Basal metabolic rate (BMR) is the rate of energy expenditure per unit time by endothermic animals at rest. It is reported in energy units per unit time Basal metabolic rate (BMR) is the rate of energy expenditure per unit time by endothermic animals at rest. It is reported in energy units per unit time ranging from watt (joule/second) to ml O2/min or joule per hour per kg body mass J/(h·kg). Proper measurement requires a strict set of criteria to be met. These criteria include being in a physically and psychologically undisturbed state and being In 1926, Raymond Pearl proposed that longevity varies inversely with basal metabolic rate (the "rate of living hypothesis"). Support for this hypothesis comes from the fact that mammals with larger body size have longer maximum life spans (large animals do have higher total metabolic rates, but the metabolic rate at the cellular level is much lower, and the breathing rate and heartbeat are slower in larger animals) and the fact that the longevity of fruit flies varies inversely with ambient temperature. Additionally, the life span of houseflies can be extended by preventing physical activity. This theory has been bolstered by several new studies linking lower basal metabolic rate to increased life expectancy, across the animal kingdom—including humans. Calorie restriction and reduced thyroid hormone levels, both of which decrease the metabolic rate, have been associated with higher longevity in animals. However, the ratio of total daily energy expenditure to resting metabolic rate can vary between 1.6 and 8.0 between species of mammals. Animals also vary in the degree of coupling between oxidative phosphorylation and ATP production, the amount of saturated fat in mitochondrial membranes, the amount of DNA repair, and many other factors that affect maximum life span. One problem with understanding the associations of lifespan and metabolism is that changes in metabolism are often confounded by other factors that may affec Basal metabolic rate (BMR) is the rate of energy expenditure per unit time by endothermic animals at rest. It is reported in energy units per unit time ranging from watt (joule/second) to ml O2/min or joule per hour per kg body mass J/(h·kg). Proper measurement requires a strict set of criteria to be met. These criteria include being in a physically and psychologically undisturbed state and being in a thermally neutral environment while in the post-absorptive state (i.e., not actively digesting food). In bradymetabolic animals, such as fish and reptiles, the equivalent term standard metabolic rate (SMR) applies. It follows the same criteria as BMR, but requires the documentation of the temperature at which the metabolic rate was measured. This makes BMR a variant of standard metabolic rate measurement that excludes the temperature data, a practice that has control and integration of activities of the autonomic nervous system (ANS) The ANS regulates contraction of smooth muscle and cardiac muscle, along with secretions of many endocrine organs such as the thyroid gland (associated with many metabolic disorders). Through the ANS, the hypothalamus is the main regulator of visceral activities, such as heart rate, movement of food through the gastrointestinal tract, and contraction of the urinary bladder. production and regulation of feelings of rage and aggression regulation of body temperature regulation of food intake, through two centers: The feeding center or hunger center is responsible for the sensations that cause us to seek food. When sufficient food or substrates have been received and leptin is high, then the satiety center is stimulated and sends impulses that inhibit the feeding center. When insufficient food is present in the stomach and ghrelin levels are high, receptors in the hypothalamus initiate the sense of hunger. The thirst center operates similarly when certain cells in the hypothalamus are stimulated by the rising osmotic pressure of the extracellular fluid. If thirst is satisfied, osmotic pressure decreases. All of these functions taken together form a survival mechanism that causes us to sustain the body processes that BMR measures. In 1926, Raymond Pearl proposed that longevity varies inversely with basal metabolic rate (the "rate of living hypothesis"). Support for this hypothesis comes from the fact that mammals with larger body size have longer maximum life spans (large animals do have higher total metabolic rates, but the metabolic rate at the cellular level is much lower, and the breathing rate and heartbeat are slower in larger animals) and the fact that the longevity of fruit flies varies inversely with ambient temperature. Additionally, the life span of houseflies can be extended by preventing physical activity. This theory has been bolstered by several new studies linking lower basal metabolic rate to increased life expectancy, across the animal kingdom—including humans. Calorie restriction and reduced thyroid hormone levels, both of which decrease the metabolic rate, have been associated with higher longevity in animals. However, the ratio of total daily energy expenditure to resting metabolic rate can vary between 1.6 and 8.0 between species of mammals. Animals also vary in the degree of coupling between oxidative phosphorylation and ATP production, the amount of saturated fat in mitochondrial membranes, the amount of DNA repair, and many other factors that affect maximum life span. One problem with understanding the associations of lifespan and metabolism is that changes in metabolism are often confounded by other factors that may affect lifespan. For example, under calorie restriction whole body metabolic rate goes down with increasing levels of restriction, but body temperature also follows the same pattern. By manipulating the ambient temperature and exposure to wind it was shown in mice and hamsters that body temperature is a more important modulator of lifespan than metabolic rate.