Basal metabolic rate
Basal metabolic rate (BMR) is the rate of energy expenditure per unit time at rest.
It is reported in energy units per unit time ranging from watt (joule/second) to ml O2/min or joule per hour per kg body mass J/(h·kg).
BMR is the amount of energy per unit of time that a person needs to keep the body functioning at rest: breathing, blood circulation, controlling body temperature, cell growth, brain and nerve function, and contraction of muscles.
The basal metabolic rate affects the rate of burning calories and ultimately whether one maintains, gains, or loses weight.
The BMR accounts for about 60 to 75% of the daily calorie expenditure.
The BMR typically declines by 1–2% per decade after age 20, mostly due to loss of fat-free mass.
There is great variability of BMR between individuals.
Increasing muscle mass has the effect of increasing BMR.
Thermogenesis refers to the generation of heat and it can be measured to determine the amount of energy expended by the body.
Aerobic fitness levels which are a product of cardiovascular exercise, is
not correlated with BMR when adjusted for fat-free body mass.
Anaerobic exercise does increase resting energy consumption
BMR can affected by overall energy expenditures by illness, food consumption, stress, and environmental temperatures.
An accurate BMR measurement requires a person to be awake and their sympathetic nervous system not be stimulated, a condition which requires complete rest.
A more common measurement is the less strict criteria, is resting metabolic rate (RMR).
BMR may be measured by gas analysis calorimetry, or though a rough estimation can be acquired through an equation using age, sex, height, and weight.
The measured respiratory quotient, 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.
BMR can be reversibly adjusted within individuals.
BMR changes in response to temperature.
The BMR is directly proportional to a person’s lean body mass.
The more lean body mass a person has, the higher their BMR.
BMR is also affected by acute illnesses and it increases with burns, fractures, infections, and fevers.
The BMR varies with the menstrual cycle: progesterone increases BMR at luteal phase
In a group of female volunteers there was an 11.5% average increase in 24 hour energy expenditure in the two weeks following ovulation, with a range of 8% to 16%.
Im a study conducted by the 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 anxiety temporarily increases BMR.
When 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.
The hypothalamus is the primary organ responsible for regulating metabolism.
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 feeding/ hunger center is responsible for the sensations that cause us to seek food.
When sufficient food has been consumed 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 cells in the hypothalamus are stimulated by the rising osmotic pressure of the extracellular fluid: If thirst is satisfied, osmotic pressure decreases.
BMR differences for men and women is mainly due to differences in body weight.
The basic metabolic rate varies between individuals.
One study of 150 adults representative reported basal metabolic rates from as low as 1027 kcal per day to as high as 2499 kcal/day, with a mean BMR of 1500 kcal/day: 62.3% of this variation was explained by differences in fat free mass.
Differences in BMR have been observed when comparing subjects with the same lean body mass.
Even with the same lean body mass, the top 5% of BMRs are 1.28–1.32 times the lowest 5% BMR, in one study.
Energy expenditure breakdown by organ:
Liver 27%
Brain 19%
Skeletal Muscle 18%
Kidneys 10%
Heart 7%
Other organs 19%
Postprandial thermogenesis increases in BMR 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.
About 20% of one’s energy expenditure comes from physical activity and another 10% from thermogenesis, or digestion of food, known as postprandial thermogenesis.
Energy expenditure requires an intake of oxygen along with coenzymes 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 to maintain fluid levels in tissues by osmoregulation, and only about one-tenth is consumed for mechanical work, such as digestion, heartbeat, and breathing.
Krebs cycle performs metabolic changes to fats, carbohydrates, and proteins as energy, defined as the ability or capacity to do work.
When the breakdown of large molecules into smaller molecules occurs, there is an associated release of energy, catabolism: the breakdown of proteins into amino acids is an example of catabolism
The process of building up large molecules is termed anabolism: 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 molecule that drives the exergonic transfer of energy to switch to endergonic anabolic reactions used in muscle contraction.
ATP 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 called adenosine, and three phosphate groups.
ATP is a high energy molecule.
ATP 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.
Oxygen consumed by the cells is used to oxidize the carbon in the carbohydrate molecule to form carbon dioxide.
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 chemical composition for fats differs from that of carbohydrates in that fats contain fewer oxygen atoms in proportion to atoms of carbon and hydrogen.
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 rate, more energy is needed to burn a saturated fatty acid than an unsaturated fatty acid.
The fatty acid molecule is broken down based on the number of carbon atoms in its structure.
Palmitic acid 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 with 106 ATP molecules produced, 4.61 ATP molecules per molecule of oxygen.
The respiratory quotient for palmitic acid is 0.696: 16CO2/23O2=.696
Proteins are composed of carbon, hydrogen, oxygen, and nitrogen form a large combination of amino acids.
There are no storage deposits of protein, like for fats.
Proteins are important parts of tissues, blood hormones, and enzymes.
These structures containing protein amino acids are continually undergoing a process of breakdown and replacement.
The respiratory quotient (RQ) for protein metabolism demonstrated by the chemical equation for oxidation of albumin:
C72H112N18O22S + 77 O2 -> 63 CO2 + 38 H2O + SO3 + 9 CO(NH2)2
The body blends the three macronutrients and metabolism is based on mitochondrial density.
Protein catabolism is estimated to supply 10% to 15% of the total energy requirement during a two-hour aerobic training session.
Protein catabolism could degrade vital protein structures: contractile properties of proteins in the heart, cellular mitochondria, myoglobin storage, and metabolic enzymes within muscles.
The aerobic oxidative system 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 to a significant extent, except during long term starvation and long bouts of exercise of greater than 90 minutes.
At rest approximately 70% of the ATP produced is derived from fats and 30% from carbohydrates.
With 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.
The level of aerobic fitness of an individual does not have any correlation with the level of resting metabolism.
Aerobic fitness levels do not improve the predictive power of fat free mass for resting metabolic rate.
Considering 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.
Compared to resistance training, aerobic training results 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.
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.
Studies suggest that the resting metabolic rate of trained muscle is around 55kJ per kilogram, per day.
A substantial increase in muscle mass, say 5 kg, would make only a minor impact on BMR.
Studies link 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.
Metabolism varies with physical condition and activity.
Weight training can have a longer impact on metabolism than aerobic training.
A decrease in food intake will typically lower the metabolic rate as the body tries to conserve energy.
It is estimated 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: antithyroid agents, propylthiouracil and methimazole.
The metabolic rate may be elevated in stress, illness, and diabetes.
Menopause may affect metabolism.
Heart rate is important for basal metabolic rate and resting metabolic rate because it drives the blood supply, stimulating the Krebs cycle.
The anaerobic threshold for energy utilization is that level of heart rate exertion that occurs without oxygen during a standardized test with a specific protocol for accuracy of measurement: the Bruce Treadmill protocol.
With targeted training the body systems can adapt to a higher mitochondrial density for increased oxygen availability for the Krebs cycle.
Training leads to a lower resting heart rate, lower blood pressure, and increased resting or basal metabolic rate.