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Abstract
1. Introduction
2. Methodological considerations2.1. Age and sex
2.2. Number of children and measurements
2.3. Weight, BMR and energy costs
2.4. Social, nutritional, national and ethnic characteristics
2.5. Description of activities
2.6. Calculations and assumptions4.1. Information gaps
4.2. Geographic or racial differences
4.3. Energy costs in relation to age and sex
4.4. Suggestions to estimate the energy cost of activities
B. TORUN *
* Institute of Nutrition of Central
America and Panama (INCAP), Apartado Postal 1188, 01901 Guatemala City, Guatemala.
Twenty-eight studies that measured the energy costs of several common activities of children were found through an extensive literature search and personal communications. The characteristics of the children, the method used to measure energy expenditure and the energy costs of the activities from each study are presented in summary tables. The children's basal metabolic rate (BMR) and the energy cost of activities per kg or as multiples of BMR (X BMR) were calculated to present the data uniformly. The activities were classified in ten categories. In most age groups there is no information about the energy cost of household work, agricultural chores, other common tasks, and many sports or games, especially for girls. Although the methodology varied between studies and the activities were not standardized, the data suggest that when energy costs are expressed as X BMR: (a) they are similar for boys and girls, (b) there are no age-related differences in sedentary activities, (c) the cost of walking and moving around increases from preschool years to mid-adolescence, and (d) energy costs from 15 years onwards are similar to those of adults. The use of energy costs of adults per kg of body weight to calculate energy costs of children leads to errors which increase with decreasing age. Suggestions are made to estimate the energy costs of children based on their known or calculated BMR and from similar activities of adults expressed as fractional multiples of BMR. Additional investigations are necessary to confirm whether there are differences related to racial, geographic and socioeconomic conditions.
The latest Consultation of international experts, convened to assess energy and protein requirements, agreed that the energy intake from food must allow a level of physical activity consistent with long-term good health, adequate growth in children, and the performance of economically necessary and socially desirable physical activity (FAO/WHO/UNU, 1985). The Consultation also concluded that energy needs are determined mainly by energy expenditure. Thus, they recommended a factorial approach to estimate the mean daily needs for dietary energy.
The factorial approach involves the summation of the energy expended during the periods that a person - or rather, a type of person - is sleeping and resting or engaged in occupational, discretional or maintenance physical activities, plus the energy needed for tissue accretion in growing children. This requires two sets of data: (a) the time allocated to the various activities, and (b) the energy cost of those activities. The latter was expressed as multiples of the basal metabolic rate (X BMR), as this accounted for inter-individual variations related to age, sex, size, body composition, physiological conditions and climate or genetic factors that might influence energy expenditure. A set of formulas was agreed upon to estimate BMR from age, sex and body weight (SCHOFIELD et al., 1985).
The mean energy requirements of adults were calculated in this manner. The same approach was followed for children older than 10 years, with the addition of energy needs associated with normal growth. Their energy needs for activity were estimated from the scarce information available and assumptions made on the time spent in those activities and their energy cost. Many of the estimates of energy expenditure were derived from the energy cost of activities performed by adults, expressed by unit of body weight. The validity of such estimates of energy costs is questionable (TORUN, 1983; TORUN et al., 1983; MENDOZA and TORUN, unpublished data), and the accurary of the estimates of energy requirements for 6- to 16-year-old children has recently been challenged (SPURR and REINA, 1989).
Factorial calculations were not done for younger children. Although much information was available on the BMR at all ages (DURNIN, 1981), the lack of information allowing to specify a desirable level of activity for young children, the scant information on the energy cost of that activity, and the variability in the energy needs for growth made it necessary to base the energy requirements on data from dietary energy intakes (FAO/WHO/UNU, 1985).
This paper summarizes most of the
information currently available on the energy cost of activities commonly performed by
children. Activities assessed under exercise laboratory conditions, such as pedalling on
an ergometer or walking on a treadmill with a fixed workload and constant speed, were not
considered because they do not reflect the way in which children act and perform in
everyday life.
2.1. Age and sex
2.2. Number of children and measurements
2.3. Weight, BMR and energy costs
2.4. Social, nutritional, national and ethnic characteristics
2.5. Description of activities
2.6. Calculations and assumptions
The activities for which energy costs have been published, unpublished data from the Philippines, kindly provided by M.P. Guzman, and additional information from our group (MENDOZA and TORUN, unpublished), were compiled in eight Tables. Data were not included if the methodology involved was judged inappropriate and highly subjective, such as the use of energy costs of activities performed by adults to calculate energy costs of children's activities vaguely categorized by teachers or parents as being more or less energy-demanding.
To determine the energy cost of activities, most studies used open-circuit indirect calorimetry with portable respirometers of the Kofranyi-Michaelis (KM) type (MÜLLER and FRANZ, 1952), or collection of exhaled gas in a Douglas bag or latex balloon; the latter are all referred to as "Douglas bag" in the Tables. Mouthpieces and nose-clips were used in some studies and respiratory masks with small valves in others. Some investigators used a ventilated hood (DUGGAN and MILNER, 1986a, b) or closed-circuit indirect calorimetry (TAYLOR et al., 1951; MAHADEVA et al., 1953) for resting or sitting activities, an indirect calorimetry chamber where CO2 production was measured (TAYLOR et al., 1948a, b, 1949, 1951), heart-rate monitoring with individual heart rate to oxygen consumption calibrations (GANDRA and BRADFIELD, 1971; BRADFIELD et al., 1971a, b) or an integrating motor pneumotachograph (McNAUGHTON and CAHN, 1970).
Several researchers did not describe
whether the respiratory quotient was measured, and they did not explain the manner in
which the volumes of oxygen uptake or energy expenditure were calculated. Most
publications describe the technique or equipment used to measure gas volume and
composition, but few indicate whether the equipment was checked, standardized or
calibrated periodically. Unless errors were clearly evident, it was assumed that the
measurements and calculations were done correctly.
The children were categorized as follows:
1. Boys and girls of preschool age (1.5-6 years)
2. Boys of school age (7-12 years)
3. Girls of school age (7-12 years)
4. Boys in early adolescence (13-14 years)
5. Boys in mid-adolescence (15-16 years)
6. Boys in late adolescence (17-19 years)
7. Girls in early and mid-adolescence (13-16 years)
8. Girls in late adolescence (17-19 years)
The small number of studies in
adolescent girls did not allow subclassifying them in three groups.
These are given in the Tables when
the information was available and when it was clearly stated whether measurements of
energy costs were done once or more than once in the same child.
These are shown in the Tables as
given by the investigators in each study, or calculated as described below. When
available, standard deviations and ranges were included. An asterisk (*) in the Tables
indicates values calculated from published data. Other symbols indicate BMRs calculated
with the formulas of Schofield et al., (#), and body weights estimated from NCHS
standards (&).
When available, they were included
in the Tables. When the information was assumed, a question mark (?) was placed next to
the presumed characteristic.
These were grouped in categories
based on the description given by the authors or the expected level of energy expenditure.
For example, sitting at ease, sitting and reading, and sitting and writing were considered
as a single group of activities, as were activities performed while standing with changes
in position but little body displacement.
Data have been published in a variety of manners, and some do not provide all methodological details or ancillary information. In order to organize the data in a uniform manner permitting analysis and comparison, the following calculations, transformations or assumptions were made in some instances:
1. Energy costs were expressed or calculated as kcal/min. When only the oxygen uptake was given by the investigators, energy expenditure was calculated assuming an equivalence of 4.82 kcal/L oxygen, STPD, for sedentary and light activities, and 4.92 kcal/L oxygen, STPD, for heavier activities.
2. When BMR was not measured, it was calculated using the formulas proposed by SCHOFIELD et al. (1985). When this involved combining data for boys and girls, weighted means were used applying the formulas for both sexes. Multiples of BMR (X BMR) were calculated from those figures.
3. When body weights were not given and could not be calculated from other data in the publication, but information was available on the children's nutritional status (e.g., "healthy, well-nourished", or "between the 50th and 75th percentile of weight-for-age"), it was assumed that the mean weight for the group was the corresponding percentile (or median for "well-nourished") in the U.S. NCHS tables of weight-for-age and sex adopted by WHO (1983).
4. When individual energy expenditures were not given per unit of body weight, the mean of the experimental group's body weight was used to estimate cal/kg/min and multiples of BMR.
5. Activities not properly described were assigned to categories of presumably similar activities. For example, "walking" was considered as "walking at free velocity on level ground", while "activities after school", in "school playground" and "games" were categorized as "leisure or play with low or moderate physical activity".
Tables 1 to 8 show the energy cost of activities for boys and girls of different age groups, from each study. The footnotes in Table 1 explain the symbols and abbreviations common to all Tables. The names of the authors and year of publication of the numerical references shown in the first column, are listed in Table 9. The full bibliographic references appear in alphabetical order at the end of this paper.
Based on their characteristics and the circumstances of their performance, activities were classified in the following categories:
- lying down awake, resting or studying
- sitting quietly, reading, writing, playing
- standing quietly or with little movement
- standing and moving moderately
- walking at free velocity on level ground
- walking faster than usual, or uphill, or carrying a load
- at school (classroom activities and recess) or sedentary and light work
- light and moderate household work
- leisure time and play of low or moderate physical intensity
- running, exercise and sports
Within each category, Tables 1 to 8 show the data from developing countries followed by those from industrialized countries, and data from rural environments followed by those in urban settings.
Table 10 combines and
summarizes the results of the studies described in Tables 1-8 for each age-and-sex
group. It shows whether the research was done in developing or industrialized countries,
the number of studies done with each type of activity, the number of children in each
study and the range of the mean body weights in the various studies. The energy cost of
each type of activity was computed as the mean of all studies performed, weighted by the
number of children in each study. When the number of children was not given in a study,
the energy cost was calculated as the arithmetic mean (i.e., non-weighted) of all studies.
The range of the mean values in the different studies is also shown.
4.1. Information gaps
4.2. Geographic or racial differences
4.3. Energy costs in relation to age and sex
4.4. Suggestions to estimate the energy cost of activities
There is still very little information on the energy cost of non-resting activities performed by children. In some age and sex groups there is an absolute lack of knowledge about activities that make an important contribution to the total daily energy expenditure. These include household work commonly done by school-age and adolescent children in many societies, agricultural chores and other tasks frequently executed by children in rural areas of developing countries, and many sports and games that children play. In general, there is even less information about girls than boys.