7.1 Basal Metabolic Rate (BMR) and Resting Energy Expenditure (REE)

Basal Metabolic Rate (BMR) and Resting Energy Expenditure (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 calories we need daily. Factors like age, 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 kilojoules (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 (weight loss, 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 muscle mass, 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
• Indirect calorimetry: 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, elderly, critically ill)
  • Equations are derived from regression analyses of large datasets, relating energy expenditure to anthropometric and demographic variables
• Harris-Benedict equation: 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)
• Mifflin-St Jeor equation: 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