What is Basal Metabolic Rate (BMR) and how is it measured?

Basal Metabolic Rate (BMR) is the rate at which an endothermic animal expends energy per unit of time while at rest. It is typically measured in units like watts or milliliters of oxygen per minute per kilogram of body mass. To ensure an accurate measurement, specific criteria must be met, including the subject being in a physically and psychologically undisturbed state, within a thermally neutral environment, and in the post-absorptive state (meaning they are not actively digesting food).

What are the key conditions required for an accurate measurement of Basal Metabolic Rate (BMR)?

For an accurate BMR measurement, several conditions must be met: the individual must be in a physically and psychologically undisturbed state, situated in a thermally neutral environment, and in the post-absorptive state, meaning they have not consumed food recently and are not actively digesting.

What is the equivalent term for BMR in bradymetabolic animals like fish and reptiles, and how does its measurement differ?

In bradymetabolic animals, such as fish and reptiles, the equivalent term is Standard Metabolic Rate (SMR). SMR follows the same criteria as BMR but additionally requires the documentation of the specific temperature at which the metabolic rate was measured, making BMR a variant of SMR that excludes this temperature data.

How much of an individual's daily calorie expenditure does the Basal Metabolic Rate account for?

The Basal Metabolic Rate accounts for a significant portion of an individual's daily energy expenditure, making up approximately 70% of the total calories burned.

How does Basal Metabolic Rate (BMR) typically change with age in humans?

In humans, BMR generally declines with age, typically decreasing by about 1-2% per decade after the age of 20. This decline is primarily attributed to the loss of fat-free mass, although individual variability is high.

What is thermogenesis in the context of metabolic rate?

Thermogenesis refers to the body's generation of heat. This process is measured to determine the amount of energy expended by the body.

What is the relationship between muscle mass and Basal Metabolic Rate (BMR)?

Increasing muscle mass has the effect of increasing an individual's Basal Metabolic Rate (BMR). Conversely, BMR generally decreases with age and the associated loss of lean body mass.

What has research in the 1990s indicated about the correlation between aerobic fitness and BMR?

Research conducted in the 1990s indicated that aerobic (resistance) fitness levels, a result of cardiovascular exercise, do not correlate with BMR when adjusted for fat-free body mass. However, anaerobic exercise has been shown to increase resting energy consumption.

Besides age and body composition, what other factors can influence an individual's energy expenditure and BMR?

An individual's overall energy expenditure and their Basal Metabolic Rate (BMR) can be influenced by several factors, including illness, the consumption of food and beverages, environmental temperature, and stress levels.

What is the difference between Basal Metabolic Rate (BMR) and Resting Metabolic Rate (RMR)?

BMR is measured under very strict conditions when a person is awake, requiring complete rest and no sympathetic nervous system stimulation. Resting Metabolic Rate (RMR) is a more commonly used measurement that uses less strict criteria but aims to approximate BMR.

How can BMR be measured, and what can be estimated using calorimetry?

BMR can be measured through gas analysis using either direct or indirect calorimetry. Calorimetry studies provide evidence for the validity of the respiratory quotient (RQ), which indicates how carbohydrates, fats, and proteins are utilized and converted into energy by the body.

What is phenotypic flexibility in relation to BMR?

Phenotypic flexibility means that BMR is a trait that can be reversibly adjusted within an individual. For example, lower temperatures generally lead to higher BMRs in birds and rodents, demonstrating this flexibility.

What are the two models proposed to explain how BMR changes in response to temperature?

The two models are the Variable Maximum Model (VMM), which suggests the maximum metabolic rate increases in response to cold while sustained metabolism remains a constant fraction of it, and the Variable Fraction Model (VFM), which proposes that maximum metabolism is constant, but sustained metabolism becomes a larger fraction of it. The VMM is generally supported in mammals and passerine birds using whole-body rates, while the VFM is supported in passerine birds using mass-specific rates.

How might BMR adjust in preparation for annual migration cycles in animals?

BMR can adjust before annual migration cycles to meet increased energetic demands. For instance, the red knot increases its BMR before migrating to support long-distance flights, likely due to increased mass in flight-related organs. The destination of migration can also influence BMR, with birds migrating northward showing higher BMRs than those migrating southward.

What is the direct relationship between lean body mass and BMR in humans?

In humans, BMR is directly proportional to a person's lean body mass. This means that individuals with more lean body mass generally have a higher Basal Metabolic Rate.

How does the menstrual cycle affect BMR in females?

In menstruating females, BMR can vary with the phases of the menstrual cycle. Due to increased progesterone levels, BMR tends to rise at the start of the luteal phase and remains elevated until the end of that phase.

What were the key variables used in the original Harris-Benedict equation to estimate BMR?

The original Harris-Benedict equation, published in 1919, used body mass (m), height (h), and age (a) to estimate total heat production (P) at complete rest, with different formulas for men and women.

What is the Mifflin St Jeor equation for estimating BMR, and what do its variables represent?

The Mifflin St Jeor equation, developed in 1990, estimates resting energy expenditure (P) using mass (m), height (h), and age (a), with a sex-specific constant (s, +5 for males, -161 for females). The formula is P = (10.0m + 6.25h - 5.0a + s) kcal/day.

How does the Katch-McArdle formula estimate resting daily energy expenditure (RDEE)?

The Katch-McArdle formula estimates RDEE (P) using lean body mass (l) in kilograms, with the formula P = 370 + 21.6 * l. Lean body mass itself is calculated from total mass (m) and body fat percentage (f) using the equation l = m(1 - f/100).

What did research on individual differences in BMR find regarding the variation among adults?

Research has shown significant variation in BMR among adults. One study reported BMRs ranging from 1,027 to 2,499 kilocalories per day, with a mean of 1,500 kcal/day. Fat-free mass was found to explain the largest portion (62%) of this variation, followed by fat mass, age, and experimental error.

What has recent research indicated about the trend of total daily energy expenditure over the past few decades?

A systematic review and meta-analysis suggest that total daily energy expenditure has declined over the past 35 years, with a decline of about 6%. This decline is attributed to a decrease in basal expenditure rather than activity expenditure.

Which organs consume the largest percentages of energy expenditure according to the provided table?

According to the table, the liver consumes the largest percentage of energy expenditure at 27%, followed by the brain at 19%, and skeletal muscle also at 18%. Kidneys account for 10%, and the heart for 7%, with other organs making up the remaining 19%.

What is the approximate breakdown of total daily energy expenditure into basal processes, physical activity, and thermogenesis?

Approximately 70% of total daily energy expenditure is attributed to basal life processes in the body's organs. Physical activity accounts for about 20%, and thermogenesis, or the digestion of food, accounts for the remaining 10%.

What is the chemical equation for the complete oxidation of a glucose molecule?

The complete oxidation of a glucose molecule is represented by the equation: C6H12O6 + 6 O2 -> 6 CO2 + 6 H2O. This process yields approximately 30-32 ATP molecules.

What is the Respiratory Quotient (R.Q.) for carbohydrate metabolism, such as glucose oxidation?

The Respiratory Quotient (R.Q.) for carbohydrate metabolism, like the oxidation of glucose, is unity or 1.0. This is because the ratio of carbon dioxide produced to oxygen consumed is equal (6 CO2 / 6 O2 = 1.0).

What is the chemical equation for the substrate utilization of palmitic acid, a saturated fatty acid?

The chemical equation for the substrate utilization of palmitic acid is: C16H32O2 + 23 O2 -> 16 CO2 + 16 H2O. This process yields approximately 106 ATP molecules.

What is the Respiratory Quotient (R.Q.) for palmitic acid metabolism?

The Respiratory Quotient (R.Q.) for palmitic acid, a saturated fatty acid, is calculated as the ratio of carbon dioxide produced to oxygen consumed, which is 16 CO2 / 23 O2, resulting in an R.Q. of approximately 0.696.

What is the chemical equation for the oxidation of albumin, representing protein metabolism?

The chemical equation for the oxidation of albumin, a protein, is: C72H112N18O22S + 77 O2 -> 63 CO2 + 38 H2O + SO3 + 9 CO(NH2)2. This process produces urea as a byproduct.

What is the Respiratory Quotient (R.Q.) for protein metabolism, such as albumin oxidation?

The Respiratory Quotient (R.Q.) for protein metabolism, exemplified by albumin oxidation, is approximately 0.818. This is derived from the ratio of carbon dioxide produced to oxygen consumed (63 CO2 / 77 O2).

What role does protein catabolism play during prolonged exercise?

Protein catabolism, or the breakdown of protein, is estimated to supply about 10% to 15% of the total energy requirement during extended aerobic training sessions (over two hours). This process can potentially degrade essential protein structures in the body.

What are the primary substrates used for ATP production at rest and during exercise intensity changes?

At rest, the body primarily uses fats (about 70%) and carbohydrates (about 30%) for ATP production. As exercise intensity increases, there is a shift towards carbohydrates, with high-intensity aerobic exercise relying almost entirely on carbohydrates if available.

What is the impact of aerobic fitness on resting metabolic rate according to studies from the early 1990s?

Studies published in 1992 and 1997 indicated that an individual's level of aerobic fitness does not have a correlation with their resting metabolic rate and does not improve the predictive power of fat-free mass for resting metabolic rate.

How do aerobic training and resistance training compare in their effects on body mass and fat mass in overweight adults?

A 2012 study comparing training modalities found that aerobic training is more effective for reducing fat mass and body mass in overweight adults. Resistance training is beneficial as a secondary factor, particularly for aging and lean mass concerns, but it carries a higher injury rate and is less impactful on fat reduction compared to aerobic exercise.

What is the potential effect of anaerobic exercise, like weight lifting, on BMR?

Anaerobic exercise, such as weight lifting, builds muscle mass, which contributes to fat-free mass. Theoretically, an increase in muscle mass could increase BMR. However, the actual impact on BMR is debated, with some studies suggesting that even a substantial increase in muscle mass might only have a minor effect on overall BMR.

What is the 'rate of living hypothesis' proposed by Raymond Pearl in 1926?

In 1926, Raymond Pearl proposed the 'rate of living hypothesis,' suggesting that longevity varies inversely with basal metabolic rate. This implies that organisms with lower metabolic rates tend to live longer.

What evidence supports the 'rate of living hypothesis' linking lower metabolic rates to increased longevity?

Support for the 'rate of living hypothesis' comes from observations like larger mammals having longer lifespans despite higher total metabolic rates (due to lower cellular metabolism and slower heart rates), and the extended lifespan of fruit flies at lower ambient temperatures. Additionally, calorie restriction and reduced thyroid hormone levels, both associated with decreased metabolic rates, have been linked to increased longevity in animals.

What factors, besides metabolic rate, can affect an animal's maximum lifespan?

Factors that can affect an animal's maximum lifespan include the ratio of total daily energy expenditure to resting metabolic rate, the degree of coupling between oxidative phosphorylation and ATP production, the amount of saturated fat in mitochondrial membranes, and the efficiency of DNA repair mechanisms.

How can changes in metabolism be confounded with other factors influencing lifespan?

Changes in metabolism can be confounded by other factors that also affect lifespan. For example, calorie restriction lowers metabolic rate but also reduces body temperature. Studies have shown that body temperature might be a more significant modulator of lifespan than metabolic rate itself in some small mammals.

How can weight training affect metabolism compared to aerobic training?

Weight training can have a longer-lasting impact on metabolism than aerobic training. However, there are no precise mathematical formulas that can accurately predict the duration and extent of metabolic increase resulting from anabolic neuromuscular training.

What is the typical effect of a significant decrease in food intake on metabolic rate?

A significant decrease in food intake typically lowers the metabolic rate. This occurs as the body attempts to conserve energy in response to reduced calorie availability.

How can antithyroid agents affect metabolic rate?

Antithyroid agents, such as propylthiouracil and methimazole, are used to treat hyperthyroidism. They work by bringing the metabolic rate down to normal levels, thereby restoring a euthyroid state.

In what medical conditions might metabolic rate be elevated?

Metabolic rate can be elevated in certain medical conditions, including stress, illness, and diabetes. Menopause can also potentially affect an individual's metabolism.

What is the primary function of the hypothalamus in relation to metabolism?

The hypothalamus plays a crucial role in regulating metabolism by controlling and integrating the autonomic nervous system, regulating body temperature, and managing food intake through centers that sense hunger and satiety.

What is the significance of Adenosine Triphosphate (ATP) in metabolic processes?

Adenosine Triphosphate (ATP) is a critical molecule that drives the transfer of energy from exergonic (energy-releasing) reactions to endergonic (energy-requiring) reactions, such as muscle contraction. It stores energy in its phosphate bonds, which are broken to release energy for cellular work.

What is the relationship between body surface area, age, sex, and BMR according to early research?

Early research by Harris and Benedict indicated that approximate values for BMR could be derived using body surface area (calculated from height and weight), age, and sex, along with oxygen and carbon dioxide measurements from calorimetry. They also found that expressing metabolic rate per unit of 'fat-free' or lean body mass eliminated sex differences, suggesting similar basal metabolism between sexes when adjusted for body composition.

What is the general principle behind the Harris-Benedict equations for estimating BMR?

The Harris-Benedict equations, both original and revised, estimate Basal Metabolic Rate (BMR) by taking into account an individual's mass, height, and age, with specific adjustments for sex. These formulas provide a way to calculate the daily caloric needs at rest.

What is the primary difference between the original and revised Harris-Benedict equations?

The revised Harris-Benedict equation, developed in 1984, was based on new data and found to be more accurate in predicting actual energy expenditure compared to the original equation published in 1919.

What is the Mifflin St Jeor equation considered to be in terms of accuracy for predicting BMR?

The Mifflin St Jeor equation, introduced in 1990, is considered to be about 5% more accurate than the revised Harris-Benedict equation in predicting basal metabolic rate, reflecting changes in lifestyle over the past century.

What is the main limitation of BMR estimation formulas like Harris-Benedict and Mifflin St Jeor?

A primary limitation of formulas like the Harris-Benedict and Mifflin St Jeor equations is that they are primarily based on total body mass. They do not fully account for the differing metabolic activity between lean body mass and body fat, which is addressed by other formulas like the Katch-McArdle and Cunningham formulas.

What is the role of the hypothalamus in regulating food intake?

The hypothalamus regulates food intake through specific centers. The feeding center, or hunger center, initiates the sensation of hunger, while the satiety center is stimulated by sufficient food intake and high leptin levels, sending signals to inhibit the feeding center. Ghrelin levels also play a role in initiating hunger signals.

Basal Metabolic Rate Wiki: Metabolic Foundations: Understanding Basal Energy Expenditure

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Defining Basal Metabolic Rate

Energy Expenditure at Rest

Basal Metabolic Rate (BMR) quantifies the rate of energy expenditure per unit of time by endothermic animals when in a state of rest. This rate is typically expressed in energy units per unit time, such as watts (joules/second) or milliliters of oxygen consumed per minute per kilogram of body mass (ml O₂/min/kg).¹

Measurement Criteria

Accurate BMR measurement necessitates strict adherence to specific conditions: the individual must be in a physically and psychologically undisturbed state, within a thermally neutral environment, and in the post-absorptive state (i.e., not actively digesting food).¹ For ectothermic animals like fish and reptiles, the equivalent term is Standard Metabolic Rate (SMR), which requires documentation of the measurement temperature.

Essential Bodily Functions

BMR represents the energy required for fundamental physiological processes that maintain life. These include breathing, circulation, temperature regulation, cellular growth and repair, brain function, and nerve activity. BMR significantly influences an individual's daily calorie expenditure, accounting for approximately 70% of total energy use, and thus plays a critical role in weight management.²

Factors Influencing BMR

Age and Body Composition

BMR generally declines with age, typically by 1-2% per decade after age 20. This decline is primarily attributed to the loss of fat-free mass. Conversely, increasing muscle mass tends to elevate BMR. While aerobic fitness itself does not correlate with BMR when adjusted for fat-free mass, anaerobic exercise (strength training) can increase resting energy expenditure.³⁴

Environmental and Physiological States

External temperature can influence BMR, with lower temperatures generally leading to higher metabolic rates in birds and rodents as they expend more energy to maintain body temperature. Illness, fever, burns, fractures, and infections can significantly increase metabolic rate due to the body's heightened physiological demands. Stress levels can also temporarily elevate BMR.⁵

Diet and Hormonal Cycles

Consumed food and beverages can affect energy expenditure through thermogenesis. For women, BMR can fluctuate throughout the menstrual cycle, often showing an increase during the luteal phase due to hormonal changes like elevated progesterone.¹³¹⁴¹⁵¹⁶¹⁷¹⁸

Physiological Regulation

The Hypothalamus: Central Regulator

The hypothalamus, situated within the diencephalon, serves as the primary organ responsible for regulating metabolism. It controls and integrates the activities of the autonomic nervous system (ANS), which governs involuntary bodily functions such as heart rate, digestion, and glandular secretions, including those from the thyroid gland, a key player in metabolic disorders.¹⁹

Survival Mechanisms

The hypothalamus also regulates body temperature, food intake (hunger and satiety centers), and thirst. These functions collectively contribute to a vital survival mechanism that sustains the bodily processes measured by BMR. The efficient functioning of these systems ensures the body's continuous energy supply and maintenance of homeostasis.¹⁹

Energy Conversion

Metabolism involves complex biochemical reactions, converting energy substrates like carbohydrates, fats, and proteins into usable energy (ATP). This process, measured through techniques like indirect calorimetry, relies on oxygen consumption and carbon dioxide production, reflecting the body's efficiency in utilizing fuel sources.¹

Estimating BMR: Key Formulas

Harris-Benedict Equation (Original, 1919)

The foundational Harris-Benedict equation, published in 1919, estimates basal heat production (P) based on mass (m), height (h), and age (a).

For Men: ${\displaystyle P=\left({\frac {13.7516m}{1~{\\text{kg}}}}+{\\frac {5.0033h}{1~{\\text{cm}}}}-{\\frac {6.7550a}{1~{\\text{year}}}}+66.4730\\right){\\frac {\\text{kcal}}{\\text{day}}},}\$

Metabolic Foundations

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Defining Basal Metabolic Rate ℹ️

⚡ Energy Expenditure at Rest

🌡️ Measurement Criteria

⚙️ Essential Bodily Functions

Factors Influencing BMR 📊

⏳ Age and Body Composition

🌡️ Environmental and Physiological States

🍎 Diet and Hormonal Cycles

Physiological Regulation 🧠

🌐 The Hypothalamus: Central Regulator

❤️ Survival Mechanisms

💡 Energy Conversion

Estimating BMR: Key Formulas 🧮

📜 Harris-Benedict Equation (Original, 1919)

🔄 Revised Harris-Benedict Equation (1984)

🎯 Mifflin-St Jeor Equation (1990)

💪 Lean Body Mass Formulas

Biochemical Basis of Metabolism 🔬

🍬 Carbohydrate Metabolism

🥑 Fat Metabolism

🥩 Protein Metabolism

Exercise and BMR 🏃

💨 Aerobic Exercise

🏋️ Anaerobic Exercise

⚖️ Time Commitment vs. Benefit

Metabolism and Longevity ⏳

💡 The Rate of Living Hypothesis

📉 Metabolic Rate and Lifespan

🔬 Modern Research Insights

Medical Considerations ⚕️

💊 Hormonal and Drug Influences

📉 Impact of Diet and Activity

🧪 Research into Metabolic Enhancement

Teacher's Corner 🧑‍🏫

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Dive in with Flashcard Learning!

Defining Basal Metabolic Rate

Energy Expenditure at Rest

Measurement Criteria

Essential Bodily Functions

Factors Influencing BMR

Age and Body Composition

Environmental and Physiological States

Diet and Hormonal Cycles

Physiological Regulation

The Hypothalamus: Central Regulator

Survival Mechanisms

Energy Conversion

Estimating BMR: Key Formulas

Harris-Benedict Equation (Original, 1919)

Revised Harris-Benedict Equation (1984)

Mifflin-St Jeor Equation (1990)

Lean Body Mass Formulas

Biochemical Basis of Metabolism

Carbohydrate Metabolism

Fat Metabolism

Protein Metabolism

Exercise and BMR

Aerobic Exercise

Anaerobic Exercise

Time Commitment vs. Benefit

Metabolism and Longevity

The Rate of Living Hypothesis

Metabolic Rate and Lifespan

Modern Research Insights

Medical Considerations

Hormonal and Drug Influences

Impact of Diet and Activity

Research into Metabolic Enhancement

Teacher's Corner

True or False?

Test Your Knowledge!

Gamer's Corner

Discover other topics to study!

References

Feedback & Support

Disclaimer

Important Notice