Basal metabolic rate

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Dayana Davidis, M.D. [2]

Overview

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.

BMR is measured under very restrictive circumstances when a person is awake, but at complete rest. An accurate BMR measurement requires that the person's sympathetic nervous system is not stimulated. 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:

1. control and integration of activities of the autonomic nervous system (ANS)
   • The ANS regulates contraction of smooth muscle and cardiac muscle, along with secretions of many endocrine organs such as the thyroid gland (associated with many metabolic disorders).
   • Through the ANS, the hypothalamus is the main regulator of visceral activities, such as heart rate, movement of food through the gastrointestinal tract, and contraction of the urinary bladder.
2. production and regulation of feelings of rage and aggression
3. regulation of body temperature
4. regulation of food intake, through two centers:
   • The feeding center or hunger center is responsible for the sensations that cause us to seek food. When sufficient food or substrates have been received and leptin is high, then the satiety center is stimulated and sends impulses that inhibit the feeding center. When insufficient food is present in the stomach and ghrelin levels are high, receptors in the hypothalamus initiate the sense of hunger.
   • The thirst center operates similarly when certain cells in the hypothalamus are stimulated by the rising osmotic pressure of the extracellular fluid. If thirst is satisfied, osmotic pressure decreases.

All of these functions taken together form a survival mechanism that causes us to sustain the body processes that BMR and RMR measure.

Resting Metabolic Rate

A metabolic rate is the number of calories needed to provide fuel for ventilation, blood circulation, temperature regulation, digestion of ingested foods, absorption of ingested foods and for daily activities. It is the minimum amount of energy required to keep the body functioning properly. Resting metabolic rate or RMR, also called resting energy expenditure or REE, is the twenty four hour estimation of the metabolic rate of an individual after three or four hours of fasting to ensure that the digestive system is inactive.

The resting metabolic rate test is performed by a professional where the individual undergoing the test should be lying down or in a reclined position, breathing through a tight mask that is hooked to a machine, which will measure the exchanges of gases through inhalation and exhalation of oxygen and carbon dioxide in order to detect the resting metabolic rate of this individual. The test takes 15 to 20 min depending on how relaxed or intense the breathing is, ideally the more relaxed it is the faster it can detect and accomplish the test.

The Harris-Benedict equations

The original equations from Harris and Benedict are:

• for men, <math>h = 66.4730+(13.7516\cdot w)+(5.0033\cdot s)-(6.7550\cdot a)</math>
• for women, <math>h = 655.0955+(9.5634\cdot w)+(1.8496\cdot s)-(4.6756\cdot a)</math>

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, and with the difference in BMR for men and women being mainly due to differences in body weight.

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 [3]

Animal BMR

Kleiber's law 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 animals fit on different curves.

Nutrition and dietary considerations

Basal metabolic rate is usually by far the largest component of total caloric expenditure. However, the Harris-Benedict equations 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. 2000 Calories is often quoted but is no more than a guideline.

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. 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. 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.

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.

References

• Tsai, Adam Gilden, MD and Wadden, Thomas A. PhD. Systematic Review: An Evaluation of Major Commercial Weightloss Programs in the United States. Ann Intern Med. 2005, 142:56-66.
• Gustafson, Deborah, PhD; Rothenburg, Elisabet, PhD; Blennow, Kaj, PhD; Steen, Bertil, MD,PhD; Skoog, Ingmar, MD, PhD. An 18-Year Follow-Up of Overweight and Risk of Alzheimer Disease. Arch Intern Med: Vol. 163, July 14, 2003, pgs 1524 through 1528.
• Expert Panel on the Identification, Evaluation and Treatment of Overweight in Adults. Clinical Guidelines on the Identification, Evaluation, and Treatment of Overweight and Obesity in Adults: Executive Summary. Downloaded from www.ajcn.org on July 9, 2006, Am J Clin Nutr 1998; 68:899-917.
• Segal, Arthur C. A Linear Diet Model. The College Mathematics Journal, January 1987.
• Pike, R.L., and Brown, M. Nutrition: An Integrated Approach, 2nd edition New York: Wiley. 1975
• Sahlin, K., Tonkonogy, and Soderlund, K. Energy Supply and muscle fatigue in humans. Acta Physiol. Scand. 162:261-266. 1998
• Saltin, B, and Gollnick. Skeletal muscle adaptability: Significance for metabolism and performance. In Handbook of Physiology, L.D. Peachey, R.H. Adrian, and S.R. Geiger, eds. Baltimore: Williams & Wilkins. 1983. pg 540 - 555.
• Thorstensson, P. Muscle strength, fiber types and enzymes in man. Acta Physiol. Scand. 102: 443. 1976
• Thorstensson, P. , Sjodin, B. Actinomyosin ATPase, myokinase, CPK and LDH in human fast and slow twitch muscle fibers. Acta Physiol. Scand. 99: 225-229. 1975
• VanHelder, W., Radomski, Goode, R. and Casey, K. Hormonal and metabolic response to three types of exercise of equal duration and external work output. Eur. J. of Appl. Physiol. 54:337-342. 1985
• Wells, J., Balke, B. and Van Fossan, D. Lactic Acid accumulation during work. A suggested standardization of work classification. J Appl. Physiol. 10: 51-55. 1957.

See also

• Thermic effect of food

External links

• The original 1918 Harris-Benedict article (free full text)
• What do BMR and RMR stand for?
• Dietary Guidelines from the U.S. Government
• Food Consumption patterns in the U.S.
• [4] Body composition Explained
• [5]Harvard University's explanation of Metabolic Equivalent Tables
• [6] Temporal Trend Shifts and the Ecological Perspectives on Weight Gain in the US
• [7] Information on the National Weight Control Registry.
• [8] Health Trends in the United States
• [9] BMR as affected by alcohol and nicotine