Basal metabolic rate (BMR) is the amount of energy expened 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 heart, lungs, brain and the rest of the nervous system, liver, kidneys, sex organs, muscles and skin. BMR decreases with age and with the loss of lean body mass. Increased muscle mass can increase BMR. Aerobic fitness level, a product of cardiovascular exercise, while previously thought to have effect on basal or resting metabolic rate (RMR), has been shown in the 1990s not to correlate with RMR, when fat-free body mass was adjusted for (see following section "Aerobic vs. anaerobic exercise" for references). Illness, previously consumed food and beverages, environmental temperature, and stress levels can affect one's overall energy expenditure as well as one's BMR. [Note: BMR can be monitored (not measured) with BMT - See Presymtec.]