Resting Metabolic Rate (RMR), is the amount of energy expended daily by humans at rest. Rest is defined as existing in a neutrally temperate environment while in the post-absorptive state. The release of energy in this state is sufficient only for the functioning of the vital organs, the heart, lungs, nervous system, kidneys, liver, intestine, sex organs, muscles, and skin.
The body’s generation of heat is known as thermogenesis and it can be measured to determine the amount of energy expended. RMR decreases with age generally (as people usually don’t maintain lean body mass) and with the loss of lean body mass. Increasing muscle mass increases RMR. Aerobic fitness level, a product of cardiovascular exercise, while previously thought to have effect on RMR, has been shown in the 1990s not to correlate with RMR, when fat-free body mass was adjusted for.
New research has come to light that suggests anaerobic exercise does increase resting energy consumption. Illness, previously consumed food and beverages, environmental temperature, and stress levels can affect one’s overall energy expenditure as well as one’s RMR.
RMR is measured when a person is awake. An accurate RMR measurement requires that the person’s sympathetic nervous system not be stimulated, a condition which requires complete rest.
RMR is 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 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 used by the body as energy.