Human consumption and expenditure of energy, alongside any change in the body’s macronutrient stores (fat, protein, and carbohydrate) is summarised by the energy balance equation:
As depicted by Figure P.1.1, total human energy expenditure consists of the following elements:
Sleeping metabolic rate and the energy cost of arousal together make up resting energy expenditure. This is similar to basal energy expenditure or basal metabolic rate, although the latter has a more strict definition of how it is measured; in the fasted state at complete rest the morning after sleep. This is the energy required to maintain cellular function. Resting metabolic rate is slightly higher than basal metabolic energy expenditure (typically within 10%).
Resting energy expenditure is normally the largest component of daily energy expenditure, and is greater in individuals of greater mass, particularly fat-free mass. Physical activity energy expenditure is the most variable day-to-day and is directly related to movement. Due to their close relationship with body size, these components are typically adjusted for body mass or fat-free mass.