Basal metabolic rate
Encyclopedia
Basal Metabolic Rate and the closely related resting metabolic rate (RMR), is the amount of daily energy
expended by humans and other animals at rest. Rest is defined as existing in a neutrally temperate
environment while in the post-absorptive state
. In plants, different considerations apply.
The release of energy in this state is sufficient only for the functioning of the vital organs, the heart
, lungs, nervous system
, kidneys, liver
, intestine
, sex organs, muscles, and skin
.
and it can be measured to determine the amount of energy expended.
BMR decreases with age generally (as people usually don't maintain lean body mass) and with the loss of lean body mass
. Increasing muscle mass increases BMR. Aerobic 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 fat-free body mass was adjusted for. New research has however come to light which suggests 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.
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 and closely related measurement, used under less strict conditions, is resting metabolic rate (RMR).
BMR and RMR are 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
(R.Q.), which measures the inherent composition and utilization of carbohydrate
s, fat
s and protein
s as they are converted to energy substrate units that can be used by the body as energy. Studies conducted by Spennewyn in 1990 found strong correlations between lean mass and metabolism based on indirect calorimetry measurements. Spennewyn discovered that lean tissue in men and women required approximately 16 calories per pound per day. Thus, once a lean mass was known it could be multiplied by 16 to reveal daily caloric needs based on the activity level of the individual. This method has been used in many health club environments to determine daily caloric needs.
s are only approximate and variation in BMR (reflecting varying body composition), in physical activity levels, and in energy expended in thermogenesis
make it difficult to estimate the dietary consumption any particular individual needs in order to maintain body weight.
(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 weight, 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:
All of these functions taken together form a survival mechanism that causes us to sustain the body processes that BMR and RMR measure.
The original equations from Harris and Benedict are:
where P is total heat production at complete rest, m is the weight, h is the height, and a is the age, and with the difference in BMR for men and women being 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 or 53 kcal/h (61.3 watts).
It was the best prediction equation until 1990, when Mifflin et al. introduced the equation:
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. showed it to be about 5% more accurate.
These formulae 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. It should be noted, however, that the Cunningham formula is used to predict RMR instead of BMR.
The Katch-McArdle Formula (BMR):
According to this formula, if the woman in the example has a body fat percentage
of 30%, her BMR would be 1263 kcal per day.
The Cunningham Formula (RMR):
Since lean body mass is metabolically active vs. fat cells which need very few calories to be sustained, these formula tend to be more accurate, especially with athletes who have above average lean mass and little body fat.
To calculate daily calorie needs, the BMR value is multiplied by a factor with a value between 1.2 and 1.9, depending on the person's activity level.
relates the BMR for animals of different sizes and the observations indicate that the BMR is proportional to the 3/4 power of body mass. Warm blooded, cold blooded and unicellular organisms fit on different curves.
About 70% of a human's total energy expenditure is due to the basal life processes within 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). 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 osmosis
, and only about one-tenth is consumed for mechanical work, such as digestion, heartbeat, and breathing.
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 reaction
s 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.
The overall equation for this reaction is:
Because the gas exchange in this reaction is equal, the respiratory quotient for carbohydrate is unity or 1.0:
, 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:
Thus the R.Q. for palmitic acid is 0.696:
The R.Q. for albumin is 63 CO2/ 77 O2 = 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 training session. However, if a person's muscle glycogen supplies are low from previous exercise sessions, the amount of energy derived from protein catabolism could increase from 15% to 45%. 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.
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.
Anaerobic exercise
— such as weight lifting
— builds additional muscle mass, which however does not or hardly affects the resting basal metabolic rate.
Some studies suggest, that a minimum of 20 to 25 minutes of cardiovascular training per day can temporarily increase the basal metabolic rate by around 10%, owing to an increase in the metabolism of the working muscles required for recovery as well as storage of glycogen and other fuel sources used by the body like ATP and Creatine.
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 span
s 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.
But the ratio of total daily energy
expenditure to resting metabolic rate can vary between 1.6 to 8.0 between species of mammal
s. Animals also vary in the degree of coupling between oxidative phosphorylation and ATP production
, the amount of saturated fat
in mitochondrial membrane
s, the amount of DNA repair
, and many other factors that affect maximum life span.
coupling, and has nothing to do with thermogenesis. Metabolic efficiency (ME) is then expressed as the efficiency of this coupling, a ratio of ampere
s 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. When ME is 100%, MR = W^3/4, what is known popularly as quarter power scaling, a version of allometric scaling premised upon unreal estimations 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. 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.
can have a longer impact on metabolism than aerobic training
, but there are no formulas currently written that can predict the length and duration of a raised metabolism from trophic changes with anabolic neuromuscular training. A decrease in food intake can lower the metabolic rate as the body tries to conserve energy. Researcher Gary Foster, Ph.D., estimates that a very low calorie diet
of fewer than 800 calories a day would reduce the metabolic rate by more than 10 percent. The metabolic rate can be affected by some drugs, such as antidepressant
s, which may produce weight gain in some individuals. Antithyroid agent
s, drugs used to treat hyperthyroidism
, such as propylthiouracil
and methimazole
, bring the metabolic rate down to normal and restore euthyroid
ism. Some research has focused on developing antiobesity drugs to raise the metabolic rate, such as drugs to stimulate thermogenesis
in skeletal muscle
. Studies of humans with 100+ year life spans have shown a link to decreased thyroid activity, their resulting lowered metabolic rate is thought to attribute to their increased life expectancy.
Some conditions can be a factor in the metabolic rate. The metabolic rate may be elevated in stress
, illness
, and diabetes
. Menopause
affects metabolism but in different ways for different people; thus hormones are sometimes used to minimize the effects of menopause.
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. 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, such as the Bruce Treadmill protocol (see Metabolic equivalent
). 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. This in turn leads to a lower resting heart rate, lower blood pressure, and increased resting or basal metabolic rate.
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. 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. The measures of basal metabolic rate and resting metabolic rate are becoming essential tools for maintaining a healthy body weight.
Food energy
Food energy is the amount of energy obtained from food that is available through cellular respiration.Food energy is expressed in food calories or kilojoules...
expended by humans and other animals at rest. Rest is defined as existing in a neutrally temperate
Temperate
In geography, temperate or tepid latitudes of the globe lie between the tropics and the polar circles. The changes in these regions between summer and winter are generally relatively moderate, rather than extreme hot or cold...
environment while in the post-absorptive state
Absorptive state
Absorptive state is the period in which the gastrointestinal tract is full and the anabolic processes exceed catabolism. The fuel used for this process is glucose.- Nutrient Processing in The Absorptive State :...
. In plants, different considerations apply.
The release of energy in this state is sufficient only for the functioning of the vital organs, the heart
Heart
The heart is a myogenic muscular organ found in all animals with a circulatory system , that is responsible for pumping blood throughout the blood vessels by repeated, rhythmic contractions...
, lungs, nervous system
Nervous system
The nervous system is an organ system containing a network of specialized cells called neurons that coordinate the actions of an animal and transmit signals between different parts of its body. In most animals the nervous system consists of two parts, central and peripheral. The central nervous...
, kidneys, liver
Liver
The liver is a vital organ present in vertebrates and some other animals. It has a wide range of functions, including detoxification, protein synthesis, and production of biochemicals necessary for digestion...
, intestine
Intestine
In human anatomy, the intestine is the segment of the alimentary canal extending from the pyloric sphincter of the stomach to the anus and, in humans and other mammals, consists of two segments, the small intestine and the large intestine...
, sex organs, muscles, and skin
Skin
-Dermis:The dermis is the layer of skin beneath the epidermis that consists of connective tissue and cushions the body from stress and strain. The dermis is tightly connected to the epidermis by a basement membrane. It also harbors many Mechanoreceptors that provide the sense of touch and heat...
.
Description
The body's generation of heat is known as thermogenesisThermogenesis
Thermogenesis is the process of heat production in organisms. It occurs mostly in warm-blooded animals, but a few species of thermogenic plants exist.-Types:...
and it can be measured to determine the amount of energy expended.
BMR decreases with age generally (as people usually don't maintain lean body mass) and with the loss of lean body mass
Body composition
In physical fitness, body composition is used to describe the percentages of fat, bone and muscle in human bodies. Because muscular tissue takes up less space in our body than fat tissue, our body composition, as well as our weight, determines leanness...
. Increasing muscle mass increases BMR. Aerobic 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 fat-free body mass was adjusted for. New research has however come to light which suggests 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.
BMR is measured under very restrictive circumstances when a person is awake. An accurate BMR measurement requires that the person's sympathetic nervous system
Sympathetic nervous system
The sympathetic nervous system is one of the three parts of the autonomic nervous system, along with the enteric and parasympathetic systems. Its general action is to mobilize the body's nervous system fight-or-flight response...
not be stimulated, a condition which requires complete rest. A more common and closely related measurement, used under less strict conditions, is resting metabolic rate (RMR).
BMR and RMR are measured by gas analysis through either direct or indirect calorimetry
Calorimetry
Calorimetry is the science of measuring the heat of chemical reactions or physical changes. Calorimetry is performed with a calorimeter. The word calorimetry is derived from the Latin word calor, meaning heat...
, though a rough estimation can be acquired through an equation using age, sex, height, and weight. Studies of energy metabolism
Metabolism
Metabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...
using both methods provide convincing evidence for the validity of the respiratory quotient
Respiratory quotient
The respiratory quotient , is a unitless number used in calculations of basal metabolic rate when estimated from carbon dioxide production. Such measurements, like measurements of oxygen uptake, are forms of indirect calorimetry...
(R.Q.), which measures the inherent composition and utilization of carbohydrate
Carbohydrate
A carbohydrate is an organic compound with the empirical formula ; that is, consists only of carbon, hydrogen, and oxygen, with a hydrogen:oxygen atom ratio of 2:1 . However, there are exceptions to this. One common example would be deoxyribose, a component of DNA, which has the empirical...
s, fat
Fat
Fats consist of a wide group of compounds that are generally soluble in organic solvents and generally insoluble in water. Chemically, fats are triglycerides, triesters of glycerol and any of several fatty acids. Fats may be either solid or liquid at room temperature, depending on their structure...
s and protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
s as they are converted to energy substrate units that can be used by the body as energy. Studies conducted by Spennewyn in 1990 found strong correlations between lean mass and metabolism based on indirect calorimetry measurements. Spennewyn discovered that lean tissue in men and women required approximately 16 calories per pound per day. Thus, once a lean mass was known it could be multiplied by 16 to reveal daily caloric needs based on the activity level of the individual. This method has been used in many health club environments to determine daily caloric needs.
Nutrition and dietary considerations
Basal metabolism is usually by far the largest component of total caloric expenditure. However, the Harris-Benedict equationHarris-Benedict equation
The Harris-Benedict equation is a method used to estimate the daily calorie requirements of an individual using their basal metabolic rate or BMR. The estimated value is then multiplied by a number that corresponds to the person's activity level. The resulting number is the recommended daily...
s are only approximate and variation in BMR (reflecting varying body composition), in physical activity levels, and in energy expended in thermogenesis
Thermogenesis
Thermogenesis is the process of heat production in organisms. It occurs mostly in warm-blooded animals, but a few species of thermogenic plants exist.-Types:...
make it difficult to estimate the dietary consumption any particular individual needs in order to maintain body weight.
Physiology
Both basal metabolic rate and resting metabolic rate are usually expressed in terms of daily rates of energy expenditure. The early work of the scientists J. Arthur Harris and Francis G. Benedict showed that approximate values could be derived using body surface areaBody surface area
In physiology and medicine, the body surface area is the measured or calculated surface of a human body. For many clinical purposes BSA is a better indicator of metabolic mass than body weight because it is less affected by abnormal adipose mass...
(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
Adipose tissue
In histology, adipose tissue or body fat or fat depot or just fat is loose connective tissue composed of adipocytes. It is technically composed of roughly only 80% fat; fat in its solitary state exists in the liver and muscles. Adipose tissue is derived from lipoblasts...
by expressing metabolic rate per unit of "fat-free" or lean body weight, the values between sexes for basal metabolism are essentially the same. Exercise physiology
Exercise physiology
Exercise physiology is the study of the acute responses and chronic adaptations to a wide-range of physical exercise conditions. In addition, many exercise physiologists study the effect of exercise on pathology, and the mechanisms by which exercise can reduce or reverse disease progression...
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
Hypothalamus
The Hypothalamus is a portion of the brain that contains a number of small nuclei with a variety of functions...
. The hypothalamus is located on the diencephalon
Diencephalon
The diencephalon is the region of the vertebrate neural tube which gives rise to posterior forebrain structures. In development, the forebrain develops from the prosencephalon, the most anterior vesicle of the neural tube which later forms both the diencephalon and the...
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 systemAutonomic nervous systemThe autonomic nervous system is the part of the peripheral nervous system that acts as a control system functioning largely below the level of consciousness, and controls visceral functions. The ANS affects heart rate, digestion, respiration rate, salivation, perspiration, diameter of the pupils,...
(ANS)- The ANS regulates contraction of smooth muscle and cardiac muscleCardiac muscleCardiac muscle is a type of involuntary striated muscle found in the walls and histologic foundation of the heart, specifically the myocardium. Cardiac muscle is one of three major types of muscle, the others being skeletal and smooth 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.
- The ANS regulates contraction of smooth muscle and cardiac muscle
- 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 leptinLeptinLeptin is a 16 kDa protein hormone that plays a key role in regulating energy intake and energy expenditure, including appetite and metabolism. It is one of the most important adipose derived hormones...
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 ghrelinGhrelinGhrelin is a 28 amino acid peptide and hormone that is produced mainly by P/D1 cells lining the fundus of the human stomach and epsilon cells of the pancreas that stimulates hunger. Ghrelin levels increase before meals and decrease after meals. It is considered the counterpart of the hormone...
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 pressureOsmotic pressureOsmotic pressure is the pressure which needs to be applied to a solution to prevent the inward flow of water across a semipermeable membrane....
of the extracellular fluid. If thirst is satisfied, osmotic pressure decreases.
- 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
- control and integration of activities of the autonomic nervous system
All of these functions taken together form a survival mechanism that causes us to sustain the body processes that BMR and RMR measure.
BMR estimation formulas
Several prediction equations exist. Historically most notable was Harris-Benedict equation, which was created in 1919.The original equations from Harris and Benedict are:
- for men,
- for women,
where P is total heat production at complete rest, m is the weight, h is the height, and a is the age, and with the difference in BMR for men and women being 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 or 53 kcal/h (61.3 watts).
It was the best prediction equation until 1990, when Mifflin et al. introduced the equation:
, 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. showed it to be about 5% more accurate.
These formulae 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. It should be noted, however, that the Cunningham formula is used to predict RMR instead of BMR.
The Katch-McArdle Formula (BMR):
, where LBM is the lean body mass in kg.
According to this formula, if the woman in the example has a body fat percentage
Body fat percentage
A person's body mass percentage is the total weight of the person's fat divided by the person's weight and consists of essential body fat and storage body fat. Essential body fat is necessary to maintain life and reproductive functions. The percentage of essential body fat for women is greater than...
of 30%, her BMR would be 1263 kcal per day.
The Cunningham Formula (RMR):
, where LBM is the lean body mass in kg
Since lean body mass is metabolically active vs. fat cells which need very few calories to be sustained, these formula tend to be more accurate, especially with athletes who have above average lean mass and little body fat.
To calculate daily calorie needs, the BMR value is multiplied by a factor with a value between 1.2 and 1.9, depending on the person's activity level.
Animal BMR
Kleiber's lawKleiber's law
Kleiber's law, named after Max Kleiber's biological work in the early 1930s, is the observation that, for the vast majority of animals, an animal's metabolic rate scales to the ¾ power of the animal's mass. Symbolically: if q0 is the animal's metabolic rate, and M the animal's mass, then Kleiber's...
relates the BMR for animals of different sizes and the observations indicate that the BMR is proportional to the 3/4 power of body mass. Warm blooded, cold blooded and unicellular organisms fit on different curves.
Biochemistry
| Energy expenditure breakdown | |
|---|---|
| Liver Liver The liver is a vital organ present in vertebrates and some other animals. It has a wide range of functions, including detoxification, protein synthesis, and production of biochemicals necessary for digestion... >27% |
|
| Brain Brain The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,... |
19% |
| Heart Heart The heart is a myogenic muscular organ found in all animals with a circulatory system , that is responsible for pumping blood throughout the blood vessels by repeated, rhythmic contractions... |
7% |
| Kidneys | 10% |
| Skeletal muscle Skeletal muscle Skeletal muscle is a form of striated muscle tissue existing under control of the somatic nervous system- i.e. it is voluntarily controlled. It is one of three major muscle types, the others being cardiac and smooth muscle... |
18% |
| Other organs | 19% |
About 70% of a human's total energy expenditure is due to the basal life processes within the organs of the body (see table). About 20% of one's energy expenditure comes from physical activity and another 10% from thermogenesis
Thermogenesis
Thermogenesis is the process of heat production in organisms. It occurs mostly in warm-blooded animals, but a few species of thermogenic plants exist.-Types:...
, or digestion of food (postprandial thermogenesis). 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 osmosis
Osmoregulation
Osmoregulation is the active regulation of the osmotic pressure of an organism's fluids to maintain the homeostasis of the organism's water content; that is it keeps the organism's fluids from becoming too diluted or too concentrated. Osmotic pressure is a measure of the tendency of water to move...
, and only about one-tenth is consumed for mechanical work, such as digestion, heartbeat, and breathing.
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 reaction
Exergonic reaction
An exergonic reaction is a chemical reaction where the change in the Gibbs free energy is negative, indicating a spontaneous reaction. Symbolically, the release of Gibbs free energy, G, in an exergonic reaction is denoted as...
s 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
Adenosine triphosphate
Adenosine-5'-triphosphate is a multifunctional nucleoside triphosphate used in cells as a coenzyme. It is often called the "molecular unit of currency" of intracellular energy transfer. ATP transports chemical energy within cells for metabolism...
(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 are produced and six molecules of oxygen are consumed.The overall equation for this reaction is:
- C6H12O6 + 6 O2 → 6 CO2 + 6 H2O
Because the gas exchange in this reaction is equal, the respiratory quotient for carbohydrate is unity or 1.0:
- R.Q. = 6 CO2 / 6 O2
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 acidPolyunsaturated fatty acid
Polyunsaturated fatty acids are fatty acids that contain more than one double bond in their backbone. This class includes many important compounds, such as essential fatty acids and those that give drying oils their characteristic property....
, monounsaturated fatty acid
Unsaturated fat
An unsaturated fat is a fat or fatty acid in which there is at least one double bond within the fatty acid chain. A fat molecule is monounsaturated if it contains one double bond, and polyunsaturated if it contains more than one double bond. Where double bonds are formed, hydrogen atoms are...
, dietary cholesterol
Cholesterol
Cholesterol is a complex isoprenoid. Specifically, it is a waxy steroid of fat that is produced in the liver or intestines. It is used to produce hormones and cell membranes and is transported in the blood plasma of all mammals. It is an essential structural component of mammalian cell membranes...
, 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
Palmitic acid
Palmitic acid, or hexadecanoic acid in IUPAC nomenclature, is one of the most common saturated fatty acids found in animals and plants. Its molecular formula is CH314CO2H. As its name indicates, it is a major component of the oil from palm trees . Palmitate is a term for the salts and esters of...
is a commonly studied example of the saturated fatty acid molecule.
The overall equation for the substrate utilization of palmitic acid is:
- C16H32O2 + 23 O2 → 16 CO2 + 16 H2O
Thus the R.Q. for palmitic acid is 0.696:
- R.Q. = 16 CO2 / 23 O2 = 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:- C72H112N18O22S + 77 O2 → 63 CO2 + 38 H2O + SO3 + 9 CO(NH2)2
The R.Q. for albumin is 63 CO2/ 77 O2 = 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 training session. However, if a person's muscle glycogen supplies are low from previous exercise sessions, the amount of energy derived from protein catabolism could increase from 15% to 45%. 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 and 1997 indicate that the level of aerobicAerobic
Aerobic is a word that means "requiring air", where "air" usually means oxygen.Aerobic may also refer to:* Aerobic exercise, prolonged exercise of moderate intensity* Aerobics, a form of aerobic exercise...
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.
Anaerobic exercise
Anaerobic exercise
Anaerobic exercise is exercise intense enough to trigger anaerobic metabolism. It is used by athletes in non-endurance sports to promote strength, speed and power and by body builders to build muscle mass...
— such as weight lifting
Weight training
Weight training is a common type of strength training for developing the strength and size of skeletal muscles. It uses the weight force of gravity to oppose the force generated by muscle through concentric or eccentric contraction...
— builds additional muscle mass, which however does not or hardly affects the resting basal metabolic rate.
Some studies suggest, that a minimum of 20 to 25 minutes of cardiovascular training per day can temporarily increase the basal metabolic rate by around 10%, owing to an increase in the metabolism of the working muscles required for recovery as well as storage of glycogen and other fuel sources used by the body like ATP and Creatine.
Longevity
In 1926 Raymond PearlRaymond Pearl
Raymond Pearl was an American biologist, regarded as one of the founders of biogerontology. He spent most of his career at Johns Hopkins University in Baltimore....
proposed that longevity
Longevity
The word "longevity" is sometimes used as a synonym for "life expectancy" in demography or known as "long life", especially when it concerns someone or something lasting longer than expected ....
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 span
Maximum life span
Maximum life span is a measure of the maximum amount of time one or more members of a population has been observed to survive between birth and death.Most living species have at least one upper limit on the number of times cells can divide...
s and the fact that the longevity of fruit flies
Drosophila melanogaster
Drosophila melanogaster is a species of Diptera, or the order of flies, in the family Drosophilidae. The species is known generally as the common fruit fly or vinegar fly. Starting from Charles W...
varies inversely with ambient temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
. 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
Basal metabolic rate
Basal Metabolic Rate , and the closely related resting metabolic rate , is the amount of daily energy expended by humans and other animals at rest. Rest is defined as existing in a neutrally temperate environment while in the post-absorptive state...
to increased life expectancy, across the animal kingdom - including humans.
But the ratio of total daily energy
Energy
In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...
expenditure to resting metabolic rate can vary between 1.6 to 8.0 between species of mammal
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...
s. Animals also vary in the degree of coupling between oxidative phosphorylation and ATP production
Chemiosmosis
Chemiosmosis is the movement of ions across a selectively permeable membrane, down their electrochemical gradient. More specifically, it relates to the generation of ATP by the movement of hydrogen ions across a membrane during cellular respiration....
, the amount of saturated fat
Saturated fat
Saturated fat is fat that consists of triglycerides containing only saturated fatty acids. Saturated fatty acids have no double bonds between the individual carbon atoms of the fatty acid chain. That is, the chain of carbon atoms is fully "saturated" with hydrogen atoms...
in mitochondrial membrane
Outer membrane
The bacterial outer membrane is found in Gram-negative bacteria. Its composition is distinct from that of the cytoplasmic membrane - among other things, the outer leaflet of the membrane includes a complex lipopolysaccharide whose lipid portion acts as an endotoxin - and it is linked to the cell's...
s, the amount of DNA repair
DNA repair
DNA repair refers to a collection of processes by which a cell identifies and corrects damage to the DNA molecules that encode its genome. In human cells, both normal metabolic activities and environmental factors such as UV light and radiation can cause DNA damage, resulting in as many as 1...
, and many other factors that affect maximum life span.
Organism longevity and basal metabolic rate
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 redoxRedox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....
coupling, and has nothing to do with thermogenesis. Metabolic efficiency (ME) is then expressed as the efficiency of this coupling, a ratio of ampere
Ampere
The ampere , often shortened to amp, is the SI unit of electric current and is one of the seven SI base units. It is named after André-Marie Ampère , French mathematician and physicist, considered the father of electrodynamics...
s 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
Kleiber's law
Kleiber's law, named after Max Kleiber's biological work in the early 1930s, is the observation that, for the vast majority of animals, an animal's metabolic rate scales to the ¾ power of the animal's mass. Symbolically: if q0 is the animal's metabolic rate, and M the animal's mass, then Kleiber's...
relating MR to W and MPLS, that appears as MR = W^ (4ME-1)/4ME. When ME is 100%, MR = W^3/4, what is known popularly as quarter power scaling, a version of allometric scaling premised upon unreal estimations 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. 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.
Medical considerations
Each person's metabolism is unique due to his or her unique physical makeup and physical behavior. Weight trainingWeight training
Weight training is a common type of strength training for developing the strength and size of skeletal muscles. It uses the weight force of gravity to oppose the force generated by muscle through concentric or eccentric contraction...
can have a longer impact on metabolism than aerobic training
Aerobic exercise
Aerobic exercise is physical exercise of relatively low intensity that depends primarily on the aerobic energy-generating process. Aerobic literally means "living in air", and refers to the use of oxygen to adequately meet energy demands during exercise via aerobic metabolism...
, but there are no formulas currently written that can predict the length and duration of a raised metabolism from trophic changes with anabolic neuromuscular training. A decrease in food intake can lower the metabolic rate as the body tries to conserve energy. Researcher Gary Foster, Ph.D., estimates that a very low calorie diet
Very Low Calorie Diet
Very low calorie diet is a diet with very or extremely low daily food energy consumption. It is defined as a diet of per day or less. VLCDs are formulated, nutritionally complete, liquid meals containing 800 kilocalories or less per day. VLCDs also contain the recommended daily requirements for...
of fewer than 800 calories a day would reduce the metabolic rate by more than 10 percent. The metabolic rate can be affected by some drugs, such as antidepressant
Antidepressant
An antidepressant is a psychiatric medication used to alleviate mood disorders, such as major depression and dysthymia and anxiety disorders such as social anxiety disorder. According to Gelder, Mayou &*Geddes people with a depressive illness will experience a therapeutic effect to their mood;...
s, which may produce weight gain in some individuals. Antithyroid agent
Antithyroid agent
An antithyroid agent is a hormone antagonist acting upon thyroid hormones.The main antithyroid drugs are carbimazole , methimazole , and propylthiouracil/PTU...
s, drugs used to treat hyperthyroidism
Hyperthyroidism
Hyperthyroidism is the term for overactive tissue within the thyroid gland causing an overproduction of thyroid hormones . Hyperthyroidism is thus a cause of thyrotoxicosis, the clinical condition of increased thyroid hormones in the blood. Hyperthyroidism and thyrotoxicosis are not synonymous...
, such as propylthiouracil
Propylthiouracil
Propylthiouracil or 6-n-Propylthiouracil is a thioamide drug used to treat hyperthyroidism by decreasing the amount of thyroid hormone produced by the thyroid gland...
and methimazole
Methimazole
Methimazole is an antithyroid drug, and part of the thioamide group.-Indications:...
, bring the metabolic rate down to normal and restore euthyroid
Euthyroid
Euthyroid is the state of having normal thyroid gland function.Examples of a nonfunctioning thyroid gland may be hypothyroidism, hyperthyroidism, or thyroiditis....
ism. Some research has focused on developing antiobesity drugs to raise the metabolic rate, such as drugs to stimulate thermogenesis
Thermogenesis
Thermogenesis is the process of heat production in organisms. It occurs mostly in warm-blooded animals, but a few species of thermogenic plants exist.-Types:...
in skeletal muscle
Skeletal muscle
Skeletal muscle is a form of striated muscle tissue existing under control of the somatic nervous system- i.e. it is voluntarily controlled. It is one of three major muscle types, the others being cardiac and smooth muscle...
. Studies of humans with 100+ year life spans have shown a link to decreased thyroid activity, their resulting lowered metabolic rate is thought to attribute to their increased life expectancy.
Some conditions can be a factor in the metabolic rate. The metabolic rate may be elevated in stress
Fight-or-flight response
The fight-or-flight response was first described by Walter Bradford Cannon....
, illness
Illness
Illness is a state of poor health. Illness is sometimes considered another word for disease. Others maintain that fine distinctions exist...
, and diabetes
Diabetes mellitus
Diabetes mellitus, often simply referred to as diabetes, is a group of metabolic diseases in which a person has high blood sugar, either because the body does not produce enough insulin, or because cells do not respond to the insulin that is produced...
. Menopause
Menopause
Menopause is a term used to describe the permanent cessation of the primary functions of the human ovaries: the ripening and release of ova and the release of hormones that cause both the creation of the uterine lining and the subsequent shedding of the uterine lining...
affects metabolism but in different ways for different people; thus hormones are sometimes used to minimize the effects of menopause.
Cardiovascular implications
Heart rate is determined by the medulla oblongataMedulla oblongata
The medulla oblongata is the lower half of the brainstem. In discussions of neurology and similar contexts where no ambiguity will result, it is often referred to as simply the medulla...
and part of the pons
Pons
The pons is a structure located on the brain stem, named after the Latin word for "bridge" or the 16th-century Italian anatomist and surgeon Costanzo Varolio . It is superior to the medulla oblongata, inferior to the midbrain, and ventral to the cerebellum. In humans and other bipeds this means it...
, 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. 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, such as the Bruce Treadmill protocol (see Metabolic equivalent
Metabolic equivalent
The metabolic equivalent of task , or simply metabolic equivalent, is a physiological concept expressing the energy cost of physical activities as multiples of resting metabolic rate and is defined as the ratio of metabolic rate during a specific physical activity to a reference rate of...
). 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. This in turn leads to a lower resting heart rate, lower blood pressure, and increased resting or basal metabolic rate.
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. 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. The measures of basal metabolic rate and resting metabolic rate are becoming essential tools for maintaining a healthy body weight.