Energy is not a nutrient but is required in the body for metabolic processes, physiological functions, muscular activity, heat production, growth and synthesis of new tissues. It is released from food components by oxidation. The main sources of energy are carbohydrates, proteins, fats and, to a lesser degree, alcohol.

The unit of energy is the kilojoule (kJ) or megajoule (1 MJ = 1,000 kJ)
4.18 kilojoules are equal to 1 kilocalorie.

Allowing for intestinal absorption and for the nitrogenous parts of protein that cannot be completely oxidised, the average amount of energy released ranges from approximately 16.7 kJ/g for carbohydrates or protein to 29.3 kJ/g for alcohol and 37.7 kJ/g for fats

Humans need energy for basal metabolism which comprises a set of functions necessary for life such as cell metabolism, synthesis and metabolism of enzymes and hormones, transport of substances around the body, maintenance of body temperature and ongoing functioning of muscles including the heart, and brain function. The amount of energy needed for this purpose in a defined period of time is called the basal metabolic rate (BMR). BMR represents about 45–70% of daily energy expenditure, depending on age, gender, body size and composition. Physical activity is the most variable determinant of energy need and is the second largest user of energy after BMR. Humans perform a number of physical activities including the obligatory demands of an individual’s economic, social and cultural environment (eg occupational, schoolwork, housework etc) or discretionary activity (eg energy expended for optional exercise or sport, or in additional social or cultural interactions).

Energy is also required to process food into nutrients resulting in increases in heat production and oxygen consumption often described by the terms 'dietary-induced thermogenesis', 'specific dynamic action of food' or 'thermic effect of feeding'. The metabolic response to food increases the BMR by about 10% over the day in people eating a mixed diet. Growth also requires energy for synthesis of tissues. In the first three months of life, growth uses about 35% of total energy needs. This falls to 5% at 12 months, less than 2% in the second year of life, 1–2% until mid-adolescence and zero by 20 years of age(FAO:WHO:UNU 2004). Additional energy is also needed in pregnancy and lactation to cover the needs of the growing foetus, the placenta and expanding maternal tissues and additional maternal effort at rest and in physical activity, as well as the production of breast milk.

As energy requirements vary with age, gender, body size and activity, recommendations are needed for each age and gender group.

Recommendations for energy intake differ from those for nutrient intake in that:

  • they are not increased to cover the needs of most members of the group or population, as this level of intake would lead to overweight or obesity in most people.

  • there are differences between the actual energy requirements needed to maintain current body size and level of physical activity and the desirable energy requirements needed to maintain body size and levels of physical activity consistent with good health. Desirable energy requirements may be lower than actual requirements for people who are overweight or obese. Desirable requirements may be higher than actual for inactive people. For people who are both overweight/obese and physically inactive, the difference between actual and desirable will depend on the balance between degree of overweight and level of inactivity.

  • they can be applied cautiously to individuals, using estimates of energy expenditure. However, predictive estimates are much less accurate for individuals than for groups, and variations in energy expenditure can be large, even between apparently similar individuals.

  • there is wide inter-individual variation in the behavioural, physiologic and metabolic components of energy needs. The average energy intake recommended for a defined group cannot be applied to other groups or individuals who differ from the defined group average in gender, age, body size, activity level and possibly other factors.

Two separate terms can therefore be used to express and determine Estimated Energy Requirements (EER):

  • The Estimated Energy Requirement for Maintenance (EERM, or actual energy requirement) is the dietary energy intake that is predicted to maintain energy balance (plus extra needs for pregnancy, lactation and growth) in healthy individuals or groups of individuals at current levels of body size and level of physical activity.

  • The Desirable Estimated Energy Requirement (DEER, or energy reference value) is the dietary energy intake that is predicted to maintain energy balance (plus extra needs for pregnancy, lactation and growth) in healthy individuals or groups of individuals of a defined gender, age, weight, height and level of physical activity consistent with good health and/or development.

Use of, and distinction between, these two terms is necessary because of the various ways in which estimates of energy requirements are used and because of the risk of over-prescription of desirable energy intakes in people who do not follow recommendations for increased physical activity. In some clinical situations, it may be necessary to estimate actual energy requirements (eg when prescribing a diet intended to produce an energy deficit leading to a 0.25–1.0 kg/week weight loss).