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Basal metabolic rate (BMR) is the amount of energy expended while an animal is 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 brain, skin, heart, muscles, liver, sex organs, lungs, nervous system, and kidneys. 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 ones overall energy expenditure, but does not affect one's BMR. 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). [1]

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 utilized by the body for 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 average values could be derived using body surface area (computed from height and weight), age, and gender, along with the oxygen and carbon dioxide measures taken from calorimetry. Studies also showed that by eliminating the gender 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 genders 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) to produce or regulate feelings of rage and aggression.

3) to regulate body temperature.

4) to regulate 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, then the satiety center is stimulated and sends impulses that inhibit the feeding center. 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.

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.

Nutrition and dietary considerations

The primary substrates that supply the body with energy for basal metabolic measurement are carbohydrates, fats, and proteins. Each of these substrates have been measured for their 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 heat needed to raise the temperature of one kilogram of water by one degree Celsius. Chemists often use a small calorie based on the gram rather than the kilogram. The large Calorie (capital "C") 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.

At restaurants, it has become popular to 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).

Biochemistry

About 70% of a human's total energy expenditure is due to the basal life processes within the organs of the body: liver, 27%; brain, 19%; heart, 7%; kidneys, 10%; skeletal muscle, 18%; other organs, 19%. About 20% of one's energy expediture comes from physical activity and another 10% from thermogenesis, or digestion of food. All of these processes require the body to process all of our substrates with oxygen and then to consume the production that occurs as carbon dioxide which is explained by the Krebs cycle. So the value of our food is certainly there for our survival, but there are relative meanings to how the body uses energy to keep our organs functioning normally based on the rate of oxygen needed to break down food biochemically.

For example, 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

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. 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 CO(2) / 23 O2 = 0.696

Exercise physiology

There are several companies testing the public for this value to assist with weight loss. It is theorized that if a person can more accurately know what amount of energy 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.

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?

Medical considerations

That is why weight management is a very difficult undertaking requiring sophisticated expertise. Each person is unique based on their structure and behavior and their metabolism is also unique. Menopause affects metabolism but in different ways for different people. 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. Careful graphing of the body's response to rest or exercise with a gas analyser that also records heart rate is one method that scientists use to measure the variation in basal metabolic rate between subjects.

Gastric bypass surgery also has implications because it affects the storage capacity of the body by reducing the contents of the stomach to about the size of a thumb and "bypasses" the majority of the stomach pouch. The new storage space for caloric content can expand back to original size if the caloric intake exceeds the newly reduced basal metabolic or resting metabolic rate. Thermogenesis is impaired significantly, the ability to consume fruits and vegetables is drastically reduced, and essential nutrients are rationed so that a healthy diet is hard to achieve. This procedure is considered to be a last alternative by the medical community when all other means are exhausted. There are over a hundred thousand of these procedures performed each year on the morbidly obese.

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. 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. By determining at what point the body switches from anaerobic to aerobic utilization, a direct correlation to how much fat is burned during exercise to the nearest calorie per second is achieved with certain analysers available in health clubs. 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

See also

External links


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