4. ENERGY REQUIREMENTS OF CHILDREN AND ADOLESCENTS

In the 20 years since the 1981 joint FAO/WHO/UNU consultation (WHO, 1985), significant experimental evidence has been collected on the TEE of children and adolescents. This makes it possible to estimate energy requirements from measurements of TEE and energy needs for growth, rather than from food intake data or from estimates of time allocation and energy costs, as were previously used. The 2001 expert consultation analysed a number of studies on TEE, growth and habitual activity patterns of children and adolescents in different parts of the world (Torun, 2001). As the objective of this report is to make recommendations for healthy, well-nourished populations - thus excluding data from undernourished, overweight and stunted groups - the analysis was restricted to information from groups of healthy, well-nourished individuals.

4.1 Measurement of total energy expenditure

Studies using the DLW technique were the starting point for estimating energy requirements of children and adolescents. However, most of the existing data on TEE measured with DLW were obtained in industrialized countries, where energy expenditure is influenced by modern technology, school environments, sedentary pastimes, mechanized transportation and social and economic support systems that demand relatively low physical effort (i.e. in developed countries or affluent societies) compared with countries and societies where cultural, economic, social and developmental circumstances require greater physical effort from an early age (i.e. in developing countries or poorer, largely rural societies). On the other hand, several investigations on TEE of healthy, well-nourished children and adolescents have been done in a broader spectrum of countries and societies using minute-by-minute heart rate monitoring (HRM) and individual calibrations of the relationship between heart rate and oxygen consumption. The mean TEE measured with this technique is comparable with the mean value obtained using DLW or whole body calorimetry (Spurr et al., 1988; Ceesay et al., 1989; Livingstone et al., 1990; Livingstone et al., 1992; Emons et al., 1992; Maffeis et al., 1995; van den Berg-Emons et al., 1996; Davidson et al., 1997; Ekelund et al., 2000). Therefore, studies using either DLW or HRM were included in this evaluation in order to encompass data on children and adolescents with a wider variety of lifestyles, and to include age groups with limited information, if based on DLW alone.

The studies reviewed for this consultation involved a total of 801 boys and 808 girls of one to 18 years of age (Torun, 2001). Most (56 percent of the boys, 68 percent of the girls) were from the United States or the United Kingdom; 18 percent of the boys and 18 percent of the girls were from Canada, Denmark, Italy, Sweden or the Netherlands; and 26 percent of the boys and 14 percent of the girls were from Brazil, Chile, Colombia, Guatemala or Mexico. The Latin American children were four to 15 years old, and all lived in urban areas. Inter-individual coefficients of variation ranged from 9 to 34 percent within studies with DLW, and from 9 to 27 percent within studies with HRM. The overall mean coefficient of variation was 19 percent for energy expenditure calculated as TEE/day, and 17 percent when calculated as TEE/kg/day. The inter-individual variability was similar to that observed with DLW among infants (18 percent for TEE/day, and 15 percent for TEE/kg/day; see section 3.1).

4.2 Equations to predict total energy expenditure

Predictive equations were derived from the studies of TEE. Because many publications did not present results on individual children, the mean values for boys or girls of a specific age, or within a reasonably narrow age range, were used in the calculations, weighting the results of each study on the number of children. Various mathematical models (e.g. linear, multiple, polynomial, etc.) were evaluated, with age and/or body weight as predictors of TEE. Age and weight were highly correlated, with a tolerance of 0.078 among boys and 0.061 among girls. Weight was selected as the single predictor, since it played a greater role than age in predicting TEE, and the exclusion of age from the predictive models did not increase the error of the estimate. The lowest errors of estimation were obtained with the following quadratic polynomial regression equations for boys and girls (Figure 4.1) (Torun, 2001):

Boys:

TEE (MJ/day) = 1.298 + 0.265 kg - 0.0011 kg²; n_weighted = 801, r = 0.982, r² = 0.964, see = 0.518
TEE (kcal/day) = 310.2 + 63.3 kg - 0.263 kg²

Girls:

TEE (MJ/day) = 1.102 + 0.273 kg - 0.0019 kg²; n_weighted = 808, r = 0.955, r² = 0.913, see = 0.650
TEE (kcal/day) = 263.4 + 65.3 kg - 0.454 kg²

The equations were validated internally by dividing the studies into model-building sub-samples (70 percent of the studies, n = 549-618 boys or girls) and validation sub-samples (30 percent of the studies, n = 183-252 boys or girls). The validation sub-samples were randomly selected for each gender after stratifying the studies on quintiles of mean body weight - the method used to measure TEE (DLW or HRM) - and categorizing according to whether the study was done in an industrialized or a developing country. The correlation coefficients of the quadratic equations derived from the model-building sub-samples ranged from 0.959 to 0.982, with standard errors of the estimate from 0.504 to 0.651 MJ/day. Mean differences between predicted and measured values among boys were within ± 1 percent, with a standard deviation of 6 percent; and among girls, within ± 3 percent, with a standard deviation of 9 percent (Torun, 2001).

4.3 Energy needs for growth

Energy needs for growth have two components: 1) the energy used to synthesize growing tissues; and 2) the energy deposited in those tissues, basically as fat and protein, because carbohydrate content is negligible. Energy spent in tissue synthesis is part of TEE measured with either DLW or HRM. Hence, only the energy deposited in growing tissues was added to TEE in order to calculate energy requirements.

Table 4.1 shows the mean weight gain of boys and girls calculated from the WHO weight-for-age standards (WHO, 1983). The composition of weight gain was based on measurements at one and two years of age (Butte et al., 2000; Butte, 2001), assuming that the composition of normally growing tissues does not change much between the end of infancy and the onset of puberty. It was estimated as 10 percent fat with an energy content of 38.7 kJ/g (9.25 kcal/g), 20 percent protein of 23.6 kJ/g (5.65 kcal/g) energy content, and 70 percent water, carbohydrate and minerals with negligible content of energy. The average energy deposited in growing tissues was then about 8.6 kJ (2 kcal) per gram of weight gain. Even if this amount of energy were an over- or underestimation as large as 50 percent, it would only produce an error of about ± 1 percent in the calculations of energy requirements in childhood and adolescence.

4.4 Calculation of energy requirements

TEE was calculated using the predictive quadratic equations and the WHO reference values of weight-for-age (Torun, 2001; WHO, 1983). The median weight at the midpoint of each year of age was used for the ages of between one and 17 years (i.e. median weights at 1.5, 2.5..., 17.5 years). At the lower end of the weight distribution, which corresponds to infants between one and two years of age, predicted values were about 7 percent higher than the actual measurements of TEE. When reduced by that percentage, TEE estimates fell in line with those of 12-month-old infants (Butte, 2001). The small transient increment in TEE/kg/day between one and three years is probably associated with the effort of children starting to walk and run.

Energy deposited in growing tissues was estimated by multiplying the mean daily weight gain at each year of age (Table 4.1), by the average energy deposited in growing tissues (8.6 kJ or 2 kcal per gram of weight gain). The sum of energy deposition and TEE is the mean daily energy requirement (MJ or kcal/day, Tables 4.2 and 4.3). This was then divided by the median weight at each year to express requirements as energy units per kilogram of body weight.

FIGURE 4.1
Quadratic polynomial regression of total energy expenditure on body weight, weighting each data point by the number of children in the study

BOYS

GIRLS

Boys: y = 1.298 + 0.265x - 0.0011x²; n_weighted = 801, r = 0.982, see = 0.518.
Girls: y = 1.102 + 0.273x - 0.0019x²; n_weighted = 808, r = 0.955, see = 0.650.
Solid circles: DLW, industrialized countries.
Solid triangles: HRM, industrialized countries.
Clear circles: DLW, developing countries.
Clear triangles: HRM, developing countries.
Source: Torun, 2001.

TABLE 4.1
Mean weight gain of boys and girls, one to 17 years of age

Source: Calculated from WHO references of weight by age (WHO,1983).

BMR was estimated by using the equations endorsed in the report of the 1985 FAO/WHO/UNU expert consultation (Schofield, 1985), and upheld by this consultation (section 5.2, Table 5.2), using the median weight for every year of age. Mean PAL was calculated as a multiple of BMR, dividing total energy expenditure by the estimated BMR. As discussed in section 3.4.2, PAL calculated in this manner is on average 1 percent lower than when daily energy requirement is divided by BMR (James and Schofield, 1990) because growth contributes that proportion to the total energy requirement in childhood and adolescence. Thus, to estimate the energy requirement, the energy accrued during growth must be added, or the PAL value of children and adolescents must be multiplied by 1.01 (i.e. to make it 1 percent higher).

4.4.1 Comparison with previous requirements

In Table 4.4 and Figure 4.2 the new requirements are compared with those of the 1985 report. The cross-over of the curves at ten to 11 years is most probably artificial and the result of the different approaches used by the 1981 consultation to calculate requirements of children under ten years of age (dietary intake) and over ten years (factorial estimate of energy expenditure) (WHO, 1985). Compared with previous estimates, energy requirements proposed by this consultation are on average 18 percent lower for boys and 20 percent lower for girls under seven years of age, and 12 and 5 percent lower, respectively, for boys and girls seven to ten years of age. From 12 years onwards, the proposed requirements are an average of 12 percent higher for both boys and girls.

4.4.2 Influence of habitual physical activity on energy requirements

Energy requirements vary with the level of habitual physical activity. Most studies of TEE were carried out on random or convenient subject samples. Children and adolescents in these samples had different levels of habitual activity, resulting in inter-individual coefficients of variability as high as ± 34 percent (Torun, 2001). Thus, the values shown in Tables 4.2 and 4.3 may be regarded as the requirements of child and adolescent populations with "average" or "moderate" (i.e. not predominantly sedentary nor vigorous) physical activity.

Children and adolescents in rural, traditional communities in developing countries are more active than their counterparts in urban areas or in developed, industrialized countries. The quantitative differences were assessed from factorial estimates of TEE, calculated from 42 studies with time allocation data that involved approximately 4 000 boys and girls in industrialized countries, and 2 400 in rural or urban areas of developing countries (Torun, 2001; 1996). On average, TEE of boys and girls five to nine, ten to 14 and 15 to 19 years of age was, respectively, about 10, 15 and 25 percent higher in rural developing countries than in cities or industrialized countries. Based on these values, on the within-study coefficients of variation of TEE measured with DLW or HRM and on the mean errors of estimation of the predictive equations for TEE, this consultation endorsed the recommendation to reduce or increase by 15 percent the requirement of population groups that are less or more active than average, starting at six years of age (Torun, 2001).

4.4.3 Requirements of populations with different levels of physical activity

Energy requirements were calculated for children over five years of age and for adolescents with lifestyles involving three levels of habitual physical activity, subtracting or adding 15 percent from the requirements shown in Tables 4.2 and 4.3 for children and adolescents with "average" physical activity. Population groups with less, similar or more than average activity were classified as leading "light", "moderate" or "vigorous" lifestyles, respectively. Their requirements are shown in Tables 4.5 and 4.6. To facilitate recollection, values were rounded to the closest 0.1 MJ (25 kcal)/day, 5 kJ (1 kcal)/kg/day, and 0.05 PAL units.

The following general descriptions may help to decide which level of energy requirement is more appropriate for a specific population group.

Examples of populations with light physical lifestyles, or that are less active than average, are children and adolescents who every day spend several hours at school or in sedentary occupations; do not practise physical sports regularly; generally use motor vehicles for transportation; and spend most leisure time in activities that require little physical effort, such as watching television, reading, using computers or playing without much body displacement.

Examples of populations with vigorous lifestyles, or that are more active than average, are children and adolescents who every day walk long distances or use bicycles for transportation; engage in high energy-demanding occupations, or perform high energy-demanding chores for several hours each day; and/or practise sports or exercise that demand a high level of physical effort for several hours, several days of the week.

Children and adolescents with habitual physical activity that is more strenuous than the examples given for a light lifestyle, but not as demanding as the examples for vigorous lifestyle, would qualify in the category of average or moderate physically active lifestyles.

4.5 Recommendations for regular physical activity

A certain amount of habitual physical activity is desirable for biological and social well-being. The regular performance of physical activity by children, in conjunction with good nutrition, is associated with health, adequate growth and well-being, and probably with lower risk of disease in adult life (Viteri and Torun, 1981; Torun and Viteri, 1994; Boreham and Riddoch, 2001). Children who are physically active explore their environment and interact socially more than their less active counterparts. There may also be a behavioural carry-over into adulthood, whereby active children are more likely to be active as adults, with the ensuing health benefits of exercise (Boreham and Riddoch, 2001).

On the other hand, sedentary lifestyles are increasing in most societies around the world, mainly owing to increased access to effort-saving technology and devices and to structural and social constraints. Examples of these are increased use of automobiles and buses for transportation, piped water and electrical appliances in the household, electronic equipment and computers in the workplace, elevators and escalators in buildings, and television sets and computers for entertainment, as well as a reduction in outdoor playing and walking caused by concerns about crime and the safety of pedestrians and cyclists. Sedentary children often eat amounts of food that exceed their relatively lower energy requirements, go into a positive energy balance and are at risk of becoming overweight or obese (Bar-Or et al., 1998; Goran and Treuth, 2001; Dietz and Gortmaker, 2001).

FIGURE 4.2
Comparison of proposed energy requirements with FAO/WHO/UNU 1985 requirements

BOYS

GIRLS

Continuous line: proposed energy requirements.
Interrupted line: 1985 requirements.
Source: Torun, 2001.

TABLE 4.2
Boy’s energy requirements calculated by quadratic regression analysis of TEE on weight, plus allowance for energy deposition in tissues during growth (E_g)

^a TEE (MJ/d) = 1.298 + 0.265 kg - 0.0011 kg².
^b 8.6 kJ or 2 kcal/g weight gain.
^c BMR_est: basal metabolic rate estimated with predictive equations on body weight (Schofield, 1985).
^d PAL_est: physical activity level = TEE/BMR_est. To calculate requirements, add E_g or multiply by 1.01 (see text).
^e Requirements for 1 to 2 years reduced by 7 percent to fit with energy requirements of infants (see text).
Source: Torun, 2001.

TABLE 4.3
Girls’ energy requirements calculated by quadratic regression analysis of TEE on weight, plus allowance for energy deposition in tissues during growth (E_g)

^a TEE (MJ/d) = 1.102 + 0.273 kg - 0.0019 kg².
^b 8.6 kJ or 2 kcal/g weight gain.
^c BMR_est: basal metabolic rate estimated with predictive equations on body weight (Schofield, 1985).
^d PAL_est: physical activity level = TEE/BMR_est. To calculate requirements, add E_g or multiply by 1.01 (see text).
^e Requirements for 1 to 2 years reduced by 7 percent to fit with energy requirements of infants (see text).
Source: Torun, 2001.

TABLE 4.4
Comparison of new proposal for daily energy requirements with the 1985 FAO/WHO/UNU report

^a % difference = new value/FAOWHOUNU × 100 - 100.
Source: Torun, 2001.

TABLE 4.5
Boys’ energy requirements in populations with three levels of habitual physical activity

Notes:

Body weight at mid-point of age interval (WHO, 1983).
Numbers rounded to the closest 0.1 MJ/d, 25 kcal/d, 5 kJ/kg/d, 1 kcal/kg/d, 0.05 PAL unit.
Moderate physical activity: MJ/d = (1.298 + 0.265 kg - 0.0011 kg²) + 8.6 kJ/g daily weight gain.
Light physical activity: 15 percent < moderate physical activity.
Vigorous physical activity: 15 percent > moderate physical activity.
PAL = TEE/(predicted BMR/d).
Source: Torun, 2001.

TABLE 4.6
Girls’ energy requirements in populations with three levels of habitual physical activity

Notes:

Body weight at mid-point of age interval (WHO, 1983).
Numbers rounded to the closest 0.1 MJ/d, 25 kcal/d, 5 kJ/kg/d, 1 kcal/kg/d, 0.05 PAL unit.
Moderate physical activity: MJ/d = (1.102 + 0.273 kg - 0.0019 kg²) + 8.6 kJ/g daily weight gain.
Light physical activity: 15 percent < moderate physical activity.
Vigorous physical activity: 15 percent > moderate physical activity.
PAL = TEE/(predicted BMR/d).
Source: Torun, 2001.

It is therefore important that recommendations for appropriate levels of physical activity accompany recommendations for dietary energy intakes. There is no direct experimental or epidemiological evidence on the minimal or optimal frequency, duration and intensity of exercise that promotes health and well-being in children, but it has been suggested that children should perform a minimum of 60 minutes per day of moderate-intensity physical activity, which may be carried out in cumulative bouts of ten or more minutes, and which should be supplemented by activities that promote flexibility, muscle strength and increase in bone mass (Boreham and Riddoch, 2001). This can be pursued by promoting walking, climbing stairs or cycling as part of everyday activities, and encouraging participation in games and sports that involve body displacement and a certain degree of physical effort. In making such recommendations, local culture, social customs and environmental characteristics must be taken into account.

4.6 Infections and mild malnutrition

Nutritional requirements and dietary energy recommendations for children who are severely malnourished or chronically ill, such as owing to HIV/AIDS, are beyond the scope of this report. It must be recognized, however, that many populations around the world include large proportions of children with some degree of weight deficit and growth retardation as a result of mild to moderate chronic malnutrition and/or repeated bouts of infections (UNICEF, 2001). As was pointed out in the report from the 1985 FAO/WHO/UNU expert consultation (WHO, 1985), when a public health problem is of such magnitude that it affects the energy and protein requirements of a significant part of the population, it may not be ignored in the assessment of such requirements or in the recommendations that are made.

Situations that promote malnutrition also favour a high incidence of infectious diseases, which in turn further contribute to the malnutrition. For many children under five - and particularly those under three - years of age who live in these conditions, being sick or convalescing from diarrhoea or a respiratory infection is part of "normal life", because they experience this several times a year, with each episode lasting two to 15 days and requiring up to twice that time to achieve full recovery, provided that an intervening new episode of disease does not interrupt the recovery process (Mata, 1978; Black and Lanata, 1995; Steinhoff, 2000). Infections of this nature often result in negative energy balance resulting from poor appetite, decreased absorption of nutrients during diarrhoeal episodes and increased metabolic rate, particularly in febrile processes (Waterlow and Tomkins, 1992; Torun, 2000). This leads to chronic mild wasting (i.e. low weight-for-height) and stunting (i.e. low height-for-age), which may be prevented, ameliorated or corrected if adequate care and food are available, especially in the periods between infectious episodes when appetite has been re-established. If, on the contrary conditions do not improve, the status quo of mild malnutrition is maintained, the possibility for catch-up is reduced and the consequences of malnutrition will continue to prevail in those societies.

Diets for catch-up weight gain must provide all nutrients and energy sources in amounts that surpass the requirements of well-nourished, healthy children. Quantitative estimates of energy requirements for catch-up are difficult to establish for two reasons: 1) the target body weight is not fixed but increases with time in a growing child, so that the longer the period of nutritional deficit, the greater the gap to be filled; and 2) a low weight for a given age may be owing either to a reduction in weight below the acceptable range of weight-for-height (i.e. wasting) or to a low height (i.e. stunting) with a concomitant decrease in weight. In the latter case, if the decrease in weight is proportional to the reduced growth in height, the child will not have a weight deficit as such, and provision of additional dietary energy may lead to overweight, as catch-up in height is much slower and less likely to be achieved than increase in weight.

The extra amounts of energy needed for catch-up growth of a child with actual weight deficit (i.e. low weight-for-height) have been estimated in studies on the rehabilitation of malnourished children as 21 kJ (5 kcal) per gram of tissue to be laid down (Fomon, 1971; Ashworth, 1969; Kerr et al., 1973; Whitehead, 1973; Spady et al., 1976; Krieger and Whitten, 1976). The recommended daily amounts of energy will depend on the rate at which catch-up is expected to occur. Under optimal clinical conditions, children with severe malnutrition can gain weight at rates of up to 20 or more times faster than normal growth. However, at the community level, catch-up rates of free-living children with mild to moderate degrees of weight deficit should realistically be expected to be no more than two or three times the normal rate.

In relation to the energy demands imposed by repeated bouts of infection, the paucity of data concerning illness, convalescence and post-convalescence does not allow estimates of energy requirements for infants and children to be based on direct measurements of energy expenditure and growth. This leads to the suggested use of a factorial estimate of theoretical needs during acute illness and/or convalescence. In addition to basal metabolism, the energy costs of normal growth and the energy needs for obligatory and discretionary activities, the factors involved in the estimate also include faecal energy losses owing to malabsorption from diarrhoeal disease, and increased energy needs imposed by fever and other responses to stress. This is no easy task owing to the variability in clinical and metabolic responses to illnesses of different aetiologies and different degrees of severity. A number of studies in different countries have attempted to quantify the proportion of the growth deficit that can be attributed to infections and the extra requirements for recovery from them, but the results have been inconsistent (WHO, 1985).

As was stated by the previous joint FAO/WHO/UNU expert consultation (WHO, 1985), it is still impossible to generalize about the amounts of additional energy needed for catch-up growth in children who have become malnourished, usually as a result of the combined effects of inadequate intake and frequent infection. The relative contributions of these two factors, and their severity, will vary in different communities and at different times. In many countries there are also important seasonal effects on food supply and incidence of infections. This consultation could not give a better recommendation than that previously offered, which was based on theoretical estimates to allow for twice the normal rate of weight gain among infants in countries with high prevalence of infant and childhood malnutrition. As shown in Table 4.7, this ranges from an increase in energy requirements - and intakes - of 14.5 percent at six to nine months of age, to 3.5 percent at 18 to 24 months. To restore growth, diets with a high energy density may be needed during the short anabolic periods following episodes of weight loss.

TABLE 4.7
Increase in energy requirements needed to allow for twice the normal growth rate of children six to 24 months old*

* It was assumed that the requirements for normal growth were 1.5 times the theoretical estimates based on weight gain.

Source: adapted from WHO, 1985.

In practice, children should be fed according to appetite, with food of good overall quality that satisfies the needs for all nutrients. To counteract the effects of anorexia and the metabolic losses that accompany infection, sufficient amounts of food should be available in periods when appetite is restored and the child is recovering from infection. It must also be recognized that supplying the child’s increased requirement is only one of the measures needed to counteract the effects of periodic infectious episodes. The primary need is for prevention through improved sanitation and other public health measures.

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