|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 adults 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?
1. Host Effects
Genetic: It is impossible, in metabolic studies, to separate the effects of genetic and environmental factors on variation in nitrogen and energy absorption. It may be possible, however, by studying different populations under similar conditions, to obtain information on this point, although this is doubtful and has not been achieved to date. This comment applies to protein. When it comes to differences in digestible energy, there is little doubt that differences in lactose hydrolysis associated with differences in intestinal lactase activity have a genetic basis.
While the net effect on digestible energy is small, because lactase-deficient individuals are not likely to have milk as a major energy source in their diet, it would be prudent to be alert for other genetic differences in utilization of energy sources within or among different populations. The fact that low lactase levels can also be induced by environmental factors does not change the preceding argument. It does suggest, however, that, where milk is used as a sole source of protein in experimental studies, this factor should be considered in the interpretation of the data.
Intestinal parasites: Data were presented at the meeting (5) to suggest that mild-tomoderate infections with intestinal helminths have minimal or no detectable effects on nitrogen and energy digestion and absorption. Severe infections, however, do measurably reduce protein absorption, and in some populations where the burden of intestinal helminths is heavy this can have public health significance. A similar general statement cannot be made for intestinal protozoa. Those not associated with the production of diarrhea probably have little effect, although impaired absorption of nitrogen has been found with Giardia infections (M. Gupta, unpublished INCAP data).
The most important infections for most developing-country populations are those associated with acute and chronic diarrhoea. Since the effect is so variable, depending on the nature, frequency, and severity of the diarrhoea, it is difficult at present to give any quantitative estimates of the consequence of diarrhoea infections on nitrogen digestibility and absorption in actual "field" situations.
2. Energy Deficits in Population Groups
Appropriateness of dietary energy standards: It is not possible to make valid global recommendations for mean energy requirements. It must be assumed that, for adult populations who are maintaining body weight and composition, energy intake and energy expenditure are in balance in spite of wide variations in individual intakes. The consequences for pregnant and lactating women, however, may be lower birth weight babies that experience greater morbidity and mortality during infancy and reduced breastmilk output during lactation.
For adults and children an adaptation of great significance is a reduction in physical activity, although growth may also be affected. However, the observation that most populations in developing countries are consuming considerably less than current FAD/WHO estimated mean calorie requirements does not necessarily mean that they are deficient in calories. It may indicate that the current requirement estimations are too high, or that the individuals have been forced to reduce their physical exertion for work, recreation, or social organization.
The biological and social consequences of low energy intakes may be partly mitigated by metabolic alterations and by altering patterns of activity, both resulting in greater efficiency of utilization of dietary energy. To some extent, individuals or societies may have adapted to a lower availability of dietary energy, and a sudden increase in dietary energy would not necessarily be wholly beneficial. For example, it might lead to a greater prevalence of obesity.
Seasonal affects: Even though long-term adaptation to low dietary energy intakes may occur, as evidenced by survival of a population, this may conceal important seasonal effects. Under the conditions prevailing among lower socio-economic groups of a number of developing countries, individuals, particularly women, experience a significant loss of weight during the season of the year when food is most scarce and costly, and regain it when the harvest season arrives (6).
This has two particularly serious adverse consequences. First, women who are in late stages of pregnancy during the adverse season have infants with lower birth weights and higher infant morbidity and mortality. Second, there may also be an adverse effect on lactation performance. Also, periods of food shortage often come just at the time when there is a need for considerable energy expenditure in agricultural labour, hence a resulting weight loss. However, under experimental conditions, change in energy intake may have significant effects on nitrogen retention. It is clear that it is extremely important to adjust the energy intake of studies designed to determine protein requirements to one that is appropriate for the subjects and not associated with any long-range change in body weight or composition. Since the latter cannot be determined precisely in periods of less than a month, this means that definitive studies to establish protein requirements must be conducted over relatively long periods of time.
3. Effect of Dietary Protein on Energy Requirements
It is known that, with an energy intake that is borderline or adequate, an increase in protein intake can result in weight gain. The functional implications of this are not understood. It does appear, however, that energy requirements for maintenance of weight and body composition may be less when dietary protein is adequate.
4. Significance of Protein-Energy Ratios: Bulk, Energy Density, and Fat Content of Diets
The protein-energy ratio of a diet has been proposed as a means of evaluating the diet's ability to meet human protein needs when consumed at levels sufficient to meet energy requirements. Unfortunately, since energy requirements of individuals and populations depend on physical, biological, and social factors in the environment, no single set of protein-energy ratios can have general validity.
A further reason why use of protein-energy ratios in diets must be approached with great caution is the lack of any fixed association between relative protein and energy requirements. In fact, the environmental circumstances-physical, biological, and social-that require an adaptation to low dietary energy intakes are likely to be associated with factors such as infections, parasites, and low digestibility of diets that lead to increased protein requirements compared with those of more privileged populations. It is particularly hazardous to assume that protein-energy ratios calculated for populations have any significance for individuals because of this disassociation between energy and protein requirements.
It is still valuable to know the concentration of each in the diet relative to the amount consumed or consumable if the distribution of protein and energy requirements for a specific population is known. It will sometimes be evident that the percentage of protein relative to energy in a diet is grossly inadequate to meet protein needs of specified groups even if enough of the diet could be consumed to meet energy needs. This is particularly likely to occur for young children during periods of recovery from severe clinical malnutrition and/or infection, when the percentage of protein calories required is further increased (7). The low protein-energy ratios of diets consisting mainly of a cereal or cassava as the major energy source must be improved by either legumes or a source of animal protein.
5. Constraints Due to Dietary Bulk for Energy and/or Protein Density
It is not necessary to define or specify a protein-energy ratio in order to examine the adequacy to meet the needs of specific target groups of the protein and energy concentrations of diets relative to their bulk. However, it is necessary to determine whether enough of the diet can be consumed to meet energy needs and whether, if this is the case, protein needs are also met. It is not uncommon for underprivileged, vulnerable groups in the developing countries to be consuming habitual diets in which sheer bulk and energy density are constraints.
It is possible that when cassava, starch, or sugar becomes a major component of the diet, protein density is a constraint. Diets consisting almost entirely of cereals are likely to be inadequate in both protein and energy density. As indicated above, children recovering from severe malnutrition and/or infection are particularly vulnerable to such constraints, and it may often be necessary to add fat to the diet to provide sufficient energy density, or a more concentrated source of protein, or both. This means that an improvement in the quality and not just the quantity of the habitual diet is required.