7.1 Basal Metabolic Rate (BMR) and Resting Energy Expenditure (REE)
6 min read•july 30, 2024
(BMR) and (REE) are key concepts in understanding our body's energy needs. They represent the minimum energy our body uses at rest, with BMR being measured under stricter conditions than REE.
These measurements are crucial for figuring out how many we need daily. Factors like , sex, and body composition affect BMR and REE. Knowing these values helps create personalized nutrition plans and spot potential metabolic issues.
Basal Metabolic Rate vs Resting Energy Expenditure
Definitions and Concepts
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- BMR is the minimum energy required to maintain vital functions in a fasted and rested state measured under highly controlled conditions (supine position, 12-hour fast, thermoneutral environment)
- REE is the energy expended by the body in a rested state under less strict conditions than BMR accounts for the majority of daily energy expenditure (60-75%)
- Both BMR and REE are expressed as kilocalories (kcal) or (kJ) per day or per 24 hours
- Conversion factor: 1 kcal = 4.184 kJ
- BMR is typically lower than REE by 10-20% due to the strict measurement conditions
Importance in Nutrition Assessment
- BMR and REE provide a baseline for estimating an individual's total daily energy expenditure (TDEE)
- Accurate estimation of BMR and REE is crucial for determining energy requirements and developing personalized nutrition plans
- Deviations from expected BMR or REE values can indicate underlying metabolic disorders or adaptations to changes in energy balance (, refeeding)
Factors Influencing BMR and REE
Biological Factors
- Age: BMR and REE decrease with age due to changes in body composition (reduced lean body mass) and metabolic processes (decreased cellular metabolism)
- BMR declines by 1-2% per decade after age 20
- Sex: Men generally have higher BMR and REE than women due to differences in body composition, particularly lean body mass (muscle)
- On average, men have 10-15% higher BMR and REE than women of the same age and weight
- Body composition: Lean body mass, especially , is a major determinant of BMR and REE
- Higher lean body mass results in higher BMR and REE (muscle tissue is more metabolically active than fat tissue)
- Each pound of muscle burns approximately 6 kcal/day at rest, while each pound of fat burns about 2 kcal/day
- Hormonal factors: Thyroid hormones (T3 and T4) and catecholamines (epinephrine and norepinephrine) increase BMR and REE
- Hyperthyroidism can increase BMR and REE by 50-100%, while hypothyroidism can decrease BMR and REE by 20-40%
- Genetics: Variations in genes involved in energy metabolism, such as uncoupling proteins (UCPs), can influence individual differences in BMR and REE
- Polymorphisms in UCP1, UCP2, and UCP3 genes have been associated with variations in BMR and REE
Environmental and Nutritional Factors
- Ambient temperature: Exposure to cold temperatures can increase BMR and REE due to adaptive thermogenic responses (shivering and non-shivering thermogenesis)
- A 1°C decrease in ambient temperature can increase BMR by 5-7%
- Altitude: High altitude exposure can increase BMR and REE due to the body's adaptive responses to hypoxia (increased red blood cell production, enhanced respiratory function)
- BMR can increase by 10-20% at altitudes above 4,000 meters
- Nutritional status: Prolonged fasting or severe calorie restriction can lead to adaptive reductions in BMR and REE to conserve energy
- BMR can decrease by 20-30% during prolonged fasting (>72 hours)
- Severe calorie restriction (50% of energy requirements) can reduce BMR by 10-15% within 2-4 weeks
BMR and REE Measurement Techniques
Direct and Indirect Calorimetry
- Direct calorimetry: Measures heat production in a specialized chamber, considered the gold standard for measuring BMR and REE, but is expensive and not widely available
- Subject remains in a sealed chamber for 24-48 hours, and heat production is measured using sensitive thermocouples
- : Measures oxygen consumption and carbon dioxide production to estimate energy expenditure, commonly used for measuring REE in clinical and research settings
- Based on the principle that energy production in the body is proportional to oxygen consumption and carbon dioxide production
- Respiratory quotient (RQ) is the ratio of CO2 produced to O2 consumed, which varies depending on the substrate being oxidized (carbohydrates, fats, proteins)
Specific Techniques for Indirect Calorimetry
- Ventilated hood system: Measures gas exchange while the subject breathes under a transparent canopy, allowing for measurement of REE in a supine or seated position
- Subject lies or sits comfortably under the canopy for 30-60 minutes, and gas exchange is measured using a metabolic analyzer
- Metabolic cart: Measures gas exchange using a mouthpiece or face mask, allowing for measurement of REE in various positions and during different activities
- Subject breathes through a mouthpiece or face mask connected to a metabolic cart, which analyzes inspired and expired gases
- Can be used to measure REE during rest and various physical activities (walking, cycling)
Predictive Equations
- Predictive equations: Estimate BMR and REE based on factors such as age, sex, weight, and height, but may have limitations in accuracy for certain populations (obese, , critically ill)
- Equations are derived from regression analyses of large datasets, relating energy expenditure to anthropometric and demographic variables
- : Widely used equation that estimates BMR based on age, sex, weight, and height
- Developed in 1919 using data from healthy, normal-weight individuals
- Separate equations for men and women:
- Men: BMR = 88.362 + (13.397 × weight in kg) + (4.799 × height in cm) - (5.677 × age in years)
- Women: BMR = 447.593 + (9.247 × weight in kg) + (3.098 × height in cm) - (4.330 × age in years)
- : Developed using a more diverse population and is considered more accurate than the Harris-Benedict equation for estimating REE in healthy individuals
- Developed in 1990 using data from normal-weight, overweight, and obese individuals
- Equation: REE = (10 × weight in kg) + (6.25 × height in cm) - (5 × age in years) + (s), where s is +5 for males and -161 for females
Interpreting BMR and REE Results
Comparison to Predictive Equations and Norms
- Measured BMR or REE values can be compared to estimates from predictive equations to assess whether an individual's energy expenditure is within the expected range
- Values within ±10% of predicted are considered normal
- Values >10% above predicted may indicate hypermetabolism, while values >10% below predicted may indicate hypometabolism
- BMR and REE values can also be compared to population norms based on age, sex, and body composition
- Norms are typically expressed as kcal/kg body weight/day or kcal/kg lean body mass/day
- Healthy adults: 20-25 kcal/kg/day or 30-35 kcal/kg lean body mass/day
Clinical Implications
- Identification of hypo- or hypermetabolism: BMR or REE values significantly below or above the predicted range may indicate underlying metabolic disorders
- Hypothyroidism: BMR and REE may be 20-40% below predicted
- Hyperthyroidism: BMR and REE may be 50-100% above predicted
- Cachexia (muscle wasting) in cancer or HIV/AIDS: REE may be 10-30% above predicted
- Determination of energy requirements: Measured or estimated BMR and REE values are used to calculate total daily energy expenditure (TDEE) by applying activity factors
- Sedentary: TDEE = BMR or REE × 1.2
- Low active: TDEE = BMR or REE × 1.375
- Active: TDEE = BMR or REE × 1.55
- Very active: TDEE = BMR or REE × 1.725
- Monitoring changes in energy expenditure: Serial measurements of BMR or REE can be used to track changes in energy expenditure over time
- Weight loss: BMR and REE may decrease by 10-20% due to adaptive thermogenesis and loss of lean body mass
- Aging: BMR and REE decline by 1-2% per decade due to changes in body composition and metabolic processes
- Medical interventions (medications, surgery): May increase or decrease BMR and REE depending on the specific intervention
- Assessment of metabolic adaptation: Comparing measured BMR or REE to predicted values can help identify adaptive changes in energy expenditure
- Prolonged calorie restriction: BMR and REE may decrease by 10-15% beyond what is expected based on changes in body weight and composition
- Refeeding after prolonged fasting or malnutrition: BMR and REE may increase by 10-20% above predicted values due to increased metabolic demands for tissue repair and growth
Key Terms to Review (19)
Age: Age refers to the length of time an individual has lived, usually measured in years. It plays a crucial role in determining various physiological and metabolic factors that influence nutritional needs, energy expenditure, and body composition, which are essential in assessing health and nutrition status.
Athletes: Athletes are individuals who engage in physical activities or sports, often requiring skill, strength, endurance, and agility. Their training regimens and nutritional needs are distinct due to their heightened energy expenditure and specific performance goals, which makes understanding their dietary habits and health status crucial for optimizing performance and recovery.
Basal Metabolic Rate: Basal metabolic rate (BMR) is the number of calories your body needs to maintain basic physiological functions while at rest, such as breathing, circulation, and cell production. BMR is a critical component of total energy expenditure and is influenced by factors like age, sex, weight, and body composition. Understanding BMR helps in assessing overall energy needs and forms the foundation for evaluating energy balance in relation to physical activity levels.
Caloric deficit: A caloric deficit occurs when an individual consumes fewer calories than their body expends over a certain period, leading to weight loss. This concept is crucial in understanding how energy balance affects body composition and health. It is influenced by various factors, including basal metabolic rate, physical activity levels, and the thermic effect of food.
Caloric Surplus: A caloric surplus occurs when an individual consumes more calories than their body expends in energy, leading to weight gain over time. This imbalance between calorie intake and expenditure can contribute to body fat accumulation and is often a key factor in discussions around weight management, energy balance, and nutritional strategies for muscle gain or weight loss.
Calories: Calories are units of energy that measure the amount of energy food provides to the body. They are essential for fueling all bodily functions, from maintaining basic metabolic processes to supporting physical activities. Understanding how calories are utilized and expended is crucial when assessing energy balance, particularly in relation to basal metabolic rate and resting energy expenditure.
Elderly: The term 'elderly' generally refers to individuals who are in the later stages of life, often considered to be aged 65 and older. This demographic is significant in nutrition assessment due to the unique dietary needs, health challenges, and social factors that impact their overall well-being and nutritional status.
Energy requirements during rest: Energy requirements during rest refer to the number of calories your body needs to maintain basic physiological functions while at rest. This concept is primarily measured through Basal Metabolic Rate (BMR) and Resting Energy Expenditure (REE), which account for energy used for essential processes like breathing, circulation, and cell production without any additional activity.
Fasting state: The fasting state refers to a metabolic condition that occurs when the body has not received food for an extended period, typically over 8 hours. In this state, the body shifts from utilizing glucose as its primary energy source to using stored fat and ketones, allowing it to maintain essential functions and energy production during times of food scarcity. This transition is critical for understanding how energy metabolism is regulated in relation to both basal metabolic rate and resting energy expenditure.
Harris-Benedict Equation: The Harris-Benedict Equation is a formula used to estimate an individual's Basal Metabolic Rate (BMR), which represents the number of calories the body needs at rest to maintain vital functions. By calculating BMR, this equation provides a foundation for understanding how much energy an individual expends throughout the day, taking into account their activity level and Total Energy Expenditure (TEE). It serves as a key tool in nutrition assessment and helps tailor dietary recommendations to meet individual energy needs.
Hormonal influences: Hormonal influences refer to the effects that hormones have on various physiological processes in the body, including metabolism, growth, and energy expenditure. These chemical messengers, produced by glands in the endocrine system, play a crucial role in regulating Basal Metabolic Rate (BMR) and Resting Energy Expenditure (REE) by affecting how the body utilizes energy, stores fat, and responds to food intake. The interplay of different hormones can significantly impact overall health and body composition.
Indirect calorimetry: Indirect calorimetry is a method used to estimate energy expenditure by measuring the amount of oxygen consumed and carbon dioxide produced during metabolic processes. This technique allows for the assessment of resting energy expenditure and overall metabolic rates, helping to provide insights into an individual's caloric needs and energy balance.
Kilojoules: Kilojoules are a unit of measurement for energy, often used in nutrition to quantify the energy content of food and the energy expended by the body. Understanding kilojoules is crucial for assessing energy balance, which refers to the relationship between energy intake and energy expenditure, particularly in relation to maintaining, losing, or gaining weight.
Mifflin-St Jeor Equation: The Mifflin-St Jeor Equation is a widely used formula for estimating an individual's Basal Metabolic Rate (BMR), which represents the number of calories burned at rest. This equation takes into account factors such as age, gender, weight, and height to provide a more personalized estimate of daily energy needs. Understanding this equation is crucial for calculating Resting Energy Expenditure (REE), assessing Physical Activity Levels (PAL), and ultimately determining Total Energy Expenditure (TEE) for dietary planning and weight management.
Muscle Mass: Muscle mass refers to the amount of muscle tissue in the body, which is a critical component of overall body composition. It plays a key role in determining an individual's metabolic rate, as muscle tissue requires more energy to maintain than fat tissue. Higher muscle mass can significantly influence Basal Metabolic Rate (BMR) and Resting Energy Expenditure (REE), as these measures reflect the energy needed to sustain vital bodily functions at rest.
Resting Energy Expenditure: Resting energy expenditure (REE) refers to the amount of energy, measured in calories, that the body requires at rest to maintain basic physiological functions such as breathing, circulation, and cell production. It represents a significant portion of an individual's total daily energy expenditure and is closely linked to basal metabolic rate (BMR), which is the energy expended while at complete rest in a thermally neutral environment. Understanding REE is crucial for nutritional assessment and can influence dietary planning and weight management.
Thermogenic Methods: Thermogenic methods refer to techniques or processes that increase energy expenditure through heat production in the body. These methods are particularly relevant when discussing factors that contribute to basal metabolic rate (BMR) and resting energy expenditure (REE), as they can significantly influence how much energy the body uses at rest and during various activities.
Weight Loss: Weight loss refers to the reduction of total body mass, which can occur due to a decrease in body fat, muscle mass, or fluids. It is often pursued for health improvement, aesthetic reasons, or due to medical conditions. Understanding weight loss is essential in relation to energy expenditure and metabolic processes, as it involves the balance between calories consumed and calories burned through activities, including basal metabolic rate and resting energy expenditure.
Weight Maintenance: Weight maintenance refers to the process of maintaining a stable body weight after achieving a desired weight loss or gain. It involves balancing calorie intake with calorie expenditure to prevent further weight loss or gain. This concept is crucial for individuals who have successfully lost weight and want to ensure that they do not regain it, as well as for those aiming to maintain their current healthy weight.