Energy intake and expenditure

Key points

• Energy is needed by the body to stay alive, grow, keep warm and move around.
• Energy is provided by food and drink. It comes from the fat, carbohydrate, protein and alcohol the diet contains.
• Energy requirements vary from one individual to the next, depending on factors such as age, sex, body composition and physical activity level.
• Energy expenditure is the sum of the basal metabolic rate (the amount of energy expended while at complete rest), the thermic effect of food (TEF, the energy required to digest and absorb food) and the energy expended in physical activity.
• To maintain bodyweight, it is necessary to balance the energy derived from food with that expended in physical activity. To lose weight, energy expenditure must exceed intake, and to gain weight, energy intake must exceed expenditure.

Energy intake and expenditure

A regular supply of dietary energy is essential for life and is required to fuel many different body processes. These include keeping the heart beating and organs functioning, maintenance of body temperature, muscle contraction and growth. However, daily energy requirements vary widely from one individual to the next. This is due to factors such as sex, body size, bodyweight, climate and physical activity levels.

Energy is obtained, from the food and drink we consume, by oxidation of carbohydrate, fat, protein and alcohol, known as macronutrients. The amount of energy that each of these macronutrients provides varies:

• Fat is the most energy dense nutrient, and provides 9kcal (37kJ)/g.
• Alcohol is the second most energy dense nutrient, providing 7kcal (29KJ)/g.
• Protein provides 4kcal (17kJ)/g
• Carbohydrate (starch and sugars) is the least energy dense nutrient, providing just 3.75kcal (16kJ)/g. [A value of 4 kcal is used for food labelling purposes.]

Application of these factors to the food and drink consumed enables energy intake to be estimated.

These days, energy intake is often measured in joules (J) (or kilojoules (kJ)) but many people are more familiar with Calories (kcal).

• 1 kilojoule (kJ) = 1,000 joules
• 1 megajoule (MJ) = 1,000,000 joules
• 1 kilocalorie (kcal) = 1,000 calories, or 1 Calorie

To convert from one unit to another:

1 kcal = 4.184 kJ, so a 1000 kcal diet provides 4.184 MJ or 4184 kJ

1 MJ = 239 kcal

Energy expenditure is the sum of the basal metabolic rate (BMR) (the amount of energy expended while at rest at a neutral temperature and in the fasting state), the thermic effect of food (TEF) (otherwise known as dietary-induced thermogenesis) and the energy expended in movement of all types.

A substantial proportion of total energy expenditure is accounted for by BMR, which is determined principally by body mass and body composition both of which vary with age and sex (see below). The TEF is the energy cost of digesting food and is rarely assessed separately.

What determines how much energy a person needs?

The actual amount of energy needed varies from person to person and depends on their basal metabolic rate (BMR) and how active they are.

Basal metabolic rate

The basal metabolic rate (BMR) is the rate at which a person uses energy to maintain the basic functions of the body – breathing, keeping warm, and keeping the heart beating – when at complete rest. An average adult will use around 1.1 kcal each minute just to maintain these functions and BMR differs from one person to the next, both within a population and between population groups. Infants and young children tend to have a proportionately high BMR for their size due to their rapid growth and development. Men usually have a higher BMR than women since they tend to have more muscle. Older adults usually have a lower BMR than younger people since their muscle mass tends to decrease with age. The BMR accounts on average for about three-quarters of an individual's energy needs.

Physical activity level

In addition to their BMR, people also use energy for movement of all types. The amount of energy a person uses to perform daily tasks varies depending on factors such as his or her weight (the heavier a person is the more energy is required for movement) and their physical activity level. An estimate of the amount of energy an individual will need can be calculated by multiplying their BMR by a factor appropriate to the amount of activity that person does (known as the Physical Activity Level (PAL)). A PAL of 1.4 is associated with a very low level of physical activity at work or during leisure time. This applies to a large proportion of the UK population. In contrast, a PAL of 1.6 for women or 1.7 for men represents moderate intensity activity, and values of 1.8 for women or 1.9 for men represent high levels of physical activity.

Energy expenditure = BMR x Physical Activity Level (PAL)

Using this approach and published data, estimates of average energy requirements for different population groups have been established.

Physical activity

Physical activity should be an important component of our daily energy expenditure. Many different types of activity contribute to our total physical activity, all of which form an integral part of everyday life. Total physical activity includes occupational activity, household chores, caregiving, leisure-time activity, transport (walking or cycling to work) and sport. Physical activity can further be categorised in terms of the frequency, duration and intensity of the activity.

Find out about how much physical activity adults and children should be doing on our page on physical activity recommendations.

How much energy do children and adults need?

The Estimated Average Requirements (EARs) for energy for the UK population were originally set by the Committee on the Medical Aspects of Food and Nutrition Policy (COMA) in 1991 and were reviewed in 2011 by the Scientific Advisory Committee on Nutrition (SACN) because the evidence base had moved on substantially, and over the same period, the levels of overweight and obesity in the UK had risen sharply. 

EARs for an individual vary throughout the life course. During infancy and childhood, it is essential that energy is sufficient to meet requirements for growth, which is rapid during some stages of childhood. Energy requirements tend to increase up to the age of 15-18 years. On average, boys have slightly higher requirements than girls and this persists throughout adulthood, being linked to body size and muscle mass. After the age of about 18 years, energy requirements tend to decrease but remain linked with the individual’s level of activity. After the age of 50 years, energy requirements are estimated to decrease further in women in particular and after age 60 years in men, which is partly due to a reduction in the basal metabolic rate (BMR), as well as a reduced level of activity and an assumed reduction in body weight.

Find out more about the EARs for the UK population on our page on nutrient requirements.

Intake versus expenditure

In order for people to maintain their bodyweight, their energy intake must equal their energy expenditure. Failure to maintain energy balance will result in weight change. Energy balance can be maintained by regulating energy intake (through the diet), energy expenditure (adjusting physical activity level to match intake) or a combination of both. Failure to compensate for an increase in energy intake with an increase in expenditure will result in weight gain (positive energy balance); conversely, a reduction in energy intake which isn’t matched by a reduction in physical activity levels will result in weight loss (negative energy balance).

The average daily energy intake of UK adults aged 19-64 years is 8630 kJ (2053 kcal) for men and 6750 kJ (1605 kcal) for women. These figures are below the EARs for both men and women and have been falling steadily, year on year, for some time. At the same time, the population has become ever more sedentary and population obesity levels are still on the increase. Assuming the estimates of intake are correct, this means that energy expenditure levels have fallen to a greater extent than the reduction in dietary energy intake. This emphasizes the need for people to become more active because as energy intake falls, the greater the likelihood that micronutrient needs will no longer be met. 

The easiest way to increase physical activity level is to incorporate more activity into daily routines, like walking or cycling instead of driving short distances and taking up more active hobbies such as gardening or rambling. Within the workplace, there are fewer opportunities for increasing activity levels, but stairs can be used instead of the lift and people can walk to speak to colleagues rather than using the phone or email.

Below are some examples of the amount of energy expended over a period of 30 minutes for a selection of activities:

Last reviewed July 2009. Revised October 2022.