FOOD AND AGRICULTURE ORGANIZATION OF THE UNITED NATIONSESN:FAO/WHO/UNU/
EPR/81/19

August 1981
WORLD HEALTH ORGANIZATION
THE UNITED NATIONS UNIVERSITY

Provisional Agenda Item 2.4.2

Joint FAO/WHO/UNU Expert Consultation on
Energy and Protein Requirements

Rome, 5 to 17 October 1981

ENERGY REQUIREMENTS OF SCHOOL-AGE
CHILDREN AND ADOLESCENTS

by

G. Debry
University of Nancy
France

In 1973, Hegsted (1) wrote: “it would be easy to say that we know practically nothing about adolescents' requirements in energy, fats, proteins and amino-acids, and then sit back. This would not be very far from the truth, but not very useful”.

What do we know eight years later?

Hegsted's remark is still applicable, because too little research has been done on this matter since the report of a Joint FAO/WHO adhoc Expert Committee which met in 1973.

The scientific meetings that have concerned themselves with this question since have called attention to many conceptual and methodological difficulties.

Evaluation of energy requirements has therefore become more and more complex.

1. CONCEPTUAL DIFFICULTIES

1.1 The concept of recommended intake

In order not to go over again the discussions on terminology by the previous expert committees (2,3,4,5), we shall agree (5) that the “recommended intake” is “the amounts considered sufficient for the maintenance of health in nearly all people”, but that it should be called (5) “safe level of intake” for proteins. The energy requirements is defined(5) as “the energy intake that is considered adequate to meet the energy needs of the average healthy person in a specified category”.

Two points need to be discussed, however:

In fact, since the 1973 report (5) the idea of minimum requirements as conveyed by the expression “amount considered adequate” has changed.

In previous reports by experts the main concern was, quite rightly, to determine the requirements for peoples living in the developing countries, and the criteria for good health were essentially attainment of the height and weight considered normal for the age in developed countries.

Today the concept of optimum health (6,7,8,9), i.e. the best possible state of physical and psychic health, is acquiring increasing importance. It is known, in fact, that an energy and protein intake which ensures a height and weight considered normal may not be identical with the optimum requirement. The many experiments carried out with animals, which it is not possible to review here, and the connection suspected to exist between the level of energy in the diet, particularly the proportion of the various nutrients comprising it, and degenerative complications, particularly cardio-vascular complaints, prompts concern with the optimum requirement. The same might apply to certain cancerous processes.

Since it is difficult to assess optimum requirements, it might be useful to try and determine minimum and maximum values for the safe level of intake.

We think these concepts are of great importance in dealing with childhood and adolescence, periods during which the body composition is being gradually established. Finally, if an increase in longevity is considered to be a desirable objective and if the work done with rodents on the connection between food patterns throughout life and longevity is of some relevance to human beings, then these considerations become very necessary.

1.2 The concept of the criterion of good health

In the approach used hitherto, the concept of the criterion of good health is not clearly defined. Each criterion expressed by a quantitative value can no longer be considered then, solely as a value signifying present good health, but also as a value predicting future good health. In an epidemiological approach such criteria could therefore be of three kinds (7):

We have intentionally avoided using the adjective “normal”, because how can a normal value be defined? The adjectives used in (a), (b) and (c) can be queried, their only merit is that they do not have a normative or predictive significance.

II. METHODOLOGICAL DIFFICULTIES

2.1 Choice of reference populations

Without being as pessimistic as Mueller (6), who affirms that “the stated RDA data base for adolescents is meagre and totally inadequate”, it must be admitted that most of the studies that have been made in the developed countries have been conducted in institutions, hospitals or schools. It is unfortunately difficult to do otherwise, but this fact must be remembered in interpreting the results. Food habits, frequency and amount of the energy intake at each meal also have en affect on the metabolic utilization of the nutrients, as has been shown particularly in the case of obese people (10, 15), when the energy intake is not too reduced. It is therefore necessary to bear in mind food habits which are extremely different in a population of children and adolescents (11, 12).

2.2 Choice of time of measurement

Study of spontaneous food intake shows that it obeys diurnal, hebdomadal and annual rhythms (11, 13, 14, 15) whose effects on the utilization of nutrients are still being discussed. No assessments have been made of requirements in children and adolescents during the course of the four seasons. In addition, the effects of energy expenditure on energy intake requirement are not immediate and many studies have shown that weight regulation is not achieved quickly. It takes a long time for intake to be adjusted to expenditure of energy.

2.3 Choice of measurement technique

In this paper it is not possible to go in detail into the advantages and disadvantages of each technique for measuring energy intake and expenditure. We will just state, simply, that the more accurate the technique, the more restrictive it is, the more it risks altering spontaneous behaviour and the more difficult it is to apply over a long period, particularly in children and adolescents.

2.4 Difficulties of evaluating energy requirements

Energy requirements may be evaluated in two ways: either by measuring the amount of energy consumed by healthy children and adolescents; or by measuring the energy expenditure direct.

a) Measuring the amount of energy consumed

In addition to the remarks already made, it is useful to ask how the results should be expressed and what is their physiological significance.

- Should the results be expressed in absolute values, by kilogramme of real weight, by kilogramme of ideal weight or by square metre of surface area? Should they be specified for each year, or can age groups be used?

All these ways have been used both in the reports of international organizations (FAO/WHO) and national agencies (NRC - National Academy of Science - INSERM) and in the surveys carried out in various countries.

The studies we have conducted, only some of which have been published so far (13, 16), have shown us that for a healthy child aged 3 to 6 years on spontaneous feeding, food consumption should be expressed either as an absolute value or by kilogramme of weight for each year of age. If it is expressed by square metre of surface area, the values are identical whatever the age. This appears to be no longer valid at the time of growth during adolescence.

- What physiological significance can be given to the results obtained?

It is difficult to know the precise physiological significance of the various ways of expressing results referred to above; or what ideal weight in a child or adolescent means. There is no doubt, of course, that an under-nourished or obese child is not in a satisfactory physiological condition. But between these two extremes, where does the limit between a physiological and a pathological situation lie?

In the Ten-State Survey (17, 18) the energy intake of black children is lower than that of white children at all ages. Yet at 16 years the height and weight of girls of the two races are very similar. It is not the same for the boys, the white ones being a little taller and heavier. But what is the ideal height? We always relate weight to height, but we have no way of deciding the ideal height.

In a survey conducted in Glasgow (19), the food consumption of adolescents was found to have fatten significantly between 1964 and 1971, but the weight of the children at each age did not vary - on the contrary, they became fatter.

In the developing countries surveys of children and adolescents of both sexes aged between 1 and 18 years have shown that the energy intake was distinctly lower than the recommended intake, yet there was no great deficit in weight for age nor signs of malnutrition (in New Guinea 20,21 - in Peru 22, 23). Younger children (2 and 3 years old), in Uganda, consume 70 percent of the recommended energy intake for British children. But their height and weight gains are identical to those of these British children (24). It is therefore possible that the body adapts to a reduced energy intake; but what does this adaptation signify in terms of good health? Does it affect resistance to effort, immune defence, longevity, etc.? What effect does excess or a moderate shortage of food have on metabolic activities? In the Ten-State Survey growth is less rapid in young black girls, but height is normal at 16 years. In many surveys it is not possible to obtain a proportional relationship between changes in weight, energy intake and energy expenditure. What are the energy dissipation processes? What is their significance in terms of health?

b) Direct measurement of energy expenditure

Basal metabolism has been expressed in terms of the absolute weight value, the square metre of surface area and the weight value 0.75. As Talbot (25) demonstrated, and despite what is stated by many authors, the basal metabolism value does not change parallel to surface area or changes in weight 0.75. The discrepancy is greatest when the body weight is between 10 and 30 kg (26). Expressed by kilogramme of weight, basal metabolism decreases with age. It is 34 Kcal/min/kg for a child of 3, 26 Kcal/min/kg for a child weighing 30 kg, and 19 Kcal/min/kg for an adolescent of 17 (27). Expressed in relation to total weight, it is highest as an absolute value at puberty (8).

- Physical activity

The apparently logical connection between muscle mass, physical activity and energy intake has, in fact, not been demonstrated. In subjects who engage in strenous physical activity, the greatest energy expenditure is not proportional to the muscle mass (26). From 4 to 18 years, increase in energy expenditure at sub-maximal level is inversely proportional to age (28, 29).

From 8 years on, the aerobic capacity per kilogramme of body weight decreases in girls. There is a clear difference from boys. This is probably due to an increase in fatty mass and a decrease in muscle mass in body composition (29, 30, 31).

Energy expenditure due to physical activity is rarely measured directly (ref. 8–27). Such expenditure is very variable in the developing countries, but generally low in the developed ones (8). Thin individuals usually have a higher energy intake than fat ones, owing not only to higher basal metabolism but also to greater physical activity (32).

2.5 Variations in energy requirements

Energy requirements vary greatly from one individual to another. This fact is obviously due to the different circumstances in which the subjects are placed, such as the level of physical activity and the climate, but also to three other factors: growth, nutritional level, individual variations.

a) Growth

This idea has been well documented in the 1973 FAO/WHO expert report (5). According to Forbes (8), the energy cost of the growth of the lean mass and the fat mass from 10 to 20 years can be estimated at 13 Kcalories per day in males and 7 Kcalories per day in females. The requirement at maximum growth is 66 Kcalories per day in males and 123 Kcalories per day in females. However, maximum growth does not occur at the same age for all adolescents, so that requirements may differ at the same age.

b) Nutritional level

The energy cost of weight gain is very different in periods of satisfactory nutrition and during periods of nutritional recuperation after considerable malnutrition. We have no data for children, but it is probable that the same is true, to a lesser degree, as for infants. In the latter (33–34), the energy cost of a gramme of tissue varies, according to the authors, from 4.4 to 10 Kcal/g. More recent studies (34) assess it at 5 Kcal/g of tissue, or 15 to 20 times the energy cost observed in well-nourished infants of the same age.

c) Individual variations in energy requirements

Children and adolescents are not equal as regards energy metabolism.

Various surveys clearly show that there is considerable variation in spontaneous food consumption by children of the same age. These differences may be due to different conditions of life within a region (35), but they are also observable in healthy children living together, with very similar physical activity and with ample food supplies (11, 13, 16). These facts prove that a satisfactory nutritional status can be obtained with energy intakes of different levels.

CONCLUSIONS

The purpose of the above data is just to call attention to a certain number of facts that have not yet been resolved and which are a nuisance in determining recommended energy intakes.

The variations in intake levels recommended both by international organizations (2, 3, 4, 5, 36) and by national agencies (8) reflect these difficulties. Finally, the fact that habits change means that recommendations should be periodically revised and modifications in the level of physical activity of children and adolescents attentively monitored.

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