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A monograph based on a workshop held in San Jose, Costa Rica, 1 - 3 February 1977, extensive review by a wide range of experts, and an editorial group meeting in Guatemala, 7 - 9 August 1978, has been completed and is now in press as the first technical publication of the United Nations University's World Hunger Programme.* The following observations on research priorities were made.
It is not always recognized that the lower energy intake of many less-privileged populations may be a handicap to their potential for economic productivity and social development, and is part of a vicious circle that needs to be broken by wise guidance of the development process. Moreover, from the standpoint of agricultural production goals, this means that more food needs to be produced, and also that a way to generate the purchasing power necessary to obtain adequate food should be part of development plans. In order to identify optimum targets for food production relative to considerations of health maintenance of populations, knowledge of the energy requirements of groups of individuals of all ages under varying environmental and cultural circumstances must be improved substantially.
Attention should be given to each age group: infants, pre-schoolers, primary-school children, adolescents, and adults of all ages. In the case of the elderly in industrialized countries, reduced physical activity and lower basal metabolic rate imply lower energy needs, but the total protein need may not decline proportionately. Thus, higher protein energy ratios than those for young adults may characterize the relationship between protein and energy requirements during the later years of life. Particular emphasis must now be given to individuals in the developing regions where work and activity patterns differ substantially from those of the subjects upon whom much of the current data are based.
At present, estimates of the energy requirements of human subjects are based largely on dietary survey data and on a limited amount of information on physical activity patterns and the energy cost of various physical activities. Apart from the difficulties involved in determining nutrient intakes precisely, it is not certain that the current recommendations are optimum for long-term maintenance of normal body composition and function. This matter can only be resolved, however, through additional research on the dietary energy requirements of humans and on the quantitative significance of factors that affect them. Thus, there is little or no information relevant to the populations of concern in this report concerning the energy cost of activities and the energy usually expended by children, non-pregnant, pregnant, and nursing women, and other adults. The impact of under-nutrition, acute and chronic infection, and of catch-up growth on these aspects of energy requirement and utilization must also be defined.
In part, this will require advances in methodology for measuring energy intake and expenditure in individuals engaged in their usual daily activities and in population groups. It also requires better understanding of the reciprocal relationships between the adequacy of dietary energy and physical activity. Because there is no single level of energy requirement independent of the activity pattern of the individual, this too must be defined in terms of the range compatible with desirable social functioning and health status. When does reduced activity as an adaptation to lower dietary energy availability become socially, economically, or medically unacceptable? What range of activity is most compatible with the maintenance of good health? What methodology can be developed to obtain answers to these questions?
Clearly, caloric intakes in excess of needs imposed by the level of physical activity will lead to over-nutrition and obesity. This is equally as undesirable as insufficient energy intakes, because of the adverse health consequences of excessive energy intake: namely, increased frequency of hypertension, heart disease, diabetes, and perhaps greater susceptibility to infection and decreased work performance. It is also a gross waste of a nation's resources. Indeed, the frequency of obesity as a public health problem in the industrialized societies, where most of the dietary data have been gathered, suggests that current recommendations may not be appropriate.
Although many studies have been carried out on the protein requirements of humans, these have been mostly short-term and largely limited to well-nourished Caucasians. Hence, knowledge of human protein and amino-acid needs is still extremely limited, especially for individuals in developing countries. It is crucial to obtain information on the protein requirements of children, pre-school and school age, non-pregnant, pregnant, and nursing women, and of the adults of populations in developing countries, considered in relation to local foods and diets, environmental conditions, and patterns of living.
Data on the requirements for individual essential amino acids are highly inadequate and unavailable for various age groups. In this context, the protein nutritional value of diets typical for population groups in developing countries, and the impact of the age of the individual on the nutritional value of habitual diets, deserve particular attention. The significance of current estimates of protein and amino-acid needs for long-term health has not been established, although a series of recent papers has raised serious doubts about their suitability even for healthy, well-nourished adult subjects.
Recent data substantiate the sensitivity of body nitrogen metabolism to the level and adequacy of energy intake and the difficulties created by this response for arriving at reliable estimates of dietary protein requirements. Furthermore, studies have also confirmed that an improved retention of nitrogen can be achieved by feeding protein of better quality, even under conditions of dietary energy inadequacy. Little is known, however, about the physiological significance of the additional nitrogen retained under these conditions, or the extra nitrogen retention brought about by excessive energy intakes. This requires, in part, the development of functional indices of the protein and energy nutritional status of individuals.
The quantitative extent to which apparently low dietary energy intakes are of practical importance in altering dietary protein utilization and requirements must be determined. If the latter are compensated for by reduced activity, there may be no significant effect on protein metabolism and utilization. It is essential that metabolic balance studies concerned with nitrogen balance in humans at any age pay meticulous attention to recording and analyzing the effect of variations in energy intake or energy expenditure on nitrogen metabolism and dietary protein utilization.
In addition to the above, the series of research recommendations made at a Joint FAO/WHO Meeting (1975) are highly relevant and should be pursued as opportunity permits. These include, (a) the need for data on the basal metabolic rate of various population groups; (b) data on the health consequences of physical inactivity; (c) the effect of climate on energy and protein requirements; (d) the degree of metabolic compensation for increased sweat losses; (e) the biological significance of seasonal variations in food intake and body weight; and (f) the digestibility of protein and energy sources and the biological value of protein in local diets.
The latter is particularly important in view of the fact that various dietary as well as host and environmental factors may modify the utilization of nitrogen and energy from diets when consumed under field conditions. Furthermore, there is an urgent need to assess the applicability of available data on the nutritive value of dietary constituents, as determined in well-nourished subjects, under the conditions of well-defined metabolic balance studies, to dietaries and conditions that characterize populations in developing regions.
A series of priority research areas were identified, as follows.
1. Energy requirements and expenditure
There is an urgent need for more information about the topics below.
a. The energy expenditures of pre-school and school-age children and men and women of differing physiological, nutritional, and health status should be determined. This, together with studies of the energy cost of various physical activities in these groups under their usual living conditions, is basic to an evaluation of the energy requirements and nutritional status of the populations in developing regions.
b. The influence of energy intake and of physical activity on the utilization and need for dietary protein should be investigated.
c. There is a need for more information on the social and health significance for populations consuming dietary energy at levels substantially below those found appropriate for healthy individuals in more privileged economic and social circumstances.
d. The costs in human welfare and economic productivity of sub-optimal calorie intakes should be determined and compared with the costs of correcting the situation.
Particularly needed are convincing, practical demonstrations of the extent to which increased dietary energy results in productivity and other social gains in free-living conditions without artificial productivity incentives that could not be maintained on a continuing basis in the society.
2. Protein requirements and allowances
There is an urgent need to determine the following matters.
a. Whether there are constitutional (genotypic) differences of practical importance in the protein requirements of different population groups. In consideration of present knowledge, university students in a developing country may provide an adequate basis for initial comparison with comparable groups studied elsewhere and in a similar manner.
b. The current estimated safe practical intakes for protein must be evaluated for the diets and food sources typical for both adults and children in developing regions.
c. The capacity of these diets to meet protein intakes necessary to maintain an adequate protein nutritional status under the prevailing environmental conditions must be determined. This involves an assessment of the chronic environmental factors in developing countries that influence the need for dietary protein (phenotypic factors), both in absolute terms and in relation to dietary energy intake and requirements. Studies of this kind should involve examination of those individuals exposed to the stresses resulting from an unsanitary tropical environment and consuming usual diets in which dietary fiber and other factors may alter protein availability. The importance of exploring the capacity of traditional diets to meet nutrient requirements is crucial.
d. The effect of repeated episodes of infectious diseases on overall protein requirements for adequate maintenance and growth.
Limited resources and common-sense priorities indicate that such studies should concentrate on the most vulnerable groups: infants and pre-school children, pregnant and lactating women, with the pregnant adolescent representing an important and particularly vulnerable group. However, for various methodological and ethical reasons, and because there are more comparative data available for adults, it is also essential to continue studies in this age group.
For studies concerned with protein needs and utilization, additional criteria of adequacy beyond nitrogen balance should be employed. These may include the measurement of body composition components, such as body cell mass by whole body 40k measurement where possible; creative and creatinine excretion as indicators of muscle-mass breakdown; serum proteins and amino-transferases (trans-aminases) as indicators of liver function; immune responses, and tests of physical capacity or endurance. The status of visceral protein should be assessed by measurements of a variety of specific types of serum proteins. These include albumin, transferrin, haptoglobin, alpha1-acid-glycoprotein, alpha2-macroglobulin, complement3, and immunoglobulins.
3. Requirements for catch-up growth and recovery from infection
The available information on the energy and protein needs (P/E ratio) necessary for recovery of weight loss and resumption of adequate growth following mild to moderate protein-energy malnutrition, with and without infection, are estimates only and are based on a number of assumptions. It is necessary that the latter be validated in order to develop sound and rational recommendations for the dietary energy intake and protein/energy ratio for populations in which there is widespread undernutrition and where chronic and acute infections are endemic.
The protein and energy deficits associated with weight loss and reduced growth in children must be measured. In addition, the adequacy of catch-up weight gain as an indicator of catch-up growth should be explored. This will involve an assessment of body composition changes in children of differing ages and with varying forms of malnutrition, to be made during and following nutritional rehabilitation.
The energy and protein/energy ratio of diets based on traditional foods that are adequate for ensuring rapid catch-up growth must be determined. Particular attention should be given to the influence of age and initial body size, because these factors need to be evaluated in the development of appropriate dietary guidelines. Initially, it is envisaged that these studies should be extended to the community in order to utilize critically the results obtained in the metabolic investigations for developing dietary guidelines and recommendations.