Basal metabolic rate
Basal metabolic rate (BMR) is the rate of
Metabolism comprises the processes that the body needs to function.
Description
The body's generation of heat is known as
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 measurement, which uses less strict criteria, is resting metabolic rate (RMR).[6]
BMR may be 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 (RQ), 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.
Phenotypic flexibility
BMR is a flexible trait (it can be reversibly adjusted within individuals), with, for example, lower temperatures generally resulting in higher basal metabolic rates for both birds[7] and rodents.[8] There are two models to explain how BMR changes in response to temperature: the variable maximum model (VMM) and variable fraction model (VFM). The VMM states that the summit metabolism (or the maximum metabolic rate in response to the cold) increases during the winter, and that the sustained metabolism (or the metabolic rate that can be indefinitely sustained) remains a constant fraction of the former. The VFM says that the summit metabolism does not change, but that the sustained metabolism is a larger fraction of it. The VMM is supported in mammals, and, when using whole-body rates, passerine birds. The VFM is supported in studies of passerine birds using mass-specific metabolic rates (or metabolic rates per unit of mass). This latter measurement has been criticized by Eric Liknes, Sarah Scott, and David Swanson, who say that mass-specific metabolic rates are inconsistent seasonally.[9]
In addition to adjusting to temperature, BMR also may adjust before annual migration cycles.[7] The red knot (ssp. islandica) increases its BMR by about 40% before migrating northward. This is because of the energetic demand of long-distance flights. The increase is likely primarily due to increased mass in organs related to flight.[10] The end destination of migrants affects their BMR: yellow-rumped warblers migrating northward were found to have a 31% higher BMR than those migrating southward.[7]
In humans, BMR is directly proportional to a person's lean body mass.[11][12] In other words, the more lean body mass a person has, the higher their BMR; but BMR is also affected by acute illnesses and increases with conditions like burns, fractures, infections, fevers, etc.[12] In menstruating females, BMR varies to some extent with the phases of their menstrual cycle. Due to the increase in progesterone, BMR rises at the start of the luteal phase and stays at its highest until this phase ends. There are different findings in research how much of an increase usually occurs. Small sample, early studies, found various figures, such as; a 6% higher postovulatory sleep metabolism,[13] a 7% to 15% higher 24 hour expenditure following ovulation,[14] and an increase and a luteal phase BMR increase by up to 12%.[15][16] A study by the American Society of Clinical Nutrition found that an experimental group of female volunteers had an 11.5% average increase in 24 hour energy expenditure in the two weeks following ovulation, with a range of 8% to 16%. This group was measured via simultaneously direct and indirect calorimetry and had standardized daily meals and sedentary schedule in order to prevent the increase from being manipulated by change in food intake or activity level.[17] A 2011 study conducted by the Mandya Institute of Medical Sciences found that during a woman's follicular phase and menstrual cycle is no significant difference in BMR, however the calories burned per hour is significantly higher, up to 18%, during the luteal phase. Increased state anxiety (stress level) also temporarily increased BMR.[18]
Physiology
The early work of the scientists
The primary
- control and integration of activities of the autonomic nervous system (ANS)
- The ANS regulates contraction of smooth muscle and cardiac muscle, along with secretions of many endocrine organs such as the thyroid gland (associated with many metabolic disorders).
- Through the ANS, the hypothalamus is the main regulator of visceral activities, such as heart rate, movement of food through the gastrointestinal tract, and contraction of the urinary bladder.
- production and regulation of feelings of rage and aggression
- regulation of body temperature
- regulation of food intake, through two centers:
- The feeding center or hunger center is responsible for the sensations that cause us to seek food. When sufficient food or substrates have been received and leptin is high, then the satiety center is stimulated and sends impulses that inhibit the feeding center. When insufficient food is present in the stomach and ghrelin levels are high, receptors in the hypothalamus initiate the sense of hunger.
- The thirst center operates similarly when certain cells in the hypothalamus are stimulated by the rising osmotic pressure of the extracellular fluid. If thirst is satisfied, osmotic pressure decreases.
All of these functions taken together form a survival mechanism that causes us to sustain the body processes that BMR measures.
BMR estimation formulas
Several equations to predict the number of calories required by humans have been published from the early 20th–21st centuries. In each of the formulas below:[19]
- P is total heat production at complete rest,
- m is mass (kg),
- h is height (cm),
- a is age (years).
- The original Harris–Benedict equation
Historically, the most notable formula was the Harris–Benedict equation, which was published in 1919:[19]
- for men,
- for women,
The difference in BMR for men and women is mainly due to differences in body mass. For example, a 55-year-old woman weighing 130 pounds (59 kg) and 66 inches (170 cm) tall would have a BMR of 1,272 kilocalories (5,320 kJ) per day.
- The revised Harris–Benedict equation
In 1984, the original Harris–Benedict equations were revised[20] using new data. In comparisons with actual expenditure, the revised equations were found to be more accurate:[21]
- for men,
- for women,
It was the best prediction equation until 1990, when Mifflin et al.[22] introduced the equation:
- The Mifflin St Jeor 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 1,204 kilocalories (5,040 kJ) per day. During the last 100 years, lifestyles have changed, and Frankenfield et al.[23] showed it to be about 5% more accurate.
These formulas are based on body mass, which does not take into account the difference in metabolic activity between lean body mass and body fat. Other formulas exist which take into account lean body mass, two of which are the Katch–McArdle formula and Cunningham formula.
- The Katch–McArdle formula (resting daily energy expenditure)
The Katch–McArdle formula is used to predict resting daily energy expenditure (RDEE).[24] The Cunningham formula is commonly cited to predict RMR instead of BMR; however, the formulas provided by Katch–McArdle and Cunningham are the same.[25]
where ℓ is the lean body mass (LBM in kg):
where f is the body fat percentage.
According to this formula, if the woman in the example has a body fat percentage of 30%, her resting daily energy expenditure (the authors use the term of basal and resting metabolism interchangeably) would be 1262 kcal per day.
Research on individual differences in BMR
The basic metabolic rate varies between individuals. One study of 150 adults representative of the population in Scotland reported basal metabolic rates from as low as 1,027 kilocalories (4,300 kJ) per day to as high as 2,499 kilocalories (10,460 kJ); with a mean BMR of 1,500 kilocalories (6,300 kJ) per day. Statistically, the researchers calculated that 62% of this variation was explained by differences in
A cross-sectional study of more than 1400 subjects in Europe and the US showed that once adjusted for differences in body composition (lean and fat mass) and age, BMR has fallen over the past 35 years.[27] The decline was also observed in a meta-analysis of more than 150 studies dating back to the early 1920s, translating into a decline in total energy expenditure of about 6%.[27]
Biochemistry
Energy expenditure breakdown[28] | |
---|---|
Liver | 27% |
Brain | 19% |
Skeletal muscle | 18% |
Kidneys
|
10% |
Heart | 7% |
Other organs
|
19% |
About 70% of a human's total energy expenditure is due to the basal life processes taking place in the organs of the body (see table). About 20% of one's energy expenditure comes from physical activity and another 10% from
For the BMR, most of the energy is consumed in maintaining fluid levels in tissues through osmoregulation, and only about one-tenth is consumed for mechanical work, such as digestion, heartbeat, and breathing.[30]
What enables the Krebs cycle to perform metabolic changes to fats, carbohydrates, and proteins is energy, which can be defined as the ability or capacity to do work. The breakdown of large molecules into smaller molecules—associated with release of energy—is catabolism. The building up process is termed anabolism. The breakdown of proteins into amino acids is an example of catabolism, while the formation of proteins from amino acids is an anabolic process.
Exergonic reactions are energy-releasing reactions and are generally catabolic. Endergonic reactions require energy and include anabolic reactions and the contraction of muscle. Metabolism is the total of all catabolic, exergonic, anabolic, endergonic reactions.
Adenosine triphosphate (ATP) is the intermediate molecule that drives the exergonic transfer of energy to switch to endergonic anabolic reactions used in muscle contraction. This is what causes muscles to work which can require a breakdown, and also to build in the rest period, which occurs during the strengthening phase associated with muscular contraction. ATP is composed of adenine, a nitrogen containing base, ribose, a five carbon sugar (collectively called adenosine), and three phosphate groups. ATP is a high energy molecule because it stores large amounts of energy in the chemical bonds of the two terminal phosphate groups. The breaking of these chemical bonds in the Krebs Cycle provides the energy needed for muscular contraction.
Glucose
Because the ratio of hydrogen to oxygen atoms in all carbohydrates is always the same as that in water—that is, 2 to 1—all of the oxygen consumed by the cells is used to oxidize the carbon in the carbohydrate molecule to form carbon dioxide. Consequently, during the complete
The overall equation for this reaction is
(30–32 ATP molecules produced depending on type of mitochondrial shuttle, 5–5.33 ATP molecules per molecule of oxygen.)
Because the gas exchange in this reaction is equal, the respiratory quotient (R.Q.) for carbohydrate is unity or 1.0:
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,
The overall equation for the substrate utilization of palmitic acid is
(106 ATP molecules produced, 4.61 ATP molecules per molecule of oxygen.)
Thus the R.Q. for palmitic acid is 0.696:
Proteins
Proteins are composed of carbon, hydrogen, oxygen, and nitrogen arranged in a variety of ways to form a large combination of
The R.Q. for albumin is 0.818:
The reason this is important in the process of understanding protein metabolism is that the body can blend the three macronutrients and based on the mitochondrial density, a preferred ratio can be established which determines how much fuel is utilized in which packets for work accomplished by the muscles. Protein catabolism (breakdown) has been estimated to supply 10% to 15% of the total energy requirement during a two-hour aerobic training session. This process could severely degrade the protein structures needed to maintain survival such as contractile properties of proteins in the heart, cellular mitochondria, myoglobin storage, and metabolic enzymes within muscles.
The oxidative system (aerobic) is the primary source of ATP supplied to the body at rest and during low intensity activities and uses primarily carbohydrates and fats as substrates. Protein is not normally metabolized significantly, except during long term starvation and long bouts of exercise (greater than 90 minutes.) At rest approximately 70% of the ATP produced is derived from fats and 30% from carbohydrates. Following the onset of activity, as the intensity of the exercise increases, there is a shift in substrate preference from fats to carbohydrates. During high intensity aerobic exercise, almost 100% of the energy is derived from carbohydrates, if an adequate supply is available.
Aerobic vs. anaerobic exercise
Studies published in 1992
However, recent research from the Journal of Applied Physiology, published in 2012,[33] compared resistance training and aerobic training on body mass and fat mass in overweight adults (STRRIDE AT/RT). When you consider time commitments against health benefits, aerobic training is the optimal mode of exercise for reducing fat mass and body mass as a primary consideration, resistance training is good as a secondary factor when aging and lean mass are a concern. Resistance training causes injuries at a much higher rate than aerobic training.[33] Compared to resistance training, it was found that aerobic training resulted in a significantly more pronounced reduction of body weight by enhancing the cardiovascular system which is what is the principal factor in metabolic utilization of fat substrates. Resistance training if time is available is also helpful in post-exercise metabolism, but it is an adjunctive factor because the body needs to heal sufficiently between resistance training episodes, whereas with aerobic training, the body can accept this every day. RMR and BMR are measurements of daily consumption of calories.[34][33] The majority of studies that are published on this topic look at aerobic exercise because of its efficacy for health and weight management.
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 (large animals do have higher total metabolic rates, but the metabolic rate at the cellular level is much lower, and the breathing rate and heartbeat are slower in larger animals) and the fact that the longevity of fruit flies varies inversely with ambient temperature.[38] Additionally, the life span of houseflies can be extended by preventing physical activity.[39] This theory has been bolstered by several new studies linking lower basal metabolic rate to increased life expectancy, across the animal kingdom—including humans. Calorie restriction and reduced thyroid hormone levels, both of which decrease the metabolic rate, have been associated with higher longevity in animals.[40][41][42][43][unreliable medical source?]
However, the ratio of total daily
One problem with understanding the associations of lifespan and metabolism is that changes in metabolism are often confounded by other factors that may affect lifespan. For example under calorie restriction whole body metabolic rate goes down with increasing levels of restriction, but body temperature also follows the same pattern. By manipulating the ambient temperature and exposure to wind it was shown in mice and hamsters that body temperature is a more important modulator of lifespan than metabolic rate.[45]
Organism longevity and basal metabolic rate
This section needs additional citations for verification. (June 2015) |
In
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.[citation needed]
Medical considerations
A person's metabolism varies with their physical condition and activity.
A decrease in food intake will typically lower the metabolic rate as the body tries to conserve energy.
The metabolic rate can be affected by some drugs, such as
The metabolic rate may be elevated in
Cardiovascular implications
Heart rate is determined by the
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.[49] This in turn can help a person to maintain an appropriate level of consumption and utilization by studying a graphical representation of the anaerobic threshold. This can be confirmed by blood tests and gas analysis using either direct or indirect calorimetry to show the effect of substrate utilization.[citation needed] The measures of basal metabolic rate and resting metabolic rate are becoming essential tools for maintaining a healthy body weight.[citation needed]
See also
References
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Further reading
- Tsai AG, Wadden TA (2005). "Systematic review: An evaluation of major commercial weight loss programs in the United States". Annals of Internal Medicine. 142 (1): 56–66. S2CID 2589699.
- Gustafson D, Rothenberg E, Blennow K, Steen B, Skoog I (2003). "An 18-Year Follow-up of Overweight and Risk of Alzheimer Disease". Archives of Internal Medicine. 163 (13): 1524–8. PMID 12860573.
- "Clinical guidelines on the identification, evaluation, and treatment of overweight and obesity in adults: Executive summary. Expert Panel on the Identification, Evaluation, and Treatment of Overweight in Adults". The American Journal of Clinical Nutrition. 68 (4): 899–917. 1998. PMID 9771869.
- Segal AC (1987). "Linear Diet Model". College Mathematics Journal. 18 (1): 44–5. JSTOR 2686315.
- Pike RL, Brown ML (1975). Nutrition: An Integrated Approach (2nd ed.). New York: Wiley. OCLC 474842663.
- Sahlin K, Tonkonogi M, Soderlund K (1998). "Energy supply and muscle fatigue in humans". Acta Physiologica Scandinavica. 162 (3): 261–6. PMID 9578371.
- Saltin B, Gollnick PD (1983). "Skeletal muscle adaptability: Significance for metabolism and performance". In Peachey LD, Adrian RH, Geiger SR (eds.). Handbook of Physiology. Baltimore: Williams & Wilkins. pp. 540–55. ISBN 978-0-470-65071-4.
- Thorstensson (1976). "Muscle strength, fibre types and enzyme activities in man". Acta Physiologica Scandinavica. Supplementum. 443: 1–45. PMID 189574.
- Thorstensson A, Sjödin B, Tesch P, Karlsson J (1977). "Actomyosin ATPase, Myokinase, CPK and LDH in Human Fast and Slow Twitch Muscle Fibres". Acta Physiologica Scandinavica. 99 (2): 225–9. PMID 190869.
- Vanhelder WP, Radomski MW, Goode RC, Casey K (1985). "Hormonal and metabolic response to three types of exercise of equal duration and external work output". European Journal of Applied Physiology and Occupational Physiology. 54 (4): 337–42. S2CID 39715173.
- Wells JG, Balke B, Van Fossan DD (1957). "Lactic acid accumulation during work; a suggested standardization of work classification". Journal of Applied Physiology. 10 (1): 51–5. PMID 13405829.
- McArdle WD, Katch FI, Katch VL (1986). Exercise Physiology: Energy, Nutrition, and Human Performance. Philadelphia: Lea & Febiger. OCLC 646595478.
- Harris JA, Benedict FG (1918). "A Biometric Study of Human Basal Metabolism". Proceedings of the National Academy of Sciences of the United States of America. 4 (12): 370–3. PMID 16576330.