Expansion
Energy expenditure at rest, mainly determined by lean body mass
Expansion
Basal metabolic rate is measured at rest, awake, fasted for 12 hours and thermoneutral. It is about 1 kcal per kg per hour, or roughly 1600 to 1800 kcal a day in an adult man, and accounts for 60 to 70 per cent of total energy expenditure.
Raised by
- Greater lean body mass (the strongest determinant)
- Thyroid hormone, catecholamines, growth hormone
- Fever: about 10 per cent per degree Celsius
- Pregnancy, lactation, growth
- Burns, sepsis and trauma
- Cold exposure
Lowered by
- Ageing, starvation and prolonged undernutrition (adaptive thermogenesis)
- Hypothyroidism
- Sleep
Organ contributions are disproportionate: liver, brain, heart and kidney together are about 5 per cent of body weight but account for 60 to 70 per cent of basal expenditure, while muscle at rest contributes relatively little despite its mass.
Total expenditure adds the thermic effect of food (about 10 per cent) and physical activity (20 to 30 per cent, the most variable component).