|Protein-Energy Requirements of Developing Countries: Evaluation of New Data (UNU, 1981, 268 p.)|
|Discussions and recommendations of the task forces|
The working group on energy requirements for children and energy-protein relationships was asked to address the following major questions:
1. What is the effect of various factors on the digestion and absorption of dietary energy?
2. What is the consequence of energy deficits for population groups; of seasonal variation in energy requirements; and of adaptation to chronic or seasonal energy deficits?
3. What is the influence of dietary energy on protein metabolism and nitrogen balance?
4. What is the influence of dietary protein on energy metabolism and nitrogen balance?
5. What is the significance of protein-energy ratios of bulk, energy density, and the fat content of diets?
The Energy Requirement of Children
1. The evidence from recent studies in Thailand and at INCAP shows that, by the classical criteria of weight gain and nitrogen balance, a net intake (measured by bomb calorimetry) of about 90 kcal/kg is adequate for children 2-3 years old.
It appears from these studies that net intake, determined by bomb calorimetry, is about 10 per cent lower than the calculated intake based on the Atwater factors (see table 1). If the FAD/WHO (8) estimated requirement of 101 kcal/kg at 1-3 years is reduced by 10 per cent, it becomes identical with the figure of 90 kcal/kg obtained by direct measurement. The consideration of net energy intake is essential if one is to compare energy intake with expenditure. When comparisons of energy intake are being made with requirement recommendation figures, or with other energy intake figures, both should be expressed in the same terms, that is, both based on Atwater factors. However, further work is needed on the comparison of conventional measurements of intake with those obtained by bomb calorimetry of food and faeces.
2. The criteria used for estimating energy requirements need to be critically reviewed. "Adequate" growth is usually assessed by reference to western standards. These may be inappropriate, not so much because of possible genetic variations in growth potential but because the average western child could be somewhat overweight.
Even in the most carefully conducted trials, the relation between nitrogen retention and weight gain appears to be quite variable. This can be explained partly by variations in the amount of fat and lean tissue deposited. It has been shown that children on the same diet do differ in the composition of tissue laid down (9). As a result, the expectation expressed at the 1977 FAD/WHO (8) meeting, that nitrogen balance might be the most sensitive criterion of the adequacy of energy intake, seems not to be substantiated by these findings.
TABLE 1. Energy Requirements and Nitrogen Balance: Mean Daily Protein and Energy Intakes
|Theoretical intake: 1.75||Actual intake: 1.73||Absorbed:* 1.14|
Source: B. Tord F. Viteri, unpublished INCAP data, 1980.
0 Coefficients of variability between 3 and 5 per cent.
* "True" N digestibility = 66 9 per cent.
** Gross intake measure by bomb calorimetry.
*** Net intake = gross intake-faecal energy (bomb calorimetry).
There also seems to be little justification for the suggestion (3) that for nitrogen retention to be considered "adequate" in children 1-3 years old it should be at least 70 mg/kg/day. The expected retention for growth at this age would be more nearly in the range of 15-25 mg/kg/day. Substantial variation from day to day is to be expected; what matters is that the average retention over a period should reach the level indicated.
3. Almost important criterion of the adequacy of energy intake is that it should support a "satisfactory" level of physical activity. There is evidence from one sutdy (INCAP) that the initial response to a fall in energy intake is a decrease in expenditure rather than in growth or nitrogen retention. Physical activity promotes not only skeletal growth but also interaction with the environment and hence stimulates mental development. Estimates of physical activity are not easy when it is measured, and the definition of what is "satisfactory" must be subjective. Nevertheless, observations of physical activity should be regarded as essential in all future studies of this kind, and a decrease in physical activity below "normal" should be avoided.
4. The rate of linear growth is, for many purposes, a better measure of the nutritional state of young children than the rate of gain in body weight. This criterion should be used whenever possible in future studies of the adequacy of energy intakes, but a longer period of observation will be necessary and it must be certain that protein is not a limiting factor. Thought needs to be given to other possible criteria, particularly those that measure outcome over an extended period.
5. It is clear from measurements of food intake that throughout the third world the energy intakes of young children frequently fall below the estimated requirement. A common estimated intake would be 75 kcal/kg/day (calculated), representing a deficit of about 25 per cent. The most obvious manifestations of this deficit are reductions in weight for height and in physical activity.
A commonly observed pattern of change in developing-country populations is that growth in weight-forheight begins to fall at 3-6 months, reaches a minimum at 12-18 months, and then returns toward normal by three years, leaving a child who is stunted but not necessarily underweight.
Weight for height is usually assessed in relation to western standards, and some percentage of the median standard is commonly taken as a cut-off point separating those who require action ("malnourished") from those who do not require action ("normal" or mildly undernourished). A cut-off point of this kind needs to be related to measures of risk-of death, morbidity, or impairment of function, A start has been made in several centres on the assessment of risk, but much more needs to be done. When weight deficits are being related to morbidity, it is essential to separate children into age groups, because the risks associated with a given deficit vary with age.
6. In all parts of the world there appear to be seasonal variations in growth. In poor countries these may be attributed to variations in food intake and in disease transmission; in rich countries the cause is not clear. It may, perhaps, be a difference in the level of physical activity. These seasonal effects must obviously be taken into account in comparative studies.
7. One of the most important questions for the future is the significance of a child's adaptation to deficits in energy intake. Adaptation is difficult to define, and this term has been used with a number of different meanings. Nevertheless, the general concept is an essential component of modern thinking in biology and nutrition.
One form of adaptation to shortage in food supply is a decrease in growth. It has been suggested, for example, that stunting is an adaptive change. Another is a decrease in physical activity. A third is a change in the efficiency of the utilization of food for metabolic and mechanical work. In all these cases the balance of advantage and disadvantage of the adaptive change to the well-being of the individual should be considered.
Energy and protein metabolism are closely interrelated. Several short-term (10-day) studies have shown that the intake of protein for zero nitrogen retention decreases with increasing energy intake, not only when the starting level is at deficient energy intake levels but also when intake reaches excess levels. A long-term study in Thailand (40 days) failed to show a decrease of nitrogen retention with decreasing energy intakes to levels considered 10 per cent below requirements and associated with lower energy expenditure and rate of weight gain. It would appear from present evidence that long-term studies are warranted that are based on energy intake and activity level immediately preceding the study.
On the other hand, a reduction of protein intake while energy intake remains constant can reduce the rate of growth in terms of weight and height even when nitrogen balance is not negative. The mechanisms for this effect are not clear. Apart from the specific dynamic action of proteins, a high protein intake does not appear to affect energy requirements.
Effects of Energy Intake on Physical Activity and Growth
Apathy and low physical activity have been observed in children of populations where energy deficiency is prevalent. Recently, quantitative estimates of physical activity and energy expenditure of children on energy intakes below estimated requirements have shown proportional decrements in the activity component of energy expenditure.
Moreover, such decrement takes place rapidly (within a week) in one- to three-year" old children when energy intake is reduced experimentally to levels slightly below estimated energy requirements. Energy conservation by a reduction in activity thus appears to be a response to deficient energy intakes in children as well as in adults.
Recent studies in animals and children in rapid growth phases and receiving sub optimal energy intakes have shown that a programme of enforced physical activity is associated with increased linear and lean body mass growth when compared with pair-fed but less active animals and children (10). These findings suggest that physical activity is necessary for adequate growth and that it enhances the efficiency of energy and/or protein utilization. They also point out the desirability of the maintenance of physical activity in children in metabolic studies aimed at exploring dietary energy protein interrelations, particularly when growth is considered a dependent variable. This has not been considered in most past studies.
Significance of Protein-Energy Ratios: Bulk, Energy Density, and Fat Contents of Diets
From all available evidence, a protein concentration above 7.5 per cent of calories when corrected for quality appears not to bring additional benefits to healthy children. For children recovering from protein energy malnutrition, infection, or other stress, this proportion should be higher, but probably need not be beyond 12 per cent. The lower limit for this value of corrected protein-energy intake should be above 5 per cent for healthy pre-school children. This should be kept in mind in attempts to increase the energy density of diets based on cereal-legume mixtures.
Studies on pre-school children fed five times a day on a free choice of corn and bean preparations common in their habitual diets have proven that these foods can be consumed in amounts that fulfil safe protein intakes, but that energy intake was inadequate and resulted in poor growth (weight) gains. An increase in fat calories from 8 per cent to near 20 per cent, and energy density from 4.5 to 4.9 kcals/g food (9 per cent increase), overcame both the energy intake deficit and the poor weight gains. Further addition of fat brought no additional improvement.
Addition of fat to infant and pre-school-child food preparations not only increases the energy density but also facilitates the swallowing of the solids and porridges that otherwise may be too gelatinous [agglutinated) for easy consumption by young children. This, and probably bulk, impair the capacity of cereal-legume mixtures, without fat added in appropriate amounts, to fulfil energy requirements.
Bulk-energy density-fat interrelationships in cereal-legume diets require very active research, including consideration of population beliefs and practices about infant feeding of such foods.
Infection and Catch-up Growth
Infection leads on the one hand to anorexia and decreased food intake, and on the other to losses of energy, nitrogen, and other body components. The extent of these changes is highly variable according to the nutritional status of the host and the severity and type of disease. In order to determine the effect of infections on the requirement for energy and nutrients, the magnitude of these losses must be known. In view of the range of variation, it will be difficult to draw realistic general conclusions from direct measurements on small numbers of children under closely controlled metabolic ward conditions.
A promising approach, which needs to be more widely pursued, is to establish the relationship under field conditions, and on a population basis, among the three variables: frequency, severity, and duration of the infection; food intake; and growth. It should then be possible to estimate the extent to which, on average, infections contribute to growth deficit, and hence to calculate the extra intake of energy and protein needed to make good the deficits, i.e., for catch-up. A beginning has been made with this kind of study in the Gambia, Peru, and Guatemala.
When one attempts to calculate the requirements for catch-up 17) two points emerge. First, relatively more protein is needed for weight gain (assuming normal balanced tissue) than for maintenance; therefore the protein-energy ratio in the diet needs to be somewhat higher than normal (the exact value will depend on the rate of catch-up aimed at). Second, catch-up growth obviously requires an increased intake of both protein and energy, but the increase is probably four to five times greater for protein than for energy. Quantitative estimates of the intakes needed for various rates of catch-up growth were tabulated in a previous UNU report 17). These estimates were based on the assumption that tissue of balanced composition is being laid down, and on observed values for the energy cost of weight gain 111). Studies on children recovering from malnutrition in hospital have shown that these estimates are realistic. For example, a net intake of 150 kcal and 3.5 9 protein/kg/day (P:E ratio about 10 per cent) will, support rates of weight gain of up to 10 g/kg/day.
In practice, the main difficulty in securing intakes adequate for catch-up growth is that even the child whose appetite is good may be physic ally unable to eat the necessary quantity of food. In this situation, the nutrient density of the food and the extent to which it is glutinous or easily swallowed become matters of great importance.
Research under metabolic ward conditions has been, and continues to be, essential in the efforts to define as accurately as possible protein and energy requirements and the mechanisms involved. Upon seclusion in metabolic wards, however, the inevitable changes that take place in living habits and very often in levels and type of energy and protein intakes, as well as the limits of time of study imposed by these conditions, make it very difficult to translate the data into "real-life, natural" conditions and into "safe levels of intake" for different population groups.
Research should be directed to develop techniques and evaluate methodology that would allow investigators to conduct field research in protein and energy nutrition as closely as possible to the accuracy and precision of metabolic ward conditions. This should allow the scientific community to obtain quantitative information on an adequate number of subjects under free-living conditions.
A series of population groups in different food and ambient ecological settings whose intakes are considered low could then be selected. A series of measurements could be made, aimed at defining their characteristics in terms of protein and energy nutrition. if the indicators used for this purpose indicate "normal population behaviour," one could try to identify populations with even greater intake deficits, if available.
In any case, the proof that energy and/or protein intakes are not limiting should be ascertained by means of nutrition interventions. The choice of indicators to detect the normal or abnormal behaviour of population groups is critical. Measurements of good health and adequate performance were proposed, such as rate of linear growth for age, lean body mass and muscle mass for age, low morbidity, milk production, birth weight, physical work capacity and fitness, and immune responses.