basal metabolic rate basal metabolic rate 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 led to problems in defining "standard" rates of metabolism for many mammals. metabolism comprises the processes that the body needs to function. basal metabolic rate is the amount of energy per unit of time that a person needs to keep the body functioning at rest. some of those processes are breathing, blood circulation, controlling body temperature, cell growth, brain and nerve function, and contraction of muscles. basal metabolic rate affects the rate that a person burns calories and ultimately whether that individual maintains, gains, or loses weight. the basal metabolic rate accounts for about 60 to 75% of the daily calorie expenditure by individuals. it is influenced by several factors. in humans, bmr typically declines by 1–2% per decade after age 20, mostly due to loss of fat-free mass, although the variability between individuals is high. description the body's generation of heat is known as thermogenesis and it can be measured to determine the amount of energy expended. bmr generally decreases with age, and with the decrease in lean body mass (as may happen with aging). increasing muscle mass has the effect of increasing bmr. aerobic (resistance) fitness level, a product of cardiovascular exercise, while previously thought to have effect on bmr, has been shown in the 1990s not to correlate with bmr when adjusted for fat-free body mass. but anaerobic exercise does increase resting energy consumption (see "aerobic vs. anaerobic exercise"). illness, previously consumed food and beverages, environmental temperature, and stress levels can affect one's overall energy expenditure as well as one's bmr. indirect calorimetry laboratory with canopy hood (dilution technique) bmr is measured under very restrictive circumstances when a person is awake. an accurate bmr measurement requires that the person's sympathetic nervous system not be stimulated, a condition which requires complete rest. a more common measurement, which uses less strict criteria, is resting metabolic rate (rmr). bmr may be 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 (rq), 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. phenotypic flexibility bmr is a flexible trait (it can be reversibly adjusted within individuals), with, for example, lower temperatures generally resulting in higher basal metabolic rates for both birds and rodents. there are two models to explain how bmr changes in response to temperature: the variable maximum model (vmm) and variable fraction model (vfm). the vmm states that the summit metabolism (or the maximum metabolic rate in response to the cold) increases during the winter, and that the sustained metabolism (or the metabolic rate that can be indefinitely sustained) remains a constant fraction of the former. the vfm says that the summit metabolism does not change, but that the sustained metabolism is a larger fraction of it. the vmm is supported in mammals, and, when using whole-body rates, passerine birds. the vfm is supported in studies of passerine birds using mass-specific metabolic rates (or metabolic rates per unit of mass). this latter measurement has been criticized by eric liknes, sarah scott, and david swanson, who say that mass-specific metabolic rates are inconsistent seasonally. in addition to adjusting to temperature, bmr also may adjust before annual migration cycles. the red knot (ssp. islandica) increases its bmr by about 40% before migrating northward. this is because of the energetic demand of long-distance flights. the increase is likely primarily due to increased mass in organs related to flight. the end destination of migrants affects their bmr: yellow-rumped warblers migrating northward were found to have a 31% higher bmr than those migrating southward. in humans, bmr is directly proportional to a person's lean body mass. in other words, the more lean body mass a person has, the higher their bmr; but bmr is also affected by acute illnesses and increases with conditions like burns, fractures, infections, fevers, etc. in menstruating females, bmr varies to some extent with the phases of their menstrual cycle. due to the increase in progesterone, bmr rises at the start of the luteal phase and stays at its highest until this phase ends. there are different findings in research how much of an increase usually occurs. small sample, early studies, found various figures, such as; a 6% higher postovulatory sleep metabolism, a 7% to 15% higher 24 hour expenditure following ovulation, and an increase and a luteal phase bmr increase by up to 12%. a study by the american society of clinical nutrition found that an experimental group of female volunteers had an 11.5% average increase in 24 hour energy expenditure in the two weeks following ovulation, with a range of 8% to 16%. this group was measured via simultaneously direct and indirect calorimetry and had standardized daily meals and sedentary schedule in order to prevent the increase from being manipulated by change in food intake or activity level. a 2011 study conducted by the mandya institute of medical sciences found that during a woman's follicular phase and menstrual cycle is no significant difference in bmr, however the calories burned per hour is significantly higher, up to 18%, during the luteal phase. increased state anxiety (stress level) also temporarily increased bmr. physiology the early work of the scientists j. arthur harris and francis g. benedict showed that approximate values for bmr could be derived using body surface area (computed from height and weight), age, and sex, along with the oxygen and carbon dioxide measures taken from calorimetry. studies also showed that by eliminating the sex differences that occur with the accumulation of adipose tissue by expressing metabolic rate per unit of "fat-free" or lean body mass, the values between sexes for basal metabolism are essentially the same. exercise physiology textbooks have tables to show the conversion of height and body surface area as they relate to weight and basal metabolic values. the primary organ responsible for regulating metabolism is the hypothalamus. the hypothalamus is located on the diencephalon and forms the floor and part of the lateral walls of the third ventricle of the cerebrum. the chief functions of the hypothalamus are: 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. bmr estimation formulas several equations to predict the number of calories required by humans have been published from the early 20th–21st centuries. in each of the formulas below: p is total heat production at complete rest, m is mass (kg), h is height (cm), a is age (years). the original harris–benedict equation historically, the most notable formula was the harris–benedict equation, which was published in 1919: for men, p = ( 13.7516 m 1 kg + 5.0033 h 1 cm − 6.7550 a 1 year + 66.4730 ) kcal day , +-+66.4730\right),} for women, p = ( 9.5634 m 1 kg + 1.8496 h 1 cm − 4.6756 a 1 year + 655.0955 ) kcal day . +-+655.0955\right).} the difference in bmr for men and women is mainly due to differences in body mass. for example, a 55-year-old woman weighing 130 pounds (59 kg) and 66 inches (170 cm) tall would have a bmr of 1,272 kilocalories (5,320 kj) per day. the revised harris–benedict equation in 1984, the original harris–benedict equations were revised using new data. in comparisons with actual expenditure, the revised equations were found to be more accurate: for men, p = ( 13.397 m 1 kg + 4.799 h 1 cm − 5.677 a 1 year + 88.362 ) kcal day , +-+88.362\right),} for women, p = ( 9.247 m 1 kg + 3.098 h 1 cm − 4.330 a 1 year + 447.593 ) kcal day . +-+447.593\right).} it was the best prediction equation until 1990, when mifflin et al. introduced the equation: the mifflin st jeor equation p = ( 10.0 m 1 kg + 6.25 h 1 cm − 5.0 a 1 year + s ) kcal day , +-+s\right),} where s is +5 for males and −161 for females. according to this formula, the woman in the example above has a bmr of 1,204 kilocalories (5,040 kj) per day. during the last 100 years, lifestyles have changed, and frankenfield et al. showed it to be about 5% more accurate. these formulas are based on body mass, which does not take into account the difference in metabolic activity between lean body mass and body fat. other formulas exist which take into account lean body mass, two of which are the katch–mcardle formula and cunningham formula. the katch–mcardle formula (resting daily energy expenditure) the katch–mcardle formula is used to predict resting daily energy expenditure (rdee). the cunningham formula is commonly cited to predict rmr instead of bmr; however, the formulas provided by katch–mcardle and cunningham are the same. p = 370 + 21.6 ⋅ ℓ , where ℓ is the lean body mass (lbm in kg): ℓ = m ( 1 − f 100 ) , \right),} where f is the body fat percentage. according to this formula, if the woman in the example has a body fat percentage of 30%, her resting daily energy expenditure (the authors use the term of basal and resting metabolism interchangeably) would be 1262 kcal per day. research on individual differences in bmr the basic metabolic rate varies between individuals. one study of 150 adults representative of the population in scotland reported basal metabolic rates from as low as 1,027 kilocalories (4,300 kj) per day to as high as 2,499 kilocalories (10,460 kj); with a mean bmr of 1,500 kilocalories (6,300 kj) per day. statistically, the researchers calculated that 62% of this variation was explained by differences in fat free mass. other factors explaining the variation included fat mass (7%), age (2%), and experimental error including within-subject difference (2%). the rest of the variation (27%) was unexplained. this remaining difference was not explained by sex nor by differing tissue size of highly energetic organs such as the brain. a cross-sectional study of more than 1400 subjects in europe and the us showed that once adjusted for differences in body composition (lean and fat mass) and age, bmr has fallen over the past 35 years. the