The early work of the scientists J. Arthur Harris and Francis G. Benedict showed that approximate values for<unk>  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<unk> <unk>  per unit of "fat-free" or lean body mass, the values between sexes for basal<unk> m 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<unk>  values. The primary organ responsible for regulating<unk> m 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<unk>  disorders). #* Through the ANS, the hypothalamus is the main regulator of visceral activities, such as heart<unk> , 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<unk>  measures.<unk>  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, : for women, The difference in<unk>  for men and women is mainly due to differences in body mass. For example, a 55-year-old woman weighing 130 lb (59 kg) and 5 feet 6 inches (168 cm) tall would have a<unk>  of 1272 kcal 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, : for women, It was the best prediction equation until 1990, when Mifflin et al. introduced the equation: ;The Mifflin St Jeor equation : where s is +5 for males and −161 for females. According to this formula, the woman in the example above has a<unk>  of 1204 kcal 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<unk>  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<unk> ; however, the formulas provided by Katch–McArdle and Cunningham are the same. : where ℓ is the lean body mass (LBM in kg): : 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<unk> m interchangeably) would be 1262 kcal per day. Causes of individual differences in<unk>  The basic<unk> <unk>  varies between individuals. One study of 150 adults representative of the population in Scotland reported<unk> s from as low as 1027 kcal per day (4301 kJ/day) to as high as 2499 kcal/day (10455 kJ/day); with a mean<unk>  of 1500 kcal/day (6279 kJ/day). Statistically, the researchers calculated that 62.3% of this variation was explained by differences in fat free mass. Other factors explaining the variation included fat mass (6.7%), age (1.7%), and experimental error including within-subject difference (2%). The rest of the variation (26.7%) was unexplained. This remaining difference was not explained by sex nor by differing tissue size of highly energetic organs such as the brain. Differences in<unk>  have been observed when comparing subjects with the same lean body mass. In one study, when comparing individuals with the same lean body mass, the top 5% of<unk> s are 1.28–1.32 times the lowest 5%<unk> . However, this study did not account for the sex, height, fasting-state, or body fat percentage of the subjects.
Basal metabolic rate
Basal body temperature
Heart rate variability
Basal rate
Electrodermal activity
Glycemic index
Cardiac reserve
Renal threshold