INTRODUCTION — Nutritional needs of children and adolescents are strongly influenced by requirements for somatic growth. This is particularly true during adolescence, when healthy individuals accrue up to 50 percent of their adult weight [1]. Recommendations for dietary energy intake are designed to maintain health, promote optimal growth and maturation, and support a desirable level of physical activity.
The physiologic determinants of energy needs and recommended energy intake for children and adolescents are reviewed here. A clinical approach to nutritional assessment and counseling is discussed in the following topic reviews:
●(See "Indications for nutritional assessment in childhood".)
●(See "Dietary recommendations for toddlers, preschool, and school-age children".)
●(See "Dietary history and recommended dietary intake in children".)
DETERMINANTS OF ENERGY NEEDS — The energy needs (estimated energy requirements [EER]) for children and adolescents are comprised of:
●Total energy expenditure (TEE), which may be partitioned into the following components:
•Basal energy expenditure – This is the largest component (approximately 45 to 70 percent of TEE) and is strongly influenced by body composition (fat-free mass)
•Physical activity energy expenditure – This accounts for approximately 30 percent of TEE and varies substantially among individuals
•Thermic effect of food (TEF) – Accounts for approximately 10 percent of the energy content of the food consumed
•Synthesis cost of growth – The energy expended in tissue synthesis
•Thermoregulation – This is negligible under thermoneutral conditions
●Energy cost of growth – The energy cost of growth consists of two components: (1) the energy deposited in newly synthesized tissues, and (2) the energy expended in the synthesis of the newly accreted tissue (included in TEE) [2].
Techniques for estimating each of these components are outlined below, followed by prediction equations for total energy requirements.
Total energy expenditure — TEE is the sum of basal energy expenditure, thermoregulation, TEF, physical activity, and synthesis costs of growth. The two largest components of energy requirements are basal energy expenditure and physical activity; the proportions of these vary substantially, depending on age, sex, and habitual activity (figure 1).
TEE can be measured by the doubly labeled water (DLW) technique. Prior to the advent of DLW, TEE was estimated by a factorial approach numerating the various components. DLW is a noninvasive technique that uses the stable isotopes deuterium and oxygen-18 to estimate the average TEE over a 10- to 14-day period [3,4]. The DLW method measures the sum of basal energy expenditure, thermoregulation, TEF, physical activity, and synthetic cost of growth.
Basal energy expenditure — Basal energy expenditure is the energy required for cellular and tissue processes that maintain life. It represents between 45 and 70 percent of daily TEE, depending on age, sex, body size, and body composition. Basal energy expenditure is strongly correlated with the fat-free mass that comprises the bulk of active metabolic tissue. As a result, basal energy expenditure relative to weight gradually declines through childhood and adolescence [5]. Moreover, there are marked sex differences in basal energy expenditure during and after adolescence because of differences in the timing, intensity, and duration of the adolescent growth spurt. By the end of puberty, boys have substantially more fat-free mass than do girls, which is associated with substantially higher basal energy expenditure and thus total energy requirements.
Basal energy expenditure can be expressed as either the basal metabolic rate (BMR) or resting metabolic rate (RMR). Although similar, the terms should not be used interchangeably. Both are measured by indirect calorimetry, performed with the subject resting comfortably, supine, awake, and motionless in a thermoneutral environment. BMR is measured after a 12- to 14-hour overnight fast and in a state of mental relaxation, whereas RMR is measured two hours after eating and the conditions are not as carefully controlled [6]. RMR is approximately 10 percent greater than BMR [7].
For clinical or research purposes, BMR can be measured by indirect calorimetry or estimated using one of several prediction equations. For children and adolescents, most authorities use the Schofield prediction equations, which take into account sex, age, and body weight (table 1) [8,9].
Energy expended for physical activity — Energy expenditure for physical activity is the most variable component of TEE and represents between 30 and 55 percent of total energy needs [10]. It depends on the time and intensity of physical activity and is typically expressed as a multiple of BMR.
Patterns of activity — The patterns of physical activities among children and adolescents vary considerably across communities. Guidelines in the United States recommend that children and adolescents ages 6 through 17 years should do 60 minutes or more of moderate to vigorous aerobic physical activity daily [11], as well as some exercise designed to strengthen muscles and bones. Mounting evidence indicates that the vast majority of children and adolescents fall short of these goals [12,13]. In addition, sedentary behaviors, including watching television and other screen devices, are common among adolescents and are associated with worse cardiovascular fitness [14,15]. Interventions to increase physical activity in children and adolescents in the United States are needed, especially for girls and for populations with limited financial resources and opportunities [16].
Techniques to measure energy cost of physical activity — For research purposes, standards for energy expended in various physical activities may be assessed by indirect calorimetry, heart rate monitoring, and accelerometry.
●Indirect calorimetry can be used to measure the energy cost of discrete physical activities. This method estimates energy expenditure and substrate utilization by measuring oxygen consumption and carbon dioxide production [17]. A number of calorimeter systems are available to measure gas exchange in the laboratory or clinical setting; these devices estimate resting energy expenditure by analyzing oxygen consumption and carbon dioxide production in breath, with variable accuracy.
●Heart rate monitoring, calibrated against indirect calorimetry, can be used to estimate energy expenditure because a linear relationship exists between heart rate and oxygen consumption during exercise. Monitoring heart rate provides a noninvasive and inexpensive alternative to calorimetry [18].
●Accelerometry uses a small portable device to record motion in one or more planes; it provides a measure of the frequency, duration, and intensity of physical activity [19,20]. Validity studies have yielded moderate to strong correlations between accelerometer data and oxygen consumption and activity energy expenditure in adults and children [21,22].
●The combination of accelerometry and heart rate monitoring generally gives more accurate and precise estimates of energy expenditure than either method alone [23].
Activity factors used in prediction equations
●Metabolic equivalents (METs) – The energy cost of various physical activities can be expressed as METs, which are multiples of the BMR and vary by age group (table 2) [24]. METs are used with activity logs to estimate the energy spent during various activities.
By convention, 1 MET is defined as an oxygen uptake of 3.5 mL/kg/min, or 1 kcal/kg/h in adults, a value derived for a 70-kg man aged 40 years [25]. The conventional MET value is not applicable to children and adolescents [26,27]. Instead, measured or predicted BMR values should be used to appropriately adjust for individual differences in body size. In addition, energy cost is affected not only by age and size but also level of training and presence or absence of obesity [28,29].
A Youth Compendium of Physical Activities describes the energy costs of 196 activities classified by MET equivalents, derived from oxygen consumption data in children and adolescents while exercising [24]. The Compendium is a valuable resource for those interested in designing and implementing physical activity programs in youth. This is similar to the adult Compendium of Physical Activities [30]. In children 8 to 18 years of age, age- and puberty-adjusted METs are generally lower than the adult compendium MET values for sedentary and moderate activities but were more varied for high-intensity activities [31].
●Physical activity levels (PALs) – The PAL is used to categorize an individual's habitual daily activity level. The average PAL over a 10- to 14-day period can be estimated by measuring TEE using the DLW method, together with an estimate of basal energy expenditure, where: PAL = TEE/BMR
The National Academy of Medicine defined the following PAL categories for healthy children [10]:
•Sedentary – PAL ≥1 to 1.4. This category ranges from virtually no voluntary physical activity (bedrest) to a lifestyle that includes only the light physical activity associated with typical daily life, such as eating, sleeping, and walking around home and school.
•Low active – PAL ≥1.4 to <1.6. This means a lifestyle that includes physical activity equivalent to walking approximately 120 min at a rate of 2.5 miles per hour or equivalent energy expenditure in other activities, in addition to that described for the sedentary level.
•Active – PAL ≥1.6 to <1.9. This means a lifestyle that includes physical activity equivalent to walking approximately 230 min at a rate of 2.5 miles per hour or equivalent energy expenditure in other activities, in addition to that described for the sedentary level.
•Very active – PAL ≥1.9 to <2.5. This means a lifestyle that includes physical activity equivalent to walking approximately 400 min at a rate of 2.5 miles per hour or equivalent energy expenditure in other activities, in addition to that described for the sedentary level.
In general, children in the active or very active PAL categories are participating in moderate or vigorous activities, in addition to walking at 2.5 miles per hour [10]. The activity level is optimally estimated by recording an activity diary and calculating the PAL by applying the Youth Metabolic Equivalent (METy) to the time spent in each activity.
Pooled data from studies that estimated PALs by activity-time allocation in a wide variety of settings reported mean PALs ranging from 1.45 to 2.06 for children 6 to 18 years engaged in light to heavy levels of physical activity, and mean daily energy expenditure ranging from 42 to 66 kcal/kg (0.18 MJ/kg to 0.28 MJ/kg) depending on sex, age, and geographical area [32]. Daily energy expenditures were higher for boys than for girls and were highest in rural areas of resource-limited countries and lowest in resource-rich countries. Energy expenditure also tended to be higher in children 10 to 14 years of age than in children 15 to 19 years of age.
Other predictors — Other factors influence the energy cost of physical activity but are not consistently captured in prediction equations. These include the person's age, size, level of training, and presence or absence of obesity [28,29]:
●Obese individuals have higher energy expenditure than do their lean counterparts in absolute terms. However, obese individuals typically have similar rates when corrected for body size and body composition [4,33].
●Energy requirements per kilogram of body weight vary indirectly with age and size at any given walking or running speed [29].
●Training can decrease the energy cost of running.
Thermic effect of food — The TEF refers to the energy utilized during ingestion, digestion, and metabolism of food. The TEF varies with the macronutrient composition of the meal and is approximately 10 percent of the energy content of a mixed diet (ie, 10 percent of the EER) [34]. Specifically, the TEF increments are 5 to 10 percent for carbohydrate, 0 to 5 percent for fat, and 20 to 30 percent for protein.
Energy cost of growth — The energy cost of growth consists of: (1) the energy deposited in newly accrued tissues, and (2) the energy expended for tissue synthesis. Except during the first months of life, the energy required for growth is small. The synthesis cost of growth is included in TEE, as measured by the DLW method. For children, the energy deposition is approximately 20 kcal/day, increasing to 30 kcal/day at peak growth velocity for adolescents [35]. Thus, energy deposition represents 1 to 2 percent of total energy requirements between early childhood and mid-adolescence [34].
The amount of energy stored in body tissue depends on the composition of the tissue synthesized:
●For fat, the energy stored is 9.25 kcal/g (0.039 MJ/g)
●For protein, the energy stored is 5.65 kcal/g (0.024 MJ/g)
During adolescence, the energy cost of growth is greater in males compared with females. This is related to the sex-related changes in body composition that occur during puberty. As an example, between age 10 and 18, males typically accumulate fat-free mass (approximately 30 kg increase), while fat mass declines from 16 to 13 percent of body weight. Conversely, girls accumulate fat mass, rising from approximately 23.5 to 25 percent of body weight [35].
Because the energy cost of growth is a small component of total energy requirements, these sex-related differences in growth have minimal direct impact on total energy requirements. However, the resultant changes in body size and composition are still important contributors to divergent total energy requirements because of their effect on basal energy expenditure and energy costs of moving body mass during physical activity. (See 'Basal energy expenditure' above.)
In the National Academy of Medicine recommendations for daily energy intake of children and adolescents [10], the energy costs of growth for males and females were computed from rates of weight gain of children enrolled in the Fels Longitudinal Study [36] and rates of protein and fat deposition for children [37] and adolescents [35].
TOTAL ENERGY REQUIREMENTS
Estimated energy requirements — The National Academy (Institute) of Medicine has published estimated energy requirements (EERs) for children and adolescents, as outlined in the table (table 3) [10]. Recommendations for energy intake in the Dietary Guidelines for Americans differ because of rounding and focus on a narrower range of typical physical activity levels (PALs) [38].
Although these are useful general estimates, marked variability exists in the energy requirements of individuals, particularly during adolescence because of variation in growth rates, pubertal timing, and PALs [39]. As an example, in a systematic review, puberty was associated with a 12 percent increase in basal metabolic rate (BMR) or resting metabolic rate (RMR) and an 18 percent increase in total energy expenditure (TEE) compared with prepuberty [40]. The higher rates of TEE were accounted for largely by the pubertal increase in fat-free mass.
These EERs also are stratified by the habitual PAL, which is expressed as a multiple of the BMR (see 'Activity factors used in prediction equations' above). Although the recommendations allow for four categories of PAL (sedentary, low active, active, and very active), the active or very active levels are encouraged for all healthy children (ie, at least 60 minutes of physical activity on most, preferably all, days of the week) [41].
Prediction equations
Children with healthy body weight — Prediction equations for the EERs of children and adolescents with healthy body weight are provided in the table (table 4) and calculators (females (calculator 1); males (calculator 2)).
These equations were developed by the National Academy (Institute) of Medicine for children with healthy body weight as a function of age, sex, height, weight, and PAL [10]. The estimates are based on TEE as measured by the doubly labeled water (DLW) method, plus an average of 20 and 25 kcal/d for the energy cost of growth based on average rates of weight gains and rates of protein and fat deposition for children and adolescents, respectively.
Overweight and obesity — Prediction equations for children who are overweight or obese are provided in the table (table 4) and calculators (females (calculator 3); males (calculator 4)). For these children, the equation reflects energy needs for weight maintenance, and thus uses TEE (which does not include a factor for the energy cost of growth) rather than EER.
To achieve weight loss, a reduction in dietary energy intake and/or an increase in physical activity can be prescribed, using the predicted TEE as the starting point.
SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Healthy diet in children and adolescents".)
SUMMARY AND RECOMMENDATIONS
●Determinants of energy needs – Energy needs (total energy requirements) in children and adolescents depend primarily on basal energy expenditure and physical activity (figure 1). (See 'Determinants of energy needs' above.)
•Basal metabolic rate (BMR) – This is the largest component (approximately 45 to 70 percent of total energy requirements) and is strongly influenced by body composition (fat-free mass). The BMR can be estimated from the Schofield prediction equations, which are based on age, sex, and body weight (table 1). (See 'Basal energy expenditure' above.)
•Physical activity – The energy expended in physical activity accounts for approximately 30 percent of total energy expenditure (TEE) and varies substantially among individuals. The energy expenditure for specific physical activities is determined by the intensity of effort and also varies with age, size, level of training, and presence or absence of obesity. (See 'Energy expended for physical activity' above.)
Typical energy expenditure during some recreational activities is shown in the table, expressed as "youth metabolic equivalents" (METy), which are expressed as multiples of the BMR and vary by age group (table 2). An individual's habitual level of physical activity is expressed as the "physical activity level" (PAL), defined as TEE/BMR. (See 'Activity factors used in prediction equations' above.)
•Thermic effect of food (TEF) – This accounts for approximately 10 percent of the energy content of a mixed meal in healthy individuals. (See 'Thermic effect of food' above.)
•Energy cost of growth – This represents approximately 1 to 2 percent of total energy requirements between early childhood and mid-adolescence. (See 'Energy cost of growth' above.)
●Estimated energy requirements (EERs) – EERs of children and adolescents are based on the sum of their TEE, taking into account habitual PALs and energy cost of growth (table 3). Recommendations for energy intake in the Dietary Guidelines for Americans are somewhat different due to rounding for simplification. Although recommendations for energy intake recognize a range of PALs, the active or very active levels of physical activity are encouraged for all healthy children. (See 'Total energy requirements' above.)
●Prediction equations – Equations provide estimates of the energy needs for an individual, based on age, sex, height, weight, and PAL (see 'Prediction equations' above):
•For children with a healthy body weight, the EER is used (table 4) (females (calculator 1); males (calculator 2)).
•For children or adolescents with overweight or obesity, the TEE is used, to reflect the energy required for weight maintenance (table 4) (females (calculator 3); males (calculator 4)).
