Human energy requirements are estimated from measures of energy expenditure plus the additional energy needs for growth, pregnancy and lactation. Recommendations for dietary energy intake from food must satisfy these requirements for the attainment and maintenance of optimal health, physiological function and well-being. The latter (i.e. well-being) depends not only on health, but also on the ability to satisfy the demands imposed by society and the environment, as well as all the other energy-demanding activities that fulfil individual needs.
Energy balance is achieved when input (i.e. dietary energy intake) is equal to output (i.e. total energy expenditure), plus the energy cost of growth in childhood and pregnancy, or the energy cost to produce milk during lactation. When energy balance is maintained over a prolonged period, an individual is considered to be in a steady state. This can include short periods during which the day-to-day balance between intake and expenditure does not occur. An optimal steady state is achieved when energy intake compensates for total energy expenditure and allows for adequate growth in children, and pregnancy and lactation in women, without imposing metabolic, physiological or behavioural restrictions that limit the full expression of a persons biological, social and economic potential.
Within certain limits, humans can adapt to transient or enduring changes in energy intake through possible physiological and behavioural responses related to energy expenditure and/or changes in growth. Energy balance is maintained, and a new steady state is then achieved. However, adjustments to low or high energy intakes may sometimes entail biological and behavioural penalties, such as reduced growth velocity, loss of lean body mass, excessive accumulation of body fat, increased risk of disease, forced rest periods, and physical or social limitations in performing certain activities and tasks. Some of these adjustments are important and may even increase the chances of survival in times of food scarcity.
An adequate, healthy diet must satisfy human needs for energy and all essential nutrients. Furthermore, dietary energy needs and recommendations cannot be considered in isolation of other nutrients in the diet, as the lack of one will influence the others. Thus, the following definitions are based on the assumption that requirements for energy will be fulfilled through the consumption of a diet that satisfies all nutrient needs.
Energy requirement is the amount of food energy needed to balance energy expenditure in order to maintain body size, body composition and a level of necessary and desirable physical activity consistent with long-term good health. This includes the energy needed for the optimal growth and development of children, for the deposition of tissues during pregnancy, and for the secretion of milk during lactation consistent with the good health of mother and child.
The recommended level of dietary energy intake for a population group is the mean energy requirement of the healthy, well-nourished individuals who constitute that group.
Based on these definitions, a main objective for the assessment of energy requirements is the prescription of dietary energy intakes that are compatible with long-term good health. Therefore, the levels of energy intake recommended by this expert consultation are based on estimates of the requirements of healthy, well-nourished individuals. It is recognized that some populations have particular public health characteristics that are part of their usual, "normal" life. Foremost among these are population groups in many developing countries where there are numerous infants and children who suffer from mild to moderate degrees of malnutrition and who experience frequent episodes of infectious diseases, mostly diarrhoeal and respiratory infections. Special considerations are made in this report for such sub-populations.
2.1.1 Daily energy requirements and daily energy intakes
Energy requirements and recommended levels of intake are often referred to as daily requirements or recommended daily intakes. These terms are used as a matter of convention and convenience, indicating that the requirement represents an average of energy needs over a certain number of days, and that the recommended energy intake is the amount of energy that should be ingested as a daily average over a certain period of time. There is no implication that exactly this amount of energy must be consumed every day, nor that the requirement and recommended intake are constant, day after day. Neither is there any biological basis for defining the number of days over which the requirement or intake must be averaged. As a matter of convenience, taking into account that physical activity and eating habits may vary on some days of the week, periods of seven days are often used when estimating the average daily energy expenditure and recommended daily intake.
2.1.2 Average requirement and inter-individual variation
Estimates of energy requirements are derived from measurements of individuals. Measurements of a collection of individuals of the same gender and similar age, body size and physical activity are grouped together to give the average energy requirement - or recommended level of dietary intake - for a class of people or a population group. These requirements are then used to predict the requirements and recommended levels of energy intake for other individuals with similar characteristics, but on whom measurements have not been made. Although individuals in a given class have been matched for characteristics that may affect requirements, such as gender, age, body size, body composition and lifestyle, there remain unknown factors that produce variations among individuals. Consequently, there is a distribution of requirements within the class or population group (WHO, 1985) (Figure 2.1).
Distribution of energy requirements of a population group or class of individuals*
* It is assumed that individual requirements are randomly distributed about the mean requirement for the class of individuals, and that the distribution is Gaussian.
Source. WHO, 1985.
For most specific nutrients, a certain excess of intake will not be harmful. Thus, when dietary recommendations are calculated for these nutrients, the variation among individuals in a class or population group is taken into account, and the recommended level of intake is an amount that will meet or exceed the requirements of practically all individuals in the group. For example, the recommended safe level of intake for proteins is the average requirement of the population group, plus 2 standard deviations. This approach cannot be applied to dietary energy recommendations, because intakes that exceed requirements will produce a positive balance, which may lead to overweight and obesity in the long term. A high level of energy intake that assures a low probability of energy deficiency for most people (e.g. the average requirement plus 2 standard deviations) also implies a high probability of obesity for most people owing to a dietary energy excess (Figure 2.2). Therefore, in agreement with earlier reports, this expert consultation concluded that the descriptor of the dietary energy intake that could be safely recommended for a population group is the estimated average energy requirement of that group.
Probability that a particular energy intake is inadequate or excessive for an individual*
* Individuals are randomly selected among a class of people or a population group. The two probability curves overlap, so the level of energy intake that assures a low probability of dietary energy deficiency is the same level that implies a high probability of obesity owing to dietary energy excess.
Source: WHO, 1985.
Energy for the metabolic and physiological functions of humans is derived from the chemical energy bound in food and its macronutrient constituents, i.e. carbohydrates, fats, proteins and ethanol, which act as substrates or fuels. After food is ingested, its chemical energy is released and converted into thermic, mechanical and other forms of energy.
This report refers to energy requirements that must be satisfied with an adequately balanced diet, and does not make specific recommendations for carbohydrates, fats or proteins. Reports from other FAO and WHO expert groups address those topics. Nevertheless, it should be noted that fats and carbohydrates are the main sources of dietary energy, although proteins also provide important amounts of energy, especially when total dietary energy intake is limited. Ethanol is not considered part of a food system, but its contribution to total energy intake cannot be overlooked, particularly among populations that regularly consume alcoholic beverages. Allowing for the mean intestinal absorption, and for the nitrogenous portion of proteins that cannot be completely oxidized, the average values of metabolizable energy provided by substrates in a mixed diet are 16.7 kJ (4 kcal) per gram of carbohydrate or protein, and 37.7 kJ (9 kcal) per gram of fat. Ethanol provides 29.3 kJ (7 kcal) per gram. The energy value of a food or diet is calculated by applying these factors to the amount of substrates determined by chemical analysis, or estimated from appropriate food composition tables. A recent related report from a FAO technical workshop provides more information on this topic (FAO, 2003).
Human beings need energy for the following:
Basal metabolism. This comprises a series of functions that are essential for life, such as cell function and replacement; the synthesis, secretion and metabolism of enzymes and hormones to transport proteins and other substances and molecules; the maintenance of body temperature; uninterrupted work of cardiac and respiratory muscles; and brain function. The amount of energy used for basal metabolism in a period of time is called the basal metabolic rate (BMR), and is measured under standard conditions that include being awake in the supine position after ten to 12 hours of fasting and eight hours of physical rest, and being in a state of mental relaxation in an ambient environmental temperature that does not elicit heat-generating or heat-dissipating processes. Depending on age and lifestyle, BMR represents 45 to 70 percent of daily total energy expenditure, and it is determined mainly by the individuals age, gender, body size and body composition.
Metabolic response to food. Eating requires energy for the ingestion and digestion of food, and for the absorption, transport, interconversion, oxidation and deposition of nutrients. These metabolic processes increase heat production and oxygen consumption, and are known by terms such as dietary-induced thermogenesis, specific dynamic action of food and thermic effect of feeding. The metabolic response to food increases total energy expenditure by about 10 percent of the BMR over a 24-hour period in individuals eating a mixed diet.
Physical activity. This is the most variable and, after BMR, the second largest component of daily energy expenditure. Humans perform obligatory and discretionary physical activities. Obligatory activities can seldom be avoided within a given setting, and they are imposed on the individual by economic, cultural or societal demands. The term "obligatory" is more comprehensive than the term "occupational" that was used in the 1985 report (WHO, 1985) because, in addition to occupational work, obligatory activities include daily activities such as going to school, tending to the home and family and other demands made on children and adults by their economic, social and cultural environment.
Discretionary activities, although not socially or economically essential, are important for health, well-being and a good quality of life in general. They include the regular practice of physical activity for fitness and health; the performance of optional household tasks that may contribute to family comfort and well-being; and the engagement in individually and socially desirable activities for personal enjoyment, social interaction and community development.
Growth. The energy cost of growth has two components: 1) the energy needed to synthesize growing tissues; and 2) the energy deposited in those tissues. The energy cost of growth is about 35 percent of total energy requirement during the first three months of age, falls rapidly to about 5 percent at 12 months and about 3 percent in the second year, remains at 1 to 2 percent until mid-adolescence, and is negligible in the late teens.
Pregnancy. During pregnancy, extra energy is needed for the growth of the foetus, placenta and various maternal tissues, such as in the uterus, breasts and fat stores, as well as for changes in maternal metabolism and the increase in maternal effort at rest and during physical activity.
Lactation. The energy cost of lactation has two components: 1) the energy content of the milk secreted; and 2) the energy required to produce that milk. Well-nourished lactating women can derive part of this additional requirement from body fat stores accumulated during pregnancy.
The total energy expenditure of free-living persons can be measured using the doubly labelled water technique (DLW) or other methods that give comparable results. Among these, individually calibrated heart rate monitoring has been successfully validated. Using these methods, measurements of total energy expenditure over a 24-hour period include the metabolic response to food and the energy cost of tissue synthesis. For adults, this is equivalent to daily energy requirements. Additional energy for deposition in growing tissues is needed to determine energy requirements in infancy, childhood, adolescence and during pregnancy, and for the production and secretion of milk during lactation. It can be estimated from calculations of growth (or weight gain) velocity and the composition of weight gain, and from the average volume and composition of breastmilk.
2.4.1 Factorial estimates of total energy expenditure
When experimental data on total energy expenditure are not available, it can be estimated by factorial calculations based on the time allocated to activities that are performed habitually and the energy cost of those activities. Factorial calculations combine two or more components or "factors", such as the sum of the energy spent while sleeping, resting, working, doing social or discretionary household activities, and in leisure. Energy spent in each of these components may in turn be calculated by knowing the time allocated to each activity, and its corresponding energy cost.
As discussed in the following sections of this report, the experimental measurement of total energy expenditure and the assessment of growth and tissue composition allow sound predictions to be made regarding energy requirements and dietary recommendations for infants and older children around the world. Special considerations and additional calculations assist the formulation of recommendations for children and adolescents with diverse lifestyles.
Total energy expenditure has also been measured in groups of adults, but this has been primarily in industrialized countries. Variations in body size, body composition and habitual physical activity among populations of different geographical, cultural and economic backgrounds make it difficult to apply the published results on a worldwide basis. Thus, in order to account for differences in body size and composition, energy requirements were initially calculated as multiples of BMR. They were then converted into energy units using a known BMR value for the population, or the mean BMR calculated from the populations mean body weight. To account for differences in the characteristic physical activity of the associated lifestyles, energy requirements of adults were estimated by factorial calculations that took into account the times allocated to activities demanding different levels of physical effort.
The extra needs for pregnancy and lactation were also calculated using factorial estimates for the growth of maternal and foetal tissues, the metabolic changes associated with pregnancy and the synthesis and secretion of milk during lactation.
2.4.2 Expression of requirements and recommendations
Measurements of energy expenditure and energy requirement recommendations are expressed in units of energy (joules, J), in accordance with the international system of units. Because many people are still used to the customary usage of thermochemical energy units (kilocalories, kcal), both are used in this report, with kilojoules given first and kilocalories second, within parenthesis and in a different font (Arial 9). In tables, values for kilocalories are given in italic type.
Gender, age and body weight are the main determinants of total energy expenditure. Thus, energy requirements are presented separately for each gender and various age groups, and are expressed both as energy units per day and energy per kilogram of body weight. As body size and composition also influence energy expenditure, and are closely related to basal metabolism, requirements are also expressed as multiples of BMR.
A certain amount of activity must be performed regularly in order to maintain overall health and fitness, to achieve energy balance and to reduce the risk of developing obesity and associated diseases, most of which are associated with a sedentary lifestyle. This expert consultation therefore endorsed the proposition that recommendations for dietary energy intake must be accompanied by recommendations for an appropriate level of habitual physical activity. This report provides guidelines for desirable physical activity levels, and for the duration, frequency and intensity of physical exercise as recommended by various organizations with expertise in physical activity and health. It also emphasizes that appropriate types and amounts of physical activity can be carried out during the performance of either obligatory or discretionary activities and that recommendations must take into account the cultural, social and environmental characteristics of the target population.
In addition to those defined in the preceding sections, the following terms and abbreviations are used in this report. They are consistent with the definitions used in other related WHO and FAO documents (FAO, 2003; James and Schofield 1990; WHO, 1995).
Basal metabolic rate (BMR): The minimal rate of energy expenditure compatible with life. It is measured in the supine position under standard conditions of rest, fasting, immobility, thermoneutrality and mental relaxation. Depending on its use, the rate is usually expressed per minute, per hour or per 24 hours.
Body mass index (BMI): The indicator of weight adequacy in relation to height of older children, adolescents and adults. It is calculated as weight (in kilograms) divided by height (in meters), squared. The acceptable range for adults is 18.5 to 24.9, and for children it varies with age.
Doubly labelled water (DLW) technique: A method used to measure the average total energy expenditure of free-living individuals over several days (usually 10 to 14), based on the disappearance of a dose of water enriched with the stable isotopes 2H and 18O.
Energy requirement (ER): The amount of food energy needed to balance energy expenditure in order to maintain body size, body composition and a level of necessary and desirable physical activity, and to allow optimal growth and development of children, deposition of tissues during pregnancy, and secretion of milk during lactation, consistent with long-term good health. For healthy, well-nourished adults, it is equivalent to total energy expenditure. There are additional energy needs to support growth in children and in women during pregnancy, and for milk production during lactation.
Heart rate monitoring (HRM): A method to measure the daily energy expenditure of free-living individuals, based on the relationship of heart rate and oxygen consumption and on minute-by-minute monitoring of heart rate.
Total energy expenditure (TEE): The energy spent, on average, in a 24-hour period by an individual or a group of individuals. By definition, it reflects the average amount of energy spent in a typical day, but it is not the exact amount of energy spent each and every day.
Physical activity level (PAL): TEE for 24 hours expressed as a multiple of BMR, and calculated as TEE/BMR for 24 hours. In adult men and non-pregnant, non-lactating women, BMR times PAL is equal to TEE or the daily energy requirement.
Physical activity ratio (PAR): The energy cost of an activity per unit of time (usually a minute or an hour) expressed as a multiple of BMR. It is calculated as energy spent in an activity/BMR, for the selected time unit.
FAO. 2003. Food energy - methods of analysis and conversion factors. Report of a technical workshop. FAO Food and Nutrition Paper No. 77. Rome.
James, W.P.T. & Schofield, E.C. 1990. Human energy requirements. A manual for planners and nutritionists. Oxford, UK, Oxford Medical Publications under arrangement with FAO.
WHO. 1985. Energy and protein requirements: Report of a joint FAO/WHO/UNU expert consultation. WHO Technical Report Series No. 724. Geneva.
WHO. 1995. Physical status: The use and interpretation of anthropometry. Report of a WHO expert committee. WHO Technical Report Series No. 854. Geneva.
 1 joule (J) is the amount
of mechanical energy required to displace a mass of 1 kg through a distance of 1
m with an acceleration of 1 m per second (1 J = 1 kg × 1 m2
× 1 sec-2). Multiples of 1 000 (kilojoules, kJ) or 1 million
(megajoules, MJ) are used in human nutrition. The conversion factors between
joules and calories are: 1 kcal = 4.184 kJ, or conversely, 1 kJ = 0.239
 The term "fitness" encompasses cardiorespiratory health, appropriate body composition (including fat distribution), muscular strength, endurance and flexibility. Fitness can generally be described as the ability to perform moderate to vigorous physical activity without becoming excessively tired.