The requirements of individuals and populations for protein and energy have been discussed extensively by experts and committees over the past few decades. Although a rigorous defination for either protein or energy needs has not been unanimously agreed upon, various operational definitions have been formulated, and these have varied with time. Early efforts focussed on the measurement of habitual protein intakes of healthy populations. Next, metabolic studies appeared to provide data that were more precise, and obligatory losses were used for the definition of requirements and establishment of recommendations. More recently, N balance response to graded levels of protein intake over long and short periods has gained acceptance.
During the last decade attention has been directed towards the limitations of these methods. While some of the questions relate to methodology, the most important issues deal with fundamental relevance of the metabolic data to individual and population requirements. The purpose of the Berkeley workshop was to review critically recent data from experimental studies in humans conducted in developing countries. It was inevitable that major issues in the field of protein-energy requirements would be raised. A listing of the more relevant of these as discussed during the meeting has been included in this introduction to indicate a sense of the meeting and to give the framework of the data review.
(1) Measurement of Nitrogen Balance
General methodological aspects fundamental to metabolic studies have been well addressed in a previous UNU publication (1), which deals with knowledge and research needs of protein energy requirements under conditions prevailing in developing countries. Further discussion is contained in the 1977 FAO/WHO report on methodology appended here, (Appendix 1) with special attention to work with children. Of special interest to the participants in the Berkeley workshop were questions of comparability of studies, necessitating some sort of standardization program, and estimation of skin and miscellaneous losses. Both these topics are addressed in the methodology section.
A major issue that continually arose was how to measure, interpret, and use the variability both within and among subjects. While an aspect of this question is examined in the methodology section, no resolution of the problem was reached. At best it was admitted that potential sources of variability included biological, analytic, genetic, and environmental factors, and that these factors affect both the metabolic responses measured and the measurements themselves.
A further question that was considered important but not discussed in depth was how to estimate nitrogen retention for growing children and how to include this in their age-specific protein needs. It was only agreed that the growth allowance should consider the mean velocity of ponderal gain, the proportion of protein in the new tissue deposited, and the variability in growth observed under real life conditions.
(2) Interpretation of Nitrogen Balance
Several concerns were expressed in interpreting the results of nitrogen balance studies.
a) The significance of zero nitrogen balance.
Day to day variations above and below zero N balance are inherent to homeostasis, and the shorter the time interval over which the data are averaged, the less physiological significance or relevance to requirements do positive or negative deviations have. Although there is a broad range in which variations in N balance above and below zero are part of normal homeostatic mechanisms, the level of protein intake sufficient for zero N balance is still an appropriate estimate of mean protein requirement, assuming all losses are accounted for in the calculation of N balance.
There is a problem in estimating all N losses; however, we can assume that over long periods of time, N balance, under steady-state conditions, must average zero. Therefore, any consistent positive deviation from zero with no indication of failure of homeostasis must be assumed to represent unmeasured losses. How much to allow for such losses when determining zero N balance for purpose of estimating requirements is still an issue that must be resolved.
b) Within individual variation. The protein requirement of an individual is not to be viewed as genetically fixed. The original genetic potential of every individual is continuously modified by environmental circumstances from the time of conception onward. The expression of this genetic potential, the phenotype, is heavily influenced by environmental factors that vary with time, place, and situation. Thus, the protein requirement of an individual may be lower or higher at some time other than the one measured. Some quantitative information is becoming available on such variation and it is thought to be similar in magnitude to the population variation itself. This suggests that metabolic balance studies of individuals over brief periods of time -- and even three months is still brief -- contribute more to population estimates than they do to reliable estimates of an individual's “requirement”.
c) Among individual variation. In most studies, one or more individuals behaves in an anomolous manner. The method of handling the data from such subjects will influence the mean and have an even greater effect on the standard deviation and confidence intervals. Since the number of subjects in all of these multilevel studies is small, reliable population variances estimates cannot be derived from any one of them. The pooled standard deviation for those studies that follow the protocol reasonably closely and appear comparable is 17%. It is reasonable to assume that the coefficient of variation of individual studies depends, at least in part, on the degree of confinement and homogeneity of the sample.
d) Variation between studies. In arriving at realistic population estimates on the basis of short-term balance studies on small samples, one obviously needs to consider many factors; for example, the degree of departure of the experimental situation from normal free-living conditions and usual diets is very important. Even more troublesome is judgement as to the extent to which recommendations to be applied to countries or regions of the world must take into account not only dietary protein quality, but also effects arising from physical, biological, and social factors in the environment. Examples of these are, respectively, high environmental temperatures, prevalence of infectious disease, and existence of subpopulations with very high physical activity levels or with consistently inadequate or excessive dietary energy intakes.
(3) Protein Requirement
The fundamental underlying question of how to estimate protein requirements of course arose continually during the week of meetings. It was felt that any recommendation of requirement needs to be based not only on metabolic and biochemical indices of protein nutrition, but also on functional and health consequences. Further, there was an evident need for better understanding of the implications of adaptations to changes and variability in dietary intake and environment. While doubts were raised regarding the longterm validity of methods that use 10 to 15 days to assess nitrogen balance response to an intake level, it was agreed that this method does contribute to a first approximation of requirements. Subject to the errors and variabilities discussed, and assuming that they are minimized, short-term, multiple-level nitrogen balance studies give a mean zero balance intercept that is a useful estimate of mean population requirements for the dietary protein fed under the conditions of the study.
