A child or adult’s energy requirement is the average amount of food energy required to maintain normal body weight, normal physical activity, and any special needs associated with development, disease recovery, pregnancy, or lactation in children. These criteria for the attainment and preservation of optimum health, physiological function and well-being must be fulfilled by guidelines for dietary energy intake from food. The latter (i.e. well-being) depends not only on health, but also on the capacity of society and the environment to meet the demands imposed, as well as all other energy-demanding activities that meet individual needs.
The energy balance is reached when the input (i.e. the consumption of dietary energy) is equal to the output (i.e. the total expenditure of energy), plus the energy costs of childhood and pregnancy development, or the energy costs of milk production during lactation. A individual is considered to be in a steady state when the energy balance is maintained over a prolonged period. This can involve brief periods during which there is no day-to-day balance between consumption and spending. When energy intake compensates for total energy expenditure and allows for adequate growth in children and pregnancy and lactation in women, an optimal steady state is achieved without imposing metabolic, physiological or behavioral constraints which restrict the full expression of the biological, social and economic potential of an individual.
Within certain limits, through possible physiological and behavioral responses related to energy expenditure and/or growth changes, humans may respond to temporary or enduring changes in energy intake. The balance of energy is restored, and a new steady state is then reached. Adjustments to low or high energy intakes, however, can often include biological and behavioral penalties, such as decreased growth velocity, loss of lean body mass, excessive body fat accumulation, increased risk of illness, forced rest periods, and physical or social limitations in the performance of certain tasks and activities. Some of these modifications are necessary and may even increase the probability of survival in times of food scarcity.
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 words are used as a matter of convention and convenience, implying that the requirement reflects the average energy requirement for a certain number of days and that the recommended energy intake is the amount of energy to be consumed over a certain period of time on a daily basis. There is no suggestion that precisely this quantity of energy must be consumed daily, nor that day after day, the requirement and prescribed consumption are unchanged. There is also no biological justification to define the number of days for which the average requirement or intake must be determined. As a matter of convenience, intervals of seven days are frequently used when calculating the average daily energy consumption and recommended daily intake, taking into account that physical activity and eating patterns may differ on certain days of the week.
Average Requirement and Inter-individual Variation
Energy requirement estimates are derived from people’s measurements. Measurements of a group of people of the same gender and comparable age, body size and physical activity are grouped together to provide the average energy requirement – or recommended dietary consumption level – for a population group or class of individuals. These criteria are then used to predict the energy consumption requirements and recommended levels for other individuals with similar characteristics, but who have not been assessed. While characteristics that may affect requirements, such as gender, age, body size, body structure and lifestyle, have been matched by individuals in a given class, there are still unknown variables that create differences between individuals. As a result, criteria within the class or population group are distributed.
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.
A certain surplus of intake would not be detrimental to most particular nutrients. Thus, the difference between individuals in a class or population group is taken into consideration when dietary recommendations are measured for these nutrients, and the prescribed level of intake is an amount that will meet or exceed the requirements of virtually all individuals in the group. The recommended acceptable level of intake for proteins, for example, is the average population group requirement plus 2 standard deviations. This approach cannot be extended to the guidelines for dietary energy, as intakes that surpass the criteria would achieve a positive balance, which can contribute to long-term overweight and obesity. A high energy consumption level that guarantees a low risk of energy deficiency for most individuals (e.g. the average requirement plus 2 standard deviations) often implies a high probability of obesity due to a dietary energy surplus for most individuals. Therefore, in accordance with previous studies, this expert consultation concluded that the approximate average energy requirement of that group is a descriptor of the dietary energy intake that could be safely prescribed for a population 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.
Sources of Dietary Energy
Fuel for human metabolic and physiological functions is extracted from food-bound chemical energy and its components of macronutrients, i.e. carbohydrates, fats, proteins, and ethanol, serving as substrates or fuels. Its chemical energy is released and converted into thermal, mechanical and other types of energy after food is consumed. Energy requirements which must be fulfilled by a well balanced diet and which do not make clear recommendations for carbohydrates, fats or proteins. Nevertheless, it should be noted that the main sources of dietary energy are fats and carbohydrates, although proteins also provide important amounts of energy, especially when the total intake of dietary energy is restricted. While ethanol is not considered part of the food system, its contribution to the overall intake of energy cannot be ignored, particularly among populations that regularly consume alcoholic beverages.
Components of Energy Requirements
Human beings need energy for the following:
- Basal metabolism
- Metabolic response to food
- Physical activity
- Growth
- Pregnancy
- Lactation
Basal Metabolism
This consists of a variety of life-critical functions, such as cell function and replacement; enzyme and hormone production, secretion and metabolism to move proteins and other substances and molecules; body temperature maintenance; cardiac and respiratory muscle uninterrupted work; and brain function. The amount of energy used for basal metabolism over a period of time is referred to as the basal metabolic rate (BMR) and is measured under normal conditions, including being awake after 10 to 12 hours of fasting and 8 hours of physical rest in the supine position and being in a state of mental relaxation at an ambient ambient temperature that does not produce heat or dissipate heat. BMR accounts for 45 to 70 percent of daily total energy expenditure, depending on age and lifestyle, and it is primarily determined by the age, gender, body size and body composition of the person.
Metabolic Response to Food
The amount of energy used for basal metabolism over a period of time is referred to as the basal metabolic rate (BMR) and is measured under normal conditions, including being awake after 10 to 12 hours of fasting and 8 hours of physical rest in the supine position and being in a state of mental relaxation at an ambient ambient temperature that does not produce heat or dissipate heat. BMR accounts for 45 to 70 percent of daily total energy expenditure, depending on age and lifestyle, and it is primarily determined by the age, gender, body size and body composition of the person.
Physical activity
This is the most variable aspect of daily energy expenditure and, after BMR, the second largest. Humans perform physical activities that are obligatory and discretionary. In a given context, obligatory tasks can rarely be avoided and economic, cultural or social demands force them on the person. 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. For health, well-being and a good quality of life in general, discretionary activities, while not socially or economically necessary, are important. They include the daily practice of exercise and wellbeing physical activity; the performance of optional household duties that can contribute to family comfort and well-being; and the involvement in activities for personal pleasure, social engagement and community growth that are personally and socially desirable.
Growth
There are two components to the energy cost of growth: 1) the energy required to synthesize developing tissues; and 2) the energy that is stored in those tissues. In the first three months of age, the energy cost of growth is about 35 percent of total energy demand, drops steadily to about 5 percent at 12 months and about 3 percent in the second year, stays at 1 to 2 percent until mid-adolescence, and is negligible in late teens.
Pregnancy
During pregnancy, the development of the fetus, placenta and various maternal tissues, such as in the uterus, breasts and fat stores, as well as improvements in maternal metabolism and an increase in maternal effort during rest and physical activity, requires additional energy.
Lactation
There are two components to the energy cost of lactation: 1) the energy content of the milk secreted; and 2) the energy needed for that milk to be produced. Body fat stores accumulated during pregnancy may derive part of this additional requirement from well-nourished lactating women.
Calculations of Energy Requirements
Using the Doubly Labelled Water Technique (DLW) or other methods which offer comparable results, the total energy expenditure of free-living individuals can be calculated. Among these, individually calibrated measurement of heart rate has been validated successfully. Using these techniques, the metabolic response to food and the energy cost of tissue synthesis provide measurements of total energy expenditure over a 24-hour span. This is tantamount to daily energy needs for adults. For the determination of energy requirements in infancy, childhood, adolescence and during pregnancy, and for the development and secretion of milk during lactation, additional energy for deposition in growing tissues is needed. It can be calculated from growth (or weight gain) velocity and weight gain composition calculations, and from breast milk’s average volume and composition.
Factorial Estimates of Total Energy Expenditure
In the absence of experimental data on overall energy consumption, factorial estimates can be calculated on the basis of the time allocated to routine tasks and the energy expense of those activities. Two or more components or ‘factors’ are combined by factorial measurements, such as the amount of energy expended while sleeping, resting, working, performing social or discretionary household tasks, and in leisure time. In turn, the energy expended on each of these components can be determined by knowing the time and related energy costs allocated to each activity.
For infants and older children around the world, the experimental calculation of total energy expenditure and the assessment of growth and tissue composition allow sound predictions to be made about energy requirements and dietary recommendations. The formulation of recommendations for children and adolescents with complex lifestyles is helped by special requirements and additional estimates.
Total expenditure on energy was also calculated in adult groups, although this was mostly in developed countries. Variations between populations of diverse geographical, cultural and economic backgrounds in body size, body structure and habitual physical activity make it difficult to apply the reported findings on a worldwide basis. Thus, energy requirements were initially measured as multiples of BMR in order to account for variations in body size and composition. They were then translated into energy units using the population’s known BMR value, or the mean BMR determined from the mean body weight of the population. 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.
Expression of Energy Requirements and Recommendations
Energy expenditure calculations and guidelines for energy needs are expressed in units of energy (joules, J), in accordance with the international system of units.
The main determinants of total energy expenditure are gender, age and body weight. Thus, for each sex and different age groups, energy requirements are presented separately and are expressed as both energy units per day and energy per kilogram of body weight. Requirements are often expressed as multiples of BMR because body size and structure also affect energy consumption and are closely linked to basal metabolism.
