Editing Basal metabolic rate (section)

==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 (anatomy)|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:<ref name="Harris1918"/>
: '''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:<ref name="Harris1918">{{cite journal |vauthors=Harris J, Benedict F |year=1918 |title=A Biometric Study of Human Basal Metabolism |journal=[[PNAS]] |volume=4 |issue=12 |pages=370–373 |bibcode=1918PNAS....4..370H |doi=10.1073/pnas.4.12.370 |pmc=1091498 |pmid=16576330|doi-access=free }}</ref>
: for men, <math>P = \left( \frac{13.7516 m}{1~\text{kg}} + \frac{5.0033 h}{1~\text{cm}} - \frac{6.7550 a}{1~\text{year}} + 66.4730 \right) \frac{\text{kcal}}{\text{day}},</math>
: for women, <math>P = \left( \frac{9.5634 m}{1~\text{kg}} + \frac{1.8496 h}{1~\text{cm}} - \frac{4.6756 a}{1~\text{year}} + 655.0955 \right) \frac{\text{kcal}}{\text{day}}.</math>

The difference in BMR for men and women is mainly due to differences in body mass. For example, a 55-year-old woman weighing {{convert|130|lb|kg}} and {{convert|66|in|cm|0}} tall would have a BMR of {{convert|1272|kcal|kJ}} per day.

;The revised Harris–Benedict equation
In 1984, the original Harris–Benedict equations were revised<ref>{{cite journal |vauthors=Roza AM, Shizgal HM |year=1984 |title=The Harris Benedict equation reevaluated: resting energy requirements and the body cell mass |url=http://www.balanseradstyrka.se/wp-content/uploads/2015/04/The-Harris-Benedict-equation-reevaluated-resting-energy-requirements-and-the-body-cell-mass..pdf |journal=[[The American Journal of Clinical Nutrition]] |volume=40 |issue=1 |pages=168–182 |pmid=6741850 |doi=10.1093/ajcn/40.1.168}}</ref> using new data. In comparisons with actual expenditure, the revised equations were found to be more accurate:<ref>{{cite journal |vauthors=Müller B, Merk S, Bürgi U, Diem P |year=2001 |title=Calculating the basal metabolic rate and severe and morbid obesity |journal=[[Praxis Journal of Philosophy|Praxis]] |volume=90 |issue=45 |pages=1955–63 |pmid=11817239}}</ref>
: for men, <math>P = \left( \frac{13.397 m}{1~\text{kg}} + \frac{4.799 h}{1~\text{cm}} - \frac{5.677 a}{1~\text{year}} + 88.362 \right) \frac{\text{kcal}}{\text{day}},</math>
: for women, <math>P = \left( \frac{9.247 m}{1~\text{kg}} + \frac{3.098 h}{1~\text{cm}} - \frac{4.330 a}{1~\text{year}} + 447.593 \right) \frac{\text{kcal}}{\text{day}}.</math>
It was the best prediction equation until 1990, when Mifflin ''et al.''<ref>{{cite journal |vauthors=Mifflin MD, St Jeor ST, Hill LA, Scott BJ, Daugherty SA, Koh YO |year=1990 |title=A new predictive equation for resting energy expenditure in healthy individuals |journal=[[The American Journal of Clinical Nutrition]] |volume=51 |issue=2 |pages=241–247 |pmid=2305711|doi=10.1093/ajcn/51.2.241 }}</ref> introduced the equation:

;The Mifflin St Jeor equation
: <math>P = \left( \frac{10.0 m}{1~\text{kg}} + \frac{6.25 h}{1~\text{cm}} - \frac{5.0 a}{1~\text{year}} + s \right) \frac{\text{kcal}}{\text{day}},</math>
where ''s'' is +5 for males and −161 for females.

According to this formula, the woman in the example above has a BMR of {{convert|1204|kcal|kJ}} per day.
During the last 100&nbsp;years, lifestyles have changed, and Frankenfield ''et al.''<ref>{{cite journal |vauthors=Frankenfield D, Roth-Yousey L, Compher C |year=2005 |title=Comparison of predictive equations for resting metabolic rate in healthy, nonobese and obese adults: A systematic review |journal=[[Journal of the American Dietetic Association]] |volume=105 |issue=5 |pages=775–789 |doi=10.1016/j.jada.2005.02.005 |pmid=15883556}}</ref> 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).<ref>{{cite book |last=McArdle |first=William |year=2006 |title=Essentials of exercise physiology |edition= 3rd |page=266 |publisher=Lippincott Williams & Wilkins |isbn=978-0-7817-4991-6}}</ref>
The Cunningham formula is commonly cited to predict RMR instead of BMR; however, the formulas provided by Katch–McArdle and Cunningham are the same.<ref>{{cite book |last1=Dunford |first1=Marie |last2=Doyle |first2=J. Andrew |year=2007 |title=Nutrition for Sport and Exercise |page=57 |publisher=Brooks/Cole |isbn=978-0-495-01483-6}}</ref>
: <math> P = 370 + 21.6 \cdot \ell,</math>
where '''''ℓ''''' is the lean body mass (''LBM'' in kg):
: <math> \ell = m \left( 1 - \frac{f}{100} \right),</math>
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&nbsp;kcal per day.

===Research on individual differences in BMR===
The basic metabolic rate varies between individuals. One study of 150&nbsp;adults representative of the population in Scotland reported basal metabolic rates from as low as {{convert|1027|kcal|kJ}} per day to as high as {{convert|2499|kcal|kJ}}, with a mean BMR of {{convert|1500|kcal|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.<ref name="Johnstone">{{cite journal |vauthors=Johnstone AM, Murison SD, Duncan JS, Rance KA, Speakman JR, Koh YO |year=2005 |title=Factors influencing variation in basal metabolic rate include fat-free mass, fat mass, age, and circulating thyroxine but not sex, circulating leptin, or triiodothyronine |journal=[[American Journal of Clinical Nutrition]] |volume=82 |issue=5 |pages=941–948 |pmid=16280423 |doi=10.1093/ajcn/82.5.941|doi-access=free }}</ref>

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.<ref name="speak">{{cite journal |vauthors=Speakman JR, et al. |year=2023 |title= Total daily energy expenditure has declined over the last 3 decades due to declining basal expenditure, not activity expenditure |volume=5 | pages=579–585 |journal=[[Nature Metabolism]] |issue=4 |doi=10.1038/s42255-023-00782-2|pmid=37100994|pmc=10445668 }}</ref> The decline was also observed in a [[meta-analysis]] of more than 150 studies dating back to the early 1920s, translating into a decline in total energy expenditure of about 6%.<ref name="speak"/>