What is the best way to determine basal metabolic rate (BMR)?

Determining Basal Metabolic Rate (BMR)

Indirect calorimetry is the gold standard for measuring BMR, but when unavailable, the Harris-Benedict equations provide the most reliable predictive formula for most individuals. 1

Understanding BMR

Basal metabolic rate represents the minimal rate of energy expenditure compatible with life, typically measured:

• In a resting, supine position
• Under strictly controlled laboratory conditions
• In a fasted state
• With controlled environmental conditions 1

BMR differs from resting metabolic rate (RMR), which is assessed in less strict conditions at any given state of rest throughout the day. RMR is typically higher than BMR 1.

Methods for Determining BMR

1. Gold Standard: Indirect Calorimetry

When available, indirect calorimetry should be used to measure BMR, as individual measurements may differ considerably from estimated values 1. This is particularly important for:

• Patients with cirrhosis (30-35% may be hypermetabolic) 1
• Individuals at extremes of BMI (severely underweight or obese) 1
• Clinical settings requiring precise energy requirements

2. Predictive Equations

When indirect calorimetry is unavailable, predictive equations can be used:

Harris-Benedict Equations (Most Widely Validated)

• For men: BMR = 66.4730 + 13.7516 × weight(kg) + 5.0033 × height(cm) - 6.7550 × age(years) 1
• For women: BMR = 655.0955 + 9.5634 × weight(kg) + 1.8496 × height(cm) - 4.6756 × age(years) 1

These equations perform reasonably well in healthy, normal-weight adults and those with moderate obesity (accurate in 68.5% of individuals with BMI 25-40 kg/m²) 1.

Special Considerations for Weight

• For patients without ascites: Use actual body weight
• For patients with ascites: Use ideal weight according to height 1
• For obese patients: The Harris-Benedict equation has been shown to have acceptable precision even in severe obesity (BMI >37) 2

Limitations and Pitfalls

1. Accuracy limitations: Even the best predictive equations are accurate in only about 60% of cases across different weight groups 1

2. Population-specific concerns:

   • Older adults: Harris-Benedict equations have reduced accuracy 1
   • Children: Adult equations are not valid; specialized pediatric equations should be used 3
   • Extreme obesity or malnutrition: Predictive accuracy drops to around 45-60% 1

3. Disease states: Acute and chronic diseases affect energy needs and may require adjustment factors 1

Total Energy Expenditure

For estimating total energy requirements:

• BMR typically represents about 73% of total energy expenditure
• Thermogenesis accounts for approximately 15%
• Physical activity accounts for approximately 12% 1

For practical purposes in clinical settings:

• For healthy individuals: Total energy requirement ≈ 1.5 × BMR
• For those with acute/chronic illness: Total energy requirement ≈ 1.3 × BMR
• For critical illness: Total energy requirement ≈ 1.1 × BMR 1

Conclusion

While indirect calorimetry remains the gold standard for determining BMR, the Harris-Benedict equations provide a reasonable alternative in most clinical and non-clinical settings. However, practitioners should be aware of their limitations, particularly in special populations and disease states.