Basal metabolic rate (BMR) is the amount of energy expended while at rest in a neutrally temperate environment, in the post-absorptive state (meaning that the digestive system is inactive, which requires about twelve hours of fasting in humans). The release of energy in this state is sufficient only for the functioning of the vital organs, such as the brain, skin, heart, muscles, liver, sex organs, lungs, nervous system, and kidneys. BMR decreases with age and with the loss of lean body mass. Increased cardiovascular exercise and muscle mass can increase BMR. Illness, previously consumed food and beverages, environmental temperature, and stress levels can affect ones overall energy expenditure, and can affect one's BMR as revealed by gas analysis. An accurate BMR measurement requires that the person's sympathetic nervous system is not stimulated. Basal metabolic rate is measured under very restrictive circumstances. A more common and closely related measurement, used under less strict conditions, is resting metabolic rate (RMR).
BMR and RMR are measured by gas analysis through either direct or indirect calorimetry, though a rough estimation can be acquired through an equation using age, sex, height, and weight. Studies of energy metabolism using both methods provide convincing evidence for the validity of the respiratory quotient (R.Q.), which measures the inherent composition and utilization of carbohydrates, fats and proteins as they are converted to energy substrate units that can be converted by the body to energy.