Tasa metabólica basal

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La tasa metabólica basal ( TMB ) es la tasa de gasto energético por unidad de tiempo de los animales endotérmicos en reposo. ^[1] Se informa en unidades de energía por unidad de tiempo que van desde vatios (julio / segundo) hasta ml O ₂ / min o julio por hora por kg de masa corporal J / (h · kg). Una medición adecuada requiere que se cumpla un estricto conjunto de criterios. Estos criterios incluyen estar en un estado de tranquilidad física y psicológica, en un ambiente térmicamente neutro , mientras que en el estado de posabsorción (es decir, sin digerir alimentos activamente ). ^[1] En animales bradimetabólicos , como peces yreptiles , se utiliza el término equivalente tasa metabólica estándar ( SMR ). Sigue los mismos criterios que la TMB, pero requiere la documentación de la temperatura a la que se midió la tasa metabólica. Esto hace que la TMB sea una variante de la medición de la tasa metabólica estándar que excluye los datos de temperatura, una práctica que ha dado lugar a problemas en la definición de tasas de metabolismo "estándar" para muchos mamíferos. ^[1]

El metabolismo comprende los procesos que el cuerpo necesita para funcionar. ^[2] La tasa metabólica basal es la cantidad de energía por unidad de tiempo que una persona necesita para mantener el funcionamiento del cuerpo en reposo. Algunos de esos procesos son la respiración , la circulación sanguínea , el control de la temperatura corporal , el crecimiento celular , la función cerebral y nerviosa y la contracción de los músculos.. La tasa metabólica basal afecta la velocidad a la que una persona quema calorías y, en última instancia, si esa persona mantiene, gana o pierde peso. La tasa metabólica basal representa alrededor del 60 al 75% del gasto calórico diario de los individuos. Está influenciado por varios factores. La TMB generalmente disminuye en un 1-2% por década después de los 20 años, principalmente debido a la pérdida de masa libre de grasa , ^[3] aunque la variabilidad entre individuos es alta. ^[4]

Descripción

La generación de calor del cuerpo se conoce como termogénesis y se puede medir para determinar la cantidad de energía gastada. La TMB generalmente disminuye con la edad y con la disminución de la masa corporal magra (como puede ocurrir con el envejecimiento). El aumento de la masa muscular tiene el efecto de aumentar la TMB. El nivel de condición física aeróbica (resistencia) , un producto del ejercicio cardiovascular , mientras que antes se pensaba que tenía un efecto sobre la TMB, se demostró en la década de 1990 que no se correlaciona con la TMB cuando se ajusta a la masa corporal libre de grasa. ^{[ cita requerida ]} Pero el ejercicio anaeróbico aumenta el consumo de energía en reposo (ver " ejercicio aeróbico vs. anaeróbico "). ^[5] Las enfermedades, los alimentos y bebidas consumidos anteriormente, la temperatura ambiental y los niveles de estrés pueden afectar el gasto energético general y la TMB.

Laboratorio de calorimetría indirecta con campana de dosel (técnica de dilución)

La TMB se mide en circunstancias muy restrictivas cuando una persona está despierta. Una medición precisa de la TMB requiere que no se estimule el sistema nervioso simpático de la persona , una condición que requiere un descanso completo. Una medida más común, que utiliza criterios menos estrictos, es la tasa metabólica en reposo (RMR) . ^[6]

La TMB se puede medir mediante análisis de gases mediante calorimetría directa o indirecta , aunque se puede obtener una estimación aproximada mediante una ecuación que utilice la edad, el sexo, la altura y el peso. Los estudios del metabolismo energético que utilizan ambos métodos proporcionan evidencia convincente de la validez del cociente respiratorio (RQ), que mide la composición inherente y la utilización de carbohidratos , grasas y proteínas a medida que se convierten en unidades de sustrato energético que el cuerpo puede utilizar como energía.

Flexibilidad fenotípica

La TMB es un rasgo flexible (se puede ajustar de forma reversible dentro de los individuos), con, por ejemplo, temperaturas más bajas que generalmente resultan en tasas metabólicas basales más altas tanto para las aves ^[7] como para los roedores. ^[8]Hay dos modelos para explicar cómo cambia la TMB en respuesta a la temperatura: el modelo de máximo variable (VMM) y el modelo de fracción variable (VFM). El VMM afirma que el metabolismo de la cumbre (o la tasa metabólica máxima en respuesta al frío) aumenta durante el invierno, y que el metabolismo sostenido (o la tasa metabólica que puede mantenerse indefinidamente) permanece como una fracción constante del primero. El VFM dice que el metabolismo de la cumbre no cambia, pero que el metabolismo sostenido es una fracción mayor del mismo. El VMM es compatible con mamíferos y, cuando se utilizan tasas de cuerpo entero, aves paseriformes. El VFM está respaldado en estudios de aves paseriformes utilizando tasas metabólicas específicas de masa (o tasas metabólicas por unidad de masa). Esta última medida ha sido criticada por Eric Liknes, Sarah Scott y David Swanson,que dicen que las tasas metabólicas específicas de la masa son inconsistentes estacionalmente.^[9]

Además de ajustarse a la temperatura, BMR también puede ajustarse antes de los ciclos de migración anual. ^[7] El nudo rojo (ssp. Islandica ) aumenta su TMB en aproximadamente un 40% antes de migrar hacia el norte. Esto se debe a la enérgica demanda de vuelos de larga distancia. Es probable que el aumento se deba principalmente al aumento de la masa en los órganos relacionados con el vuelo. ^[10] El destino final de los migrantes afecta su TMB: se encontró que las currucas de rabadilla amarilla que migran hacia el norte tienen un TMB 31% más alto que las que migran hacia el sur. ^[7]

En los seres humanos, la TMB es directamente proporcional a la masa corporal magra de una persona . ^{[11] [12]} En otras palabras, cuanto más masa corporal magra tiene una persona, mayor es su TMB; pero la TMB también se ve afectada por enfermedades agudas y aumenta con afecciones como quemaduras, fracturas, infecciones, fiebres, etc. ^[12] En las mujeres que menstrúan, la TMB varía en cierta medida con las fases de su ciclo menstrual . Debido al aumento de la progesterona , la TMB aumenta al comienzo de la fase lútea.y permanece en su punto más alto hasta que finaliza esta fase. Hay diferentes hallazgos en la investigación sobre cuánto aumento suele ocurrir. Una pequeña muestra, los primeros estudios, encontraron varias figuras, tales como; un 6% más de metabolismo postovulatorio del sueño, ^[13] un 7% a 15% más de gasto en 24 horas después de la ovulación, ^[14] y un aumento y un aumento de la TMB en la fase lútea hasta en un 12%. ^{[15] [16]} 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.^[17] 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.^[18]

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:

1. 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.
2. production and regulation of feelings of rage and aggression
3. regulation of body temperature
4. 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), and
• a   is age (years),^[19]

The original Harris-Benedict equation

Historically, the most notable formula was the Harris–Benedict equation, which was published in 1919.

• for men, ${\displaystyle P=\left({\frac {13.7516m}{1~{\mbox{kg}}}}+{\frac {5.0033h}{1~{\mbox{cm}}}}-{\frac {6.7550a}{1~{\mbox{year}}}}+66.4730\right){\frac {\mbox{kcal}}{\mbox{day}}}}$
• for women, ${\displaystyle P=\left({\frac {9.5634m}{1~{\mbox{kg}}}}+{\frac {1.8496h}{1~{\mbox{cm}}}}-{\frac {4.6756a}{1~{\mbox{year}}}}+655.0955\right){\frac {\mbox{kcal}}{\mbox{day}}}}$^[19]

The difference in BMR for men and women is mainly due to differences in body weight. For example, a 55-year-old woman weighing 130 lb (59 kg) and 5 feet 6 inches (168 cm) tall would have a BMR of 1272 kcal per day.

The revised Harris-Benedict equation

In 1984, the original Harris-Benedict equations were revised^[20] using new data. In comparisons with actual expenditure, the revised equations were found to be more accurate.^[21]

• for men, ${\displaystyle P=\left({\frac {13.397m}{1~{\mbox{kg}}}}+{\frac {4.799h}{1~{\mbox{cm}}}}-{\frac {5.677a}{1~{\mbox{year}}}}+88.362\right){\frac {\mbox{kcal}}{\mbox{day}}}}$
• for women, ${\displaystyle P=\left({\frac {9.247m}{1~{\mbox{kg}}}}+{\frac {3.098h}{1~{\mbox{cm}}}}-{\frac {4.330a}{1~{\mbox{year}}}}+447.593\right){\frac {\mbox{kcal}}{\mbox{day}}}}$

It was the best prediction equation until 1990, when Mifflin et al.^[22] introduced the equation:

The Mifflin St Jeor equation

• ${\displaystyle P=\left({\frac {10.0m}{1~{\mbox{kg}}}}+{\frac {6.25h}{1~{\mbox{cm}}}}-{\frac {5.0a}{1~{\mbox{year}}}}+s\right){\frac {\mbox{kcal}}{\mbox{day}}}}$, where s is +5 for males and −161 for females.

According to this formula, the woman in the example above has a BMR of 1204 kcal per day. During the last 100 years, lifestyles have changed and Frankenfield et al.^[23] showed it to be about 5% more accurate.

These formulas are based on body weight, 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).^[24]The Cunningham formula is commonly cited to predict RMR instead of BMR; however, the formulas provided by Katch-McArdle and Cunningham are the same.^[25]

• ${\displaystyle P=370+\left(21.6\cdot \ell \right)}$

where ℓ is the lean body mass (LBM in kg)

• ${\displaystyle \ell =m\left(1-{\frac {f}{100}}\right)}$

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.

Causes of 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 1027 kcal per day (4301 kJ/day) to as high as 2499 kcal/day (10455 kJ/day); with a mean BMR 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.^[26]

Differences in BMR 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 BMRs are 1.28–1.32 times the lowest 5% BMR.^[27] Additionally, this study reports a case where two individuals with the same lean body mass of 43 kg had BMRs of 1075 kcal/day (4.5 MJ/day) and 1790 kcal/day (7.5 MJ/day). This difference of 715 kcal/day (67%) is equivalent to one of the individuals completing a 10 kilometer run every day.^[27] However, this study did not account for the sex, height, fasting-state, or body fat percentage of the subjects.

Biochemistry

Energy expenditure breakdown[28]
Liver	27%
Brain	19%
Skeletal Muscle	18%
Kidneys	10%
Heart	7%
Other organs	19%

Postprandial thermogenesis increases in basal metabolic rate occur at different degrees depending on consumed food composition.

About 70% of a human's total energy expenditure is due to the basal life processes taking place in the organs of the body (see table). About 20% of one's energy expenditure comes from physical activity and another 10% from thermogenesis, or digestion of food (postprandial thermogenesis).^[29] All of these processes require an intake of oxygen along with coenzymes to provide energy for survival (usually from macronutrients like carbohydrates, fats, and proteins) and expel carbon dioxide, due to processing by the Krebs cycle.

For the BMR, most of the energy is consumed in maintaining fluid levels in tissues through osmoregulation, and only about one-tenth is consumed for mechanical work, such as digestion, heartbeat, and breathing.^[30]

What enables the Krebs cycle to perform metabolic changes to fats, carbohydrates, and proteins is energy, which can be defined as the ability or capacity to do work. The breakdown of large molecules into smaller molecules—associated with release of energy—is catabolism. The building up process is termed anabolism. The breakdown of proteins into amino acids is an example of catabolism, while the formation of proteins from amino acids is an anabolic process.

Exergonic reactions are energy-releasing reactions and are generally catabolic. Endergonic reactions require energy and include anabolic reactions and the contraction of muscle. Metabolism is the total of all catabolic, exergonic, anabolic, endergonic reactions.

Adenosine Triphosphate (ATP) is the intermediate molecule that drives the exergonic transfer of energy to switch to endergonic anabolic reactions used in muscle contraction. This is what causes muscles to work which can require a breakdown, and also to build in the rest period, which occurs during the strengthening phase associated with muscular contraction. ATP is composed of adenine, a nitrogen containing base, ribose, a five carbon sugar (collectively called adenosine), and three phosphate groups. ATP is a high energy molecule because it stores large amounts of energy in the chemical bonds of the two terminal phosphate groups. The breaking of these chemical bonds in the Krebs Cycle provides the energy needed for muscular contraction.

Glucose

Because the ratio of hydrogen to oxygen atoms in all carbohydrates is always the same as that in water—that is, 2 to 1—all of the oxygen consumed by the cells is used to oxidize the carbon in the carbohydrate molecule to form carbon dioxide. Consequently, during the complete oxidation of a glucose molecule, six molecules of carbon dioxide and six molecules of water are produced and six molecules of oxygen are consumed.

The overall equation for this reaction is:

${\displaystyle {\ce {C6H12O6 + 6 O2 -> 6 CO2 + 6 H2O}}}$

(30-32 ATP molecules produced depending on type of mitochondrial shuttle, 5-5.33 ATP molecules per molecule of oxygen)

Because the gas exchange in this reaction is equal, the respiratory quotient (R.Q.) for carbohydrate is unity or 1.0:

${\displaystyle R.Q.={\frac {6\ CO_{2}}{6\ O_{2}}}=1.0}$

Fats

The chemical composition for fats differs from that of carbohydrates in that fats contain considerably fewer oxygen atoms in proportion to atoms of carbon and hydrogen. When listed on nutritional information tables, fats are generally divided into six categories: total fats, saturated fatty acid, polyunsaturated fatty acid, monounsaturated fatty acid, dietary cholesterol, and trans fatty acid. From a basal metabolic or resting metabolic perspective, more energy is needed to burn a saturated fatty acid than an unsaturated fatty acid. The fatty acid molecule is broken down and categorized based on the number of carbon atoms in its molecular structure. The chemical equation for metabolism of the twelve to sixteen carbon atoms in a saturated fatty acid molecule shows the difference between metabolism of carbohydrates and fatty acids. Palmitic acid is a commonly studied example of the saturated fatty acid molecule.

The overall equation for the substrate utilization of palmitic acid is:

${\displaystyle {\ce {C16H32O2 + 23 O2 -> 16 CO2 + 16 H2O}}}$

(106 ATP molecules produced, 4.61 ATP molecules per molecule of oxygen)

Thus the R.Q. for palmitic acid is 0.696:

${\displaystyle R.Q.={\frac {16\ CO_{2}}{23\ O_{2}}}=0.696}$

Proteins

Proteins are composed of carbon, hydrogen, oxygen, and nitrogen arranged in a variety of ways to form a large combination of amino acids. Unlike fat the body has no storage deposits of protein. All of it is contained in the body as important parts of tissues, blood hormones, and enzymes. The structural components of the body that contain these amino acids are continually undergoing a process of breakdown and replacement. The respiratory quotient for protein metabolism can be demonstrated by the chemical equation for oxidation of albumin:

${\displaystyle {\ce {C72H112N18O22S + 77 O2 -> 63 CO2 + 38 H2O + SO3 + 9 CO(NH2)2}}}$

The R.Q. for albumin is 0.818:

${\displaystyle R.Q.={\frac {63\ CO_{2}}{77\ O_{2}}}=0.818}$

The reason this is important in the process of understanding protein metabolism is that the body can blend the three macronutrients and based on the mitochondrial density, a preferred ratio can be established which determines how much fuel is utilized in which packets for work accomplished by the muscles. Protein catabolism (breakdown) has been estimated to supply 10% to 15% of the total energy requirement during a two-hour aerobic training session. This process could severely degrade the protein structures needed to maintain survival such as contractile properties of proteins in the heart, cellular mitochondria, myoglobin storage, and metabolic enzymes within muscles.

The oxidative system (aerobic) is the primary source of ATP supplied to the body at rest and during low intensity activities and uses primarily carbohydrates and fats as substrates. Protein is not normally metabolized significantly, except during long term starvation and long bouts of exercise (greater than 90 minutes.) At rest approximately 70% of the ATP produced is derived from fats and 30% from carbohydrates. Following the onset of activity, as the intensity of the exercise increases, there is a shift in substrate preference from fats to carbohydrates. During high intensity aerobic exercise, almost 100% of the energy is derived from carbohydrates, if an adequate supply is available.

Aerobic vs. anaerobic exercise

Studies published in 1992^[31] and 1997^[32] indicate that the level of aerobic fitness of an individual does not have any correlation with the level of resting metabolism. Both studies find that aerobic fitness levels do not improve the predictive power of fat free mass for resting metabolic rate.

However, recent research from the Journal of Applied Physiology, published in 2012,^[33] compared resistance training and aerobic training on body mass and fat mass in overweight adults (STRRIDE AT/RT). When you consider time commitments against health benefits, aerobic training is the optimal mode of exercise for reducing fat mass and body mass as a primary consideration, resistance training is good as a secondary factor when aging and lean mass are a concern. Resistance training causes injuries at a much higher rate than aerobic training.^[33] Compared to resistance training, it was found that aerobic training resulted in a significantly more pronounced reduction of body weight by enhancing the cardiovascular system which is what is the principal factor in metabolic utilization of fat substrates. Resistance training if time is available is also helpful in post-exercise metabolism, but it is an adjunctive factor because the body needs to heal sufficiently between resistance training episodes, whereas with aerobic training, the body can accept this every day. RMR and BMR are measurements of daily consumption of calories.^[34][33] The majority of studies that are published on this topic look at aerobic exercise because of its efficacy for health and weight management.

Anaerobic exercise, such as weight lifting, builds additional muscle mass. Muscle contributes to the fat-free mass of an individual and therefore effective results from anaerobic exercise will increase BMR.^[35] However, the actual effect on BMR is controversial and difficult to enumerate. Various studies^[36][37] suggest that the resting metabolic rate of trained muscle is around 55kJ per kilogram, per day. Even a substantial increase in muscle mass, say 5 kg, would make only a minor impact on BMR.

Longevity

See also: Heartbeat hypothesis

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.^[38] Additionally, the life span of houseflies can be extended by preventing physical activity.^[39] 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.^{[40][41][42][43]}

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.^[44]

Organism longevity and basal metabolic rate

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In allometric scaling, maximum potential life span (MPLS) is directly related to metabolic rate (MR), where MR is the recharge rate of a biomass made up of covalent bonds. That biomass (W) is subjected to deterioration over time from thermodynamic, entropic pressure. Metabolism is essentially understood as redox coupling, and has nothing to do with thermogenesis. Metabolic efficiency (ME) is then expressed as the efficiency of this coupling, a ratio of amperes^{[clarification needed]} captured and used by biomass, to the amperes available for that purpose. MR is measured in watts, W is measured in grams. These factors are combined in a power law, an elaboration on Kleiber's law relating MR to W and MPLS, that appears as MR = W^ (4ME-1)/4ME.^{[clarification needed]} When ME is 100%, MR = W^3/4; this is popularly known as quarter power scaling, a version of allometric scaling that is premised upon unrealistic estimates of biological efficiency.

The equation reveals that as ME drops below 20%, for W < one gram, MR/MPLS increases so dramatically as to endow W with virtual immortality by 16%. The smaller W is to begin with, the more dramatic is the increase in MR as ME diminishes. All of the cells of an organism fit into this range, i.e., less than one gram, and so this MR will be referred to as BMR.

But the equation reveals that as ME increases over 25%, BMR approaches zero. The equation also shows that for all W > one gram, where W is the organization of all of the BMRs of the organism's structure, but also includes the activity of the structure, as ME increases over 25%, MR/MPLS increases rather than decreases, as it does for BMR. An MR made up of an organization of BMRs will be referred to as an FMR.^{[clarification needed]} As ME decreases below 25%, FMR diminishes rather than increases as it does for BMR.

The antagonism between FMR and BMR is what marks the process of aging of biomass W in energetic terms. The ME for the organism is the same as that for the cells, such that the success of the organism's ability to find food (and lower its ME), is key to maintaining the BMR of the cells driven, otherwise, by starvation, to approaching zero; while at the same time a lower ME diminishes the FMR/MPLS of the organism.^{[citation needed]}

Medical considerations

A person's metabolism varies with their physical condition and activity. Weight training can have a longer impact on metabolism than aerobic training, but there are no known mathematical formulas that can exactly predict the length and duration of a raised metabolism from trophic changes with anabolic neuromuscular training.

A decrease in food intake will typically lower the metabolic rate as the body tries to conserve energy.^[45] Researcher Gary Foster estimates that a very low calorie diet of fewer than 800 calories a day would reduce the metabolic rate by more than 10 percent.^[46]

The metabolic rate can be affected by some drugs, such as antithyroid agents, drugs used to treat hyperthyroidism, such as propylthiouracil and methimazole, bring the metabolic rate down to normal and restore euthyroidism. Some research has focused on developing antiobesity drugs to raise the metabolic rate, such as drugs to stimulate thermogenesis in skeletal muscle.

The metabolic rate may be elevated in stress, illness, and diabetes. Menopause may also affect metabolism.

Cardiovascular implications

Heart rate is determined by the medulla oblongata and part of the pons, two organs located inferior to the hypothalamus on the brain stem. Heart rate is important for basal metabolic rate and resting metabolic rate because it drives the blood supply, stimulating the Krebs cycle.^{[citation needed]} During exercise that achieves the anaerobic threshold, it is possible to deliver substrates that are desired for optimal energy utilization. The anaerobic threshold is defined as the energy utilization level of heart rate exertion that occurs without oxygen during a standardized test with a specific protocol for accuracy of measurement,^{[citation needed]} such as the Bruce Treadmill protocol (see metabolic equivalent of task). With four to six weeks of targeted training the body systems can adapt to a higher perfusion of mitochondrial density for increased oxygen availability for the Krebs cycle, or tricarboxylic cycle, or the glycolitic cycle.^{[citation needed]} This in turn leads to a lower resting heart rate, lower blood pressure, and increased resting or basal metabolic rate.^{[citation needed]}

By measuring heart rate we can then derive estimations of what level of substrate utilization is actually causing biochemical metabolism in our bodies at rest or in activity.^{[citation needed]} This in turn can help a person to maintain an appropriate level of consumption and utilization by studying a graphical representation of the anaerobic threshold. This can be confirmed by blood tests and gas analysis using either direct or indirect calorimetry to show the effect of substrate utilization.^{[citation needed]} The measures of basal metabolic rate and resting metabolic rate are becoming essential tools for maintaining a healthy body weight.

See also

• Harris-Benedict equation
• Hypothyroidism
• Metabolic age
• Metabolic syndrome
• Schofield equation
• Thermic effect of food

References

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Further reading

• Tsai, AG; Wadden, TA (2005). "Systematic review: An evaluation of major commercial weight loss programs in the United States". Annals of Internal Medicine. 142 (1): 56–66. doi:10.7326/0003-4819-142-1-200501040-00012. PMID 15630109. S2CID 2589699.
• Gustafson, D.; Rothenberg, E; Blennow, K; Steen, B; Skoog, I (2003). "An 18-Year Follow-up of Overweight and Risk of Alzheimer Disease". Archives of Internal Medicine. 163 (13): 1524–8. doi:10.1001/archinte.163.13.1524. PMID 12860573.
• "Clinical guidelines on the identification, evaluation, and treatment of overweight and obesity in adults: Executive summary. Expert Panel on the Identification, Evaluation, and Treatment of Overweight in Adults". The American Journal of Clinical Nutrition. 68 (4): 899–917. 1998. doi:10.1093/ajcn/68.4.899. PMID 9771869.
• Segal, Arthur C. (1987). "Linear Diet Model". College Mathematics Journal. 18 (1): 44–5. doi:10.2307/2686315. JSTOR 2686315.
• Pike, Ruth L; Brown, Myrtle Laurestine (1975). Nutrition: An Integrated Approach (2nd ed.). New York: Wiley. OCLC 474842663.
• Sahlin, K.; Tonkonogi, M.; Soderlund, K. (1998). "Energy supply and muscle fatigue in humans". Acta Physiologica Scandinavica. 162 (3): 261–6. doi:10.1046/j.1365-201X.1998.0298f.x. PMID 9578371.
• Saltin, Bengt; Gollnick, Philip D. (1983). "Skeletal muscle adaptability: Significance for metabolism and performance". In Peachey, Lee D; Adrian, Richard H; Geiger, Stephen R (eds.). Handbook of Physiology. Baltimore: Williams & Wilkins. pp. 540–55. OCLC 314567389. Republished as: Saltin, Bengt; Gollnick, Philip D. (2011). "Skeletal Muscle Adaptability: Significance for Metabolism and Performance". Comprehensive Physiology. doi:10.1002/cphy.cp100119. ISBN 978-0-470-65071-4.
• Thorstensson (1976). "Muscle strength, fibre types and enzyme activities in man". Acta Physiologica Scandinavica. Supplementum. 443: 1–45. PMID 189574.
• Thorstensson, Alf; Sjödin, Bertil; Tesch, Per; Karlsson, Jan (1977). "Actomyosin ATPase, Myokinase, CPK and LDH in Human Fast and Slow Twitch Muscle Fibres". Acta Physiologica Scandinavica. 99 (2): 225–9. doi:10.1111/j.1748-1716.1977.tb10373.x. PMID 190869.
• Vanhelder, W. P.; Radomski, M. W.; Goode, R. C.; Casey, K. (1985). "Hormonal and metabolic response to three types of exercise of equal duration and external work output". European Journal of Applied Physiology and Occupational Physiology. 54 (4): 337–42. doi:10.1007/BF02337175. PMID 3905393. S2CID 39715173.
• Wells, JG; Balke, B; Van Fossan, DD (1957). "Lactic acid accumulation during work; a suggested standardization of work classification". Journal of Applied Physiology. 10 (1): 51–5. doi:10.1152/jappl.1957.10.1.51. PMID 13405829.
• McArdle, William D; Katch, Frank I; Katch, Victor L (1986). Exercise Physiology: Energy, Nutrition, and Human Performance. Philadelphia: Lea & Febiger. OCLC 646595478.
• Harris, JA; Benedict, FG (1918). "A Biometric Study of Human Basal Metabolism". Proceedings of the National Academy of Sciences of the United States of America. 4 (12): 370–3. Bibcode:1918PNAS....4..370H. doi:10.1073/pnas.4.12.370. PMC 1091498. PMID 16576330.

External links

• Harris-Benedict study. Detailed discussion of antecedents, data, measurements, statistics (Published by The Carnegie Institution of Washington 1919)
• BMR as affected by alcohol
• BMR and personality
• TDEECalculator.net