Energy expenditure refers to the amount of energy a person uses to complete all regular bodily functions, from movement to breathing to digestion. Energy expenditure is measured in calories, a unit of heat. In the app, the momentary rate of energy expenditure is shown in kcal per day.
Energy expenditure is mainly a sum of internal heat produced and external work. The internal heat produced is, in turn, mainly a sum of basal metabolic rate and the thermic effect of food, which is related to digestion. The basal metabolic rate generally accounts for 50-70% of total energy expenditure, and is roughly 50-60 kcal per hour in females and 60-75 kcal per hour in males. The basal metabolic rate is influenced by genes, age, illness, exercise, muscle mass, and can drop by up to 45% during starvation or crash diets.
External work may be estimated by measuring the physical activity level. External work often accounts for about one-fifth of energy expenditure but may account for the majority of expenditure in some endurance athletes. Energy expenditure can vary widely from person to person and for an individual will also vary by activity. For instance, energy expenditure during sleep is ~10% lower than sitting quietly. Light intensity activities such as desk work or slow walking may expend up to three times as much energy as sitting quietly. Moderate-intensity activities such as leisurely cycling or sexual activity may expend 3-6 times as much energy as sitting quietly, while vigorous-intensity activities such as jogging or doing push-ups expend more than 6 times as much energy as sitting quietly. For those of you that can run at 22.5 km per hour it is estimated to use 23 times the amount of energy as sitting quietly!
Total energy expenditure, also known as metabolic rate, is affected by such factors as the amount of physical activity a person does, the number of hours spent sleeping, the types of foods consumed, exposure to extreme temperatures, and the amount of muscle mass a person carries.