balance promotes negative N balance because lean tissue is mobilized along with fat and glycogen, and positive energy balance promotes positive N balance (Calloway and Spector, 1954; Inoue et al, 1973; Garza et al, 1978). Even very small discrepancies in energy balance can have a major impact on N balance, with an energy imbalance of only 1 kcal/kg influencing N balance to the extent of 1 mg/kg (Pellett & Young, 1992). Because of the large normal variation in daily weight, it is extremely difficult to be sure that energy intakes are appropriate in short-term balance studies.
(3) Effect of study duration on nitrogen balance. Since usual protein intakes are higher than recommended protein needs, subjects must be adapted to the lower levels used in nitrogen balance studies. It is possible that the reported negative N balances in several studies of older subjects were influenced by inadequate equilibration periods.
(4) Miscellaneous nitrogen losses. The 1985 consultation suggested that mean miscellaneous N losses are 8 mg/kg in adults, an increase from the value of 5 mg/kg used previously. It is currently assumed that there is no effect of aging on miscellaneous N losses, although a comparison of data from younger and older men (Calloway et al, 1971; Zanni et al, 1979) indicates that older individuals have only 50% of the N loss in nails and less than 25% in hair. While it is not possible to predict the extent to which miscellaneous losses may be lower in older subjects until more information becomes available, the direction of this potential bias is to overestimate negative N balance.
(5) Effects of age-related loss of lean body mass. Older individuals exhibit a gradual loss of lean body mass with age, although this may be minimized by exercise. Thus the observation of negative N balance in an older person could be influenced by the 'inevitable' loss of N associated with aging, although the extent of the loss is unknown.
Concern was expressed that in some elderly, the energy flux may decrease so much that their protein and micronutrient needs are no longer met. It was also pointed out that almost nothing is known about the protein needs of unhealthy elderly who are a significant proportion of the total. Digestibility of protein does not appear to be affected in healthy elderly, but could be affected by disease.
It was noted that N balance may not be the best approach to assessing adequacy and making recommendations for protein intakes of the elderly. Higher protein intakes may be protective by enhancing immune function and producing other benefits. Reeds urged looking at functional indicators in addition to nitrogen balance. An improvement in some functions was noted at 0.9 g compared with 0.5 g protein per kg/d in healthy elderly (Roberts).
It was suggested that the possibility of a desirable upper level of protein intake should also be considered. Excessive protein intakes can accelerate a decline in kidney function in the elderly. In recent US and European reports, upper levels for protein intake are specified. It was also noted that with deteriorating kidney function more protein is lost and the protein requirement increased.
Calloway DH, Odell ACF & Margen S (1971): Sweat and miscellanous nitrogen losses in human balance studies. J. Nutr. 101, 775786.
Calloway DH & Spector H (1954): Nitrogen balance is related to caloric and protein intake in active young men. Am. J. Clin. Nutr. 2 405-412.
Durkin N. Ogar DA, Tilve SG & Margen S (1984): Human protein requirements: autocorrelation and adaptation to a low-protein diet. In Protein-energy-reguirement studies in developing countries: results of international research, eds WM Rand, R Uany & NS Scrimshaw Fd Nutr. Bull., Suppl. 10, pp. 57-62.
FAO/WHO/UNU (1985): Energy and protein requirements. Technical Report Series 724. Geneva: World Health Organization.
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Garza C, Scrimshaw NS & Young VR (1977a): Human protein requirements: a long term metabolic nitrogen balance study in young men to evaluate the 1973 FAO/WHO safe level of egg protein intake. J. Nutr. 107, 335352.
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Pellett PL & Young VR (1992): The effects of different levels of energy intake on protein metabolism and of different levels of protein intake on energy metabolism: a statistical evaluation from the published literature. In Protein-energy interactions, eds NS Scrimshaw & B Schürch Lausanne, Switzerland: international Dietary Energy Consultative Group.
Rand WM, Uauy R & Scrimshaw NS (1984): Protein energy requirement studies in developing countries: results of international research. Fd Nutr. Bull., Suppl. 10.
Zanni E, Calloway D & Zezulka A (1979): Protein requirement of elderly men. J. Nutr. 109, 513-524.