Cover Image
close this bookEnergy and Protein requirements, Proceedings of an IDECG workshop, November 1994, London, UK, Supplement of the European Journal of Clinical Nutrition (International Dietary Energy Consultative Group - IDECG, 1994, 198 pages)
close this folderProtein requirements of elderly people
View the document(introductory text...)
View the documentIntroduction: current dietary protein requirement recommendations
View the documentProtein requirement studies in elderly people: previously reviewed data
View the documentProtein requirement studies in elderly people: reassessment of previously reviewed data
View the documentSummary
View the documentReferences
View the documentDiscussion
View the documentReferences

(introductory text...)

WW Campbell and WJ Evans

Noll Physiological Research Center, The Pennsylvania State University, University Park, PA 16802, USA

Descriptors: protein, nitrogen balance, ageing

Introduction: current dietary protein requirement recommendations

the current mean dietary protein requirement for healthy adult men and women of all ages, as set by the 1985 joint fao/who/unu expert consultation, is estimated to be 0.6 g protein/kg/d, with a suggested safe level of intake set at 0.75 g protein/kg/d. these protein requirements and allowance estimates were established using a standard method of nitrogen (n) balance in young men, and making extrapolations for older adults and the elderly. while acknowledging that various changes (including progressive changes in body composition, physiological functional capacity, physical activity, total food intake, and frequency of disease) occur with advancing chronological age, all of which might be expected to contribute to altered protein requirements, the consultation felt that insufficient data were available from studies conducted with elderly subjects to determine protein requirements with confidence. it noted that the suggested safe level of protein intake should not be lower than the recommendations for younger adults and suggested that the elderly may utilize dietary protein less efficiently..

Correspondence to: William J. Evans, Noll Physiological Research Center, 119 Noll Laboratory, The Pennsylvania State University, University Park, PA 16802, USA.

Protein requirement studies in elderly people: previously reviewed data

Protein requirement studies in elderly subjects, utilizing the N balance technique as the standard method of assessment, have been conducted by several research groups during the past 16 years (Table 1). Data that were available to and reviewed by the 1985 Consultation included the studies by Cheng et al, Uany et al, Zanni et al and Gersovitz et al. These studies did indeed provide conflicting results, with some studies (Cheng et al, 1978; Zanni et al, 1979) concluding that 0.75 g protein/kg/d is adequate, and others (Gersovitz et al, 1982; Uany et al, 1978) concluding that this protein intake is inadequate for N equilibrium in elderly individuals. Differences in results and conclusions among these four studies may be due, in part, to differences in physiologic and metabolic status of subjects within each study and between the studies, as well as differences in study design and the assumptions used in calculating N balance. Indeed, each study used a somewhat different formula to calculate N balance, none of which was exactly the formula recommended for use by the 1985 Consultation.

Cheng et al (1978) estimated that a mean protein intake of 0.77 g protein/kg/d was required for N equilibrium in seven elderly male nursing home residents. They concluded that 0.8 g protein/kg/d high-biological-value protein was adequate for these elderly men.

Table 1 Protein requirement studies in elderly people: subjects and protein intake information


Subjects

Age

Protein intake

Nitrogen balance

Reference

n, sex

(y)

(g/kg/d)

(study days)

Cheng et al (1978)

7, M

63-77

0.4, 0.8, 1.6

7-11

Uauy et al (1978)

7, M

68-74

0.57, 0.70, 0.85

6-10


7, F

71-78

0.52, 0.70, 0.80

6-10

Zanni et al (1979)

6, M

63-77

0.36-0.41, 0.42-0.46

11-15

Gersovitz et al (1982)

7, M

70-82

0.8

6-10, 16-20, 26-30


8, F

71-99

0.8


Bunker et al (1987)

24, M F

70-86

variable, 0.55-1.25

1-5

Campbell et al (1994)

12, M F

56-80

0.8 or 1.6

6-11

Castaneda et al (1995)

12, F

66-79

0.45 or 0.92

16-21, 56-61

M = male, F = female.

However, each subject's N balance was overestimated by not accounting for miscellaneous N losses, and no upward adjustment was made in the mean protein requirement to provide a safe level of protein intake for nearly all elderly people.

Uauy et al (1978) measured N balance in seven elderly women and seven elderly men provided weight maintenance diets for 10-day periods with various quantities of whole-egg as the protein source. Mean N equilibrium was calculated to be 0.83 g protein/kg/d in the elderly women, and estimated to range from 0.7 to 0.85 g protein/kg/d in elderly men. The authors concluded that these elderly women and men would obtain adequate protein with diets providing 12-14% of total energy intake as protein. This roughly corresponded to protein intakes of 0.95-1.10 g protein/kg/d for these elderly women and 0.830.97 g protein/kg/d for these elderly men, based on calculated mean energy intake data provided in the original publication. Uany et al did not specifically adjust the mean protein requirement upward to provide most elderly people with a safe level of protein intake.

Zanni et al (1979) estimated a mean protein requirement of 0.46 g protein/kg/d based on data from six elderly men, and concluded that the 1973 FAO/WHO safe intake of 0.57 g protein/kg/d was adequate for apparently healthy elderly men. Nitrogen balance was determined while each man consumed either a protein free diet for 17 days, or diets providing either about 1.44 or 1.73 times measured endogenous N losses (15 days each period). Several factors may have contributed to an underestimation of actual protein requirements in this study, including the measurement of differences in N balance over a small range of inadequate protein intakes (0.36-0.46 g protein/kg/d), including data from the protein-free balance period in the N balance calculations when it is generally recognized that this will lower the protein requirement estimate due to non-linearity of the N balance response in this submaintenance range (FAO/WHO/UNU, 1985), and increasing N retention by not providing a restabilization period between the 17-day protein-free balance period and the other balance periods.

Gersovitz et al (1982) provided weight-maintaining diets with ().8 g protein/kg/d to seven elderly men and eight elderly women continuously for 30 days. Nitrogen balance was assessed on study days 6-10, 16-20 and 26-30. During the initial 10-day period, all seven men and four of eight women were in negative balance. Mean N balance was improved (i.e. N equilibrium reached) for both men and women on days 26-30, but three men (43%) and four women (50%) were in negative N balance. These long-term N balance data indicated that 0.8 g protein/kg/d was not adequate for a majority of elderly men and women.

Table 2 Protein requirement assessments in elderly people



Protein intake for nitrogen balance


Subjects

Authors calculation

1985 FAO/WHO/UNU calculation

Reference

n, sex

(g/kg/d)

Cheng et al (1978)

7, M

0.77

0.93 ± 0.05a

Uauy et al (1978)

7, M

0.7-0.85

0.81 ± 0.09a


7, F

0.83

M & F combined

Zanni et al (1979)

6, M

0.46

0.65 ± 0.19a

Campbell et al (1994)

12, M F

1.00 (0.72-1.18)b

samec

Castaneda et al (1995)

12, F

0.78 (days 16-21)

samec



0.82 (days 56-61)

samec

M = male, F == female.
a Recalculated data presented by Campbell et al (1994).
b Lower and upper limits of 95% confidence interval.
c 1985 FAO/WHO/UNU nitrogen balance formula used for original calculation of protein requirement.

Protein requirement studies in elderly people: reassessment of previously reviewed data

When comparing results and conclusions of these studies (Cheng et al, 1978, Gersovitz et al, 1982; Uauy et al, 1978; Zanni et al, 1979), it would be helpful if the same N balance formula was used and the same assumptions were applied to the calculations. To this end, Campbell et al (1994), recently reassessed the N balance data from each of these studies using the N balance formula recommended by the 1985 Consultation. This reassessment was possible because individual subjects' N balance data had been published in each of the original articles. The results are shown in Table 2. Recalculation of the data from Cheng et al (1978) increased the mean protein requirement by 21%, to 0.93 ± 0.05 g protein/kg/d, with the increase mainly due to the inclusion of a factor for miscellaneous N losses in the balance formula. Data from the elderly women and men studied by Uauy et al (1978) were combined and a mean protein requirement of 0.81 ± 0.09 g protein/kg/d calculated. The similarity of this value to the original protein requirement estimates by Uauy et al was expected, since an increase in the estimated miscellaneous N losses from 5 mg to 8 mg N/kg/d was the only change in the N balance formula used. Upon recalculation of the data of Zanni et al (1979), the mean protein intake required for N equilibrium increased by 41%, to 0.65 ± 0.19 g protein/kg/d. This dramatic increase was due mainly to the exclusion of data collected during the 17-day protein-free period from the regression. When the recalculated data from all three studies were combined by weighted mean averaging, a mean protein requirement of 0.89 ± 0.05 g protein/kg/d was estimated (Campbell et al, 1994).

Protein requirement studies in elderly people: new data

In addition to the retrospective reassessment of data from these N balance studies of elderly subjects (Cheng et al, 1978; Uauy et al, 1978; Zanni et al, 1979), Campbell et al (1994) also provided N balance data on 12 men and women, aged 56-80 years, who consumed weight-maintenance diets providing either 0.8 or 1.6 g protein/kg/d. A mean protein requirement for N equilibrium of 1.0 g protein/kg/d was estimated based on N balance data collected during days 6-11 of this short term N balance study. This study was limited by the assessment of N balance at only two levels of protein intake, and the lack of opportunity to calculate separate protein requirements for each subject, because each subject consumed only one of the experimental diets. However, it is noteworthy that the lower limit of the 95% confidence interval for the 1.0 g protein/kg/d mean protein requirement is 0.72 g protein/kg/d, a value considerably greater than the 0.6 g protein/kg/d mean protein requirement established by the 1985 Consultation for adults of all ages.

The 1.0 g protein/kg/d mean protein requirement estimated by Campbell et al (1994) is similar to the 0.97 g protein/kg/d mean protein intake that maintained 24 healthy men and women, aged 70-86 years, in N equilibrium (Bunker et al, 1987). Homebound elderly people, aged 7086 years, were reported to be in marked negative N balance at a mean protein intake of 0.67 g protein/kg/d. These data by Bunker et al (1987) actually overestimate N balance because no factor for miscellaneous N losses was used in the balance calculation.

A summary of the new data from Campbell et al (1994), and the recalculation of data from Cheng et al (1978), Uauy et al (1978) and Zanni et al (1979), is provided in Figure 1. A weighted mean protein requirement for elderly people of 0.91 ± 0.043 g protein/kg/d was estimated from data of these four studies combined. As shown, the estimated mean protein requirement in three of the four studies is higher than the safe level of 0.75 g protein/kg/d recommended by the 1985 Consultation for adults of all ages.

After a mean protein requirement is established, an upward adjustment should be made to establish a safe level of intake (recommended allowance) to meet the metabolic needs of virtually all adults. The Consultation (1985) estimated the coefficient of variation of protein requirements of young adults at 12.5%, and suggested that the dietary protein needs of 97.5% of individuals in this population group would be met with a protein intake 25% above the mean protein requirement. Based on this estimate, a safe protein intake for elderly men and women would be 1.0 g protein/kg/d or more high quality protein.

Table 3 Conclusions from protein requirement studies in elderly people

Reference

Original conclusions of the authors

Conclusions based on 1985 FAO/WHO/UNU calculations

Cheng et al (1978)

0.8 g protein/kg/d

0.8 g protein/kg/d


adequate intake

not adequate intake

Uauy et al (1978)

0.8 g protein/kg d

0.8 g protein/kg/d


not adequate intake

not adequate intake

Zanni et al (1979)

1973 FAO/WHO safe intake of 0.57 g protein/kg/d
adequate intake

Mean protein requirement 41% higher than original estimate. But 0.57 g protein kg/d still considered adequate intake

Gersovitz et al (1982)

0.8 g protein/kg d
not adequate intake

Conclusions the same. 0.8 g protein/kg/d not adequate intake

Campbell et al (1994)

0.8 g protein/kg d
not adequate intake

Conclusions the same. 1985 Consultation N balance formula used

Castaneda et al (1995)


0.8 g protein/kg d not adequate intake 1985 Consultation N balance formula used


Figure 1 Dietary protein intakes required to achieve nitrogen equilibrium in elderly subjects. Individual subject (open circles) and group mean protein intakes (±2 s.e.m.s; solid bars) at nitrogen equilibrum were calculated from data by Zanni et al (1979), Cheng et al (1978), Uauy et al (1978) and Campbell et al (1994), based on the 1985 FAO/ WHO/UNU nitrogen balance formula (FAO/WHO/UNU, 1985). The dashed line represents the current estimated mean protein requirement for all adults (0.60 g protein/kg/d), and the dotted line represents the current suggested safe level of protein intake for elderly men and women (0.75 g protein/kg/d) (FAO/WO/UNU, 1985). Figure adapted from Campbell et al (1994).

The 1985 report acknowledged that no longer-term N balance data in elderly subjects were available when the Consultation met in October 1981, other than the 30-day balance data of Gersovitz et al (1982). Recently Castenada et al (1995) completed a study assessing longer-term (9 weeks) adaptation to marginal protein intakes in 12 elderly women aged 66-79 years. Subjects consumed weight-maintenance diets providing either 0.45 or 0.92 g protein/kg/d, and N balance was measured for 6-day periods during study weeks 3 and 9. All six women provided 0.45 g protein/kg/d were in negative N balance during week 3 (group mean -15.3 ± 3.3 mg N/kg/d), and the mean negative N balance moved significantly toward N equilibrium at week 9 (group mean - 5.2 ± 1.3 mg N/kg/d). However, this shift in N balance occurred only after these elderly women had accommodated with an 8% decline in body cell mass (assessed by potassium-40 scanning) and an 8% decline in muscle mass (assessed by urinary creatinine). Five of these six women remained in negative balance at week 9. The six women provided 0.92 g protein/kg/d achieved N equilibrium throughout the study, with a mean balance of 7.5 ± 6.3 and 1.7 ± 2.5 mg N/kg/d at weeks 3 and 9, respectively, and maintained body cell mass and muscle mass.

At week 3, a mean protein requirement of 0.78 g protein/kg/d for N equilibrium may be estimated from regression analysis of data from both the 0.45 and 0.92 g protein/kg/d groups. A similar estimated mean protein requirement may be calculated at week 9 (0.82 g protein/ kg/d). Thus, both short- and longer-term N balances support previous reports (Campbell et al, 1994; Gersovitz et al, 1982; Uauy et al, 1978) demonstrating higher protein requirements in healthy elderly subjects.

Short-term N balance studies (1-3 week study periods) have criticized for allowing inadequate time to fully adapt to steady-state at a given protein intake level. Indeed, these data (Castaneda, 1995), and those of Gersowitz et al (1982), show that N balance tends to move toward equilibrium (i.e. zero balance) when extended past 3 weeks in elderly men and women. That the mean protein requirement estimates in elderly women were similar at both 3 and 9 weeks, despite this shift, however, supports the conclusion that short-term balance studies are predictive of protein requirements that would be estimated from longer-term studies (Campbell, 1994). This is fully consistent with the close similarity of mean protein requirement estimates from short- and longer-term balance studies, 0.63 and 0.58 g protein/kg/d, respectively in studies reviewed by the 1985 Consultation in young men. In addition, the data of Castaneda et al demonstrate that N equilibrium shown in longer-term balance studies may be established as a result of significant detrimental changes in body composition. Thus, N balance data generated from longer-term studies do not provide an accurate assessment for protein requirements of subjects in the metabolic state in which they entered the study.

Thus available assessments of protein requirements utilizing both short- and longer-term N balance techniques suggest that the mean protein requirement of elderly men and women may be ³ 0.8 g protein/kg/d. This would indicate a safe level of intake for most elderly people of ³ 1.0 g protein/kg/d.

Summary

Based on N balance calculated in accord with the recommendations of the 1985 Joint FAO/WHO/UNU Expert Consultation, a summary of conclusions from the six N balance studies in elderly people is presented in Table 3. The data, with one flawed exception, indicate that the safe level of protein intake for older adults may be higher than currently recommended.

In the only study directly comparing elderly and young subjects (Cheng et al, 1978), no differences in protein requirement were found. However, the data suggest that the 0.8 g protein/kg/d would be inadequate for both groups. Additional direct comparisons between younger and older adults are desirable as a basis for any revised recommendations.

In the only study directly comparing the protein requirements of both elderly men and women (Uauy et al, 1978), the variability was too great to allow adequate assessment of possible gender differences. More research is needed to determine whether gender-related differences occur.

References

Bunker VW, Lawson MS, Stanfield MF & Clayton BE (1987): Nitrogen balance studies in apparently healthy elderly people and those who are housebound. Br. J. Nutr. 47, 211-221.

Campbell WW, Crim MC, Dallal GE, Young VR & Evans WJ (1994): Increased protein requirements in elderly people: new data and retrospective reassessments. Am. J. Clin. Nutr. 60, 501-509.

Castaneda C, Charnley JM, Evans WJ & Crim MC (1995): Elderly women accommodate to a low-protein diet with losses of body cell mass, muscle function, and immune response. Am. J. Clin. Nutr. 62, 30-39.

Cheng A, Gomez A, Bergan, J. Tung-Ching L, Monckeberg F & Chichester C (1978): Comparative nitrogen balance study between young and aged adults using three levels of protein intake from a combination wheat-soy-milk mixture. Am. J. Clin. Nutr. 31, 12-22.

FA/WHO/UNU (1985): Energy and protein requirements. Technical Report Series 724. Geneva: World Health Organization.

Gersovitz M, Munro H. Scrimshaw N & Young V (1982: Human protein requirements: assessment of the adequacy of the current recommended dietary allowance for dietary protein in elderly men and women. Am. J. Clin. Nutr. 35, 6 14.

Uauy R. Scrimshaw N & Young V (1978): Human protein requirements: nitrogen balance response to graded levels of egg protein in elderly men and women. Am. J. Clin. Nutr. 31, 779-785.

Zanni E, Calloway D & Zezulka A (1979): Protein requirement of elderly men. J. Nutr. 109, 513-524.

Discussion

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.

References

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.

Garza C, Scrimshaw NS & Young VR (1976): Human protein requirements: The effect of variations in energy intake within the maintenance range. Am. J. Clin. Nutr. 29, 280-287.

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.

Garza C, Scrimshaw NS & Young VR (1977b): Human protein requirements: evaluation of the 1973 FAO/WHO safe level of protein intake for young men at high energy intakes. Br. J. Nutr. 37, 403-420.

Inoue G. Fujita Y & Niiyama Y (1973): Studies on protein requirements of young men fed egg protein and rice protein with excess and maintenance energy intakes. J. Nutr. 103, 1673-1687.

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.