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 body weight, 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.
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.
British Nutrition Foundation