Basal metabolic rate Homepage (BMRH)
Basal metabolic rate (BMR) is the amount of energy expended while at rest in
a neutrally temperate environment, in the post-absorptive state (meaning that
the digestive system is inactive, which requires about twelve hours of fasting
in humans). The release of energy in this state is sufficient only for the
functioning of the vital organs, such as the heart, lungs, brain and the rest of
the nervous system, liver, kidneys, sex organs, muscles and skin. BMR decreases
with age and with the loss of lean body mass. Increased cardiovascular exercise
and muscle mass can increase BMR. Illness, previously consumed food and
beverages, environmental temperature, and stress levels can affect one's overall
energy expenditure, and can affect one's BMR as revealed by gas analysis. It is
measured when the person is at complete rest, but awake. An accurate BMR
measurement requires that the person's sympathetic nervous system is not
stimulated. Basal metabolic rate is measured under very restrictive
circumstances. 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 carbohydrates, fats
and proteins as they are converted to energy substrate units that can be used by
the body as energy.
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 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 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 brain stem 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 and RMR measure.
The Harris-Benedict equations
The original equations from Harris and Benedict are:
for men,
for women,
where h = total heat production per 24 hours at complete rest in kcals, w =
weight in kilograms, s = stature (height) in centimeters, and a = age in years,
Example calculation
As an example, for a 55-year-old woman, an estimated BMR might be 32
kilocalories (kcal) per square meter per hour. If her body surface area were 1.4
m2, the hourly energy expenditure would be 44.8 kcal/h (32 kcal/(m2·h) x 1.4
m2). This amounts to an energy expenditure of 1075 kcal per day (44.8 kcal x
24). The value of 1075 kilocalories, then, is the resting metabolic rate; or, if
the more stringent measurement conditions were met, it could also be the basal
metabolic rate. A detailed non-metric formula for BMR can be found at this link
Nutrition and dietary considerations
One's basal metabolic rate is usually by far the highest form of caloric
expenditure. Considering this, it is easier to know how much energy one should
consume to either gain, maintain, or lose weight if one is aware of one's BMR.
The ubiquitous 2000 calorie diet shown on nutrition information labels could be
more accurately replaced by one's BMR plus exercise and thermogenetic
expenditure.
The primary substrates that supply the body with energy for basal metabolic
measurement are carbohydrates, fats, and proteins. Each of these substrates has
been measured for its caloric values in a bomb calorimeter, which determines
exact values for energy in units of heat that are expressed as calories. A
calorie is the amount of energy needed to raise the temperature of one
millilitre of water by one degree Celsius. Chemists often use a small calorie
based on the gram rather than the kilogram. The large calorie is often called a
kilocalorie (kcal), which is one thousand small calories. The "calorie" content
of food is actually expressed in terms of large calories, whether called
calories or kilocalories.
Some restaurants provide customers with nutrition facts that explain the caloric
content of each menu item. One popular restaurant chain describes its hamburger
as having a serving size of 105 grams and containing 280 calories. Ninety
calories are described as being from fat and four of those calories from
saturated fat. The list is further subdivided into where the grams come from in
the total weight content: 30 milligrams of cholesterol, 550 milligrams of
sodium, 36 grams of carbohydrates, 2 grams of dietary fiber, and 7 grams of
sugar. If a person knew their BMR or RMR, they could calculate what amount of
caloric content and weight would satisfy their body's basic survival needs, and
what excess or deficit would render a weight gain or weight loss (ignoring the
thermic effect of food, and effect from activity).
As an example if a person knew that their BMR was 1,610 kcal and they fasted and
rested eating only a Double Quarter Pounder with cheese, at 730 kcal, and 280
grams, large fries at 520 kcal and 170 grams, baked apple pie for dessert at 250
kcal and 77 grams, with a 12 fl oz (355 mL) soda at 110 kcal, the person would
expect to weigh the same in a 24 hour period if no activity occurred and we
added a 10% factor for thermogenesis, 150 kcal for another 16 fl oz (473 mL)
beverage and 10 kcal at 10 grams for ketchup on the fries.
Then with a pedometer that accounts for bodyweight, we could begin to estimate
what level of activity would cause weight loss or weight gain along with the
value of BMR and thermogenesis. This would address the mathematical aspect of
weight management.
Biochemistry
Energy expenditure breakdown
liver 27%
brain 19%
heart 7%
kidneys 10%
skeletal 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, or
digestion of food. 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, which is explained
by the Krebs cycle.
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. Various forms of energy exist: mechanical, chemical,
electromagnetic, heat, and nuclear energy. In BMR or RMR, the conversion of
chemical energy to mechanical energy is necessary for movement.
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 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 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. When palmitic acid is
broken down, more oxygen is needed and more carbon dioxide is produced, but the
respiratory quotient moves below unity to account for the increased energy
required to burn fat molecules (generally nine calories per gram of fat versus
four calories for a gram of carbohydrate or protein.)
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 depots 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:
C72H112N2O22S + 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 why this is important in the process of understanding protein
metabolism is because 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.
It is estimated that 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.
Exercise physiology
There are several companies testing the public for the respiratory quotient that
identifies heart rates attributed to substrate utilization to assist with weight
loss. It is theorized that if a person can more accurately know what amount of
energy from carbohydrates, fats and proteins is needed to survive, then a person
can select consumption patterns to more efficiently match what is required by
the body for daily activities. Thus the emphasis shifts from caloric
restriction, which slows the BMR or RMR and causes frustration of weight
management goals, to substrate utilization, which focuses on what the body needs
to stay healthy. By measuring the carbon dioxide expended (VCO2) in ml/min and
dividing that by oxygen consumed (VO2) in ml/min you can determine the R.Q.,
which can then be compared to heart rate for purposes of application. The Balke
VO 2 Max running test could help to estimate what cardiac output level could be
achieved by a 15 minute level of exertion using the following equation: (((Total
distance covered ÷ 15) - 133) × 0.172) + 33.3. For a 50 year old male, weighing
150 pounds (68 kg), standing 69.75 inches (177 cm), that would be 47 ml/kg/min
if he ran 3200 meters in 15 min. However, the same test using gas analysis would
reveal more accurate information such as a peak VO 2 of 51.8 ml/kg/min at an
anaerobic threshold of 126 beats per minute, at 30.2 ml/kg/min and 58% of VO 2
max. This would be 1725 meters in 15 minutes according to the Balke formula. But
only gas analysis could determine the value accurately for purposes of losing
weight successfully if that was an objective. So if a person had a measured BMR
or RMR of 1610 kcal by gas analysis, and they walked around a track for 10
minutes with a heart rate at 94 beats per minute, they would consume all 25
grams of fat in a single quarter pounder with cheese with a previously
determined anaerobic threshold of 126 beats per minute from a Peak VO 2 of 51.8
ml/kg/minute. This analysis is precisely what is lacking from the current regime
of dieting programs that stress caloric restriction, total calorie management
from scale measure, and RMR or BMR from formulas using height, weight, age,
activity level. These methods fail to appreciate the Krebs cycle and the ability
of the body to adapt to lifestyle choices through BMR and RMR adjustment. By
measuring the body with gas analysis as the principal determinant of BMR under
strict fasting conditions, or RMR using less stringent measures, a person who
wants to achieve a more optimal level of conditioning is more accurately
directed to energy utilization patterns that are effective.
The reason why it's important to understand this difference with exercise
testing is because it's essential to take into consideration whether or not the
heart is capable of providing exercise stressed muscles with enough oxygen.
Conditions such as obesity will affect the ability of formulas to accurately
predict external work because the need to move a larger body changes the oxygen
cost during exercise at least 5.8 ml/min for each kg of body weight.
Longevity
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 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.
But the ratio of resting metabolic rate to total daily energy expenditure can
vary between 1.6 to 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.
Medical considerations
Each person's metabolism is unique due to their unique physical makeup and
physical behavior. For some, this makes weight management a very difficult
undertaking requiring sophisticated expertise. There are a number of medical
adjustments to natural human processes that can affect one's metabolism.
Menopause affects metabolism but in different ways for different people, thus
hormones are sometimes used to minimize the effects of menopause. Weight
training can have a longer impact on metabolism than aerobic training, but there
are no formulas currently written which can predict the length and duration of a
raised metabolism from trophic changes with anabolic neuromuscular training.
Gastric bypass surgery is used to reduce the content capacity of the stomach,
bringing caloric intake down and lowering thermogenesis. Because the surgery
significantly reduces caloric consumption, it will decrease BMR and RMR over
time in the same fashion as aging, because the volume of the stomach is reduced.
The stomach along with the rest of the digestive tract is a major contributor to
BMR and RMR.
Celiac disease is fairly common, occurring in 1% of the U.S. population, with 2
million undiagnosed. The symptoms include unexplained weight loss, fatigue, and
general lethargy. Sometimes symptoms are accompanied by a ravenous appetite or
abdominal cramping, bloating, and gas because of continued decomposition of food
and partially digested bowel contents. Celiac disease is caused by an autoimmune
response to certain proteins found in grains, including wheat, rye, and barley.
The cells in the small intestine are most affected. The small intestine is
important in absorbing food nutrients and various body fluids. Healthy small
intestines are lined with small projections (villi) that increase surface area
and absorption. Damage to these projections causes malnutrition, diarrhea, and
dehydration. Totally eliminating gluten proteins from the diet will prevent
irritation to the villi and cause the symptoms to cease. Celiac disease along
with other disease processes lower and reduce BMR and RMR.
Controversies
What brings interest to the study of basal metabolism or resting metabolism are
the paradoxes. For example, there are formulas for prediction which have many
contradictory outcomes. If muscle is the principle determinant of resting
metabolism, why does metabolic rate go up when we gain weight, including fat,
and become weaker physically due to loss of muscle mass from caloric
restriction? Why does metabolism go up when we drink coffee which has no
appreciable effect on muscle gain? Why is metabolism perceived to be different
between cultures, requiring different formulas to be devised by scientists with
equipment that measures the rate with extreme precision? Why do we assume that
2,000 kilocalories daily is the standard amount of energy needed for a woman to
survive, and 2,500 for a man, when the basal metabolic rates are so different in
all the studies that are performed on this topic each year? Do the formulas of
Harris and Benedict apply to seniors, when the subjects of the original work
done at the Carnegie Institute of Washington D.C. in 1914 were college-age
students? At what exact point does the basal metabolic rate change with aging
processes, caloric restriction, menopause, varicose vein anomalies, exercise and
is it discernible? Why is the standard for measurement 24 hours of fasting from
food the benchmark for assessing basal metabolic rate or index when certain
foods take longer than 24 hours to digest? Does it make a difference if the VCO2
estimate is defaulted?
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. 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.
Knowing what the body burns at rest or through exercise yields (via heart rate
monitoring) a targeted program of energy utilization based on metabolic
performance. The resting heart rate is correlated to the resting metabolic rate
because of the singular contribution made by the heart to survival. 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..
Contact Information
Call our office today to set up an appointment. Learn more about how we can
help you, and learn more about the other services that we can offer you. All
messages we receive will be answered as soon as possible. We look forward to
hearing from you.
- Electronic mail
- General Information:
