Study guide
Study guide

Basal metabolic rate

Define basal metabolic rate and list the factors that affect it

Definition and normal values

Basal metabolic rate (BMR) represents the energy required to sustain life at rest. It is more specifically defined as the rate of energy expenditure per unit of time under standard conditions:

  • immediately after waking up
  • at physical and mental rest
  • comfortable environmental temperature
  • at least 12 hours after a meal

Normal BMR is approximately 100 watts or 2000 kcal/day for a 70kg adult male.

Resting energy expenditure (REE) measures energy expenditure under relaxed, more practical conditions:

  • resting comfortably
  • at least 2-4 hours after a meak
  • not necessarily soon after waking

REE and BMR typically differ by less than 10%, with REE being higher.

Determinants of basal metabolic rate

A terrible mnemonic - MNM SLAP SET

Core metabolic factors - Metabolic demand, Neurohormonal factors, Muscle mass

Physiological states - Sex, Lactation, Age, Pregnancy

Stressors - Starvation, Eating, Temperature

Determinant

Effect

Metabolic demand

Increased metabolic demand states (e.g. fever, sepsis, inflammation, trauma, exercise) raise BMR.

Neurohormonal factors

Thyroid hormones, catecholamines, and sympathetic activity increase BMR by increasing the rate of metabolic reactions.

Lean muscle mass

Increased lean muscle mass → increased BMR.

Sex
  • Women have a lower BMR due to a lower proportion of lean muscle mass.
  • Adjusted for lean body mass, BMR is similar between sexes.
Lactation

Increases BMR due to energy requirements for milk production.

Age
  • BMR decreases with advanced age due to reduced metabolically active tissue.
  • Children have a substantially higher BMR per unit body mass.
Pregnancy

BMR increases by ~20% due to:

  • additional foetal energy requirements
  • growth of placenta, uterus and breast tissue
  • increased cardiac and respiratory work to maintain increased cardiac output and minute ventilation
Starvation

Decreases BMR due to:

  • Atrophy of metabolically active tissues
  • Reduced metabolic activity (↓ thyroid hormones and sympathetic tone)
Eating
  • BMR increases for ~6 hours after eating, as energy is required for digestion, absorption, storage and metabolism of nutrients; a concept known as specific dynamic action.
  • Deamination of amino acids accounts for much of this increase.
Temperature
  • Acclimatisation to warm environments → decreased BMR
  • Fever - hypothalamus mediates an increase in BMR which generates heat to facilitate fever

Describe the ways it may be measured

Direct Calorimetry

The subject lives in a calorimeter chamber for several days.

The chamber is surrounded by water, which absorbs the heat produced by the body.

Heat production is proportionate to BMR. Hence the temperature increase in the water is directly proportional to the metabolic rate.

If the quantity of O₂ used and CO₂ produced is also measured, the respiratory quotient can be calculated.

Advantages: Measures actual heat production. Gold standard technique.

Limitations: Expensive, complex and impractical for clinical use.

Direct calorimetry

Direct calorimetry works by quantifying heat production.

Image by Topend Sports

Indirect Calorimetry

Based on the principle that heat production is proportional to oxygen consumption.

Assumes 1L O₂ consumed = 4.8 kJ of energy produced.

There are multiple techniques, the following is the spirometer technique.

  • Patient breathes from a closed-circuit system containing a known volume of O₂.
  • Inspired air passes through an inspiratory valve; expired air passes through an expiratory valve where CO₂ is removed before being inspired again.
  • As O₂ is consumed, the gas volume in the circuit decreases.
  • The rate of volume reduction reflects the rate of O₂ consumption, which correlates with BMR.
  • The respiratory quotient can be calculated if the rate of CO₂ exhalation is also measured.

Advantages: Less complex and more practical than direct calorimetry. Variations in this technique can be used in-line with a ventilator. Highly accurate despite indirect estimate.

Limitations: Requires careful calibration and assumptions about substrate utilization.

Indirect calorimetry

Indirect calorimetry metabolic cart using a canopy hood.

Image from Cosmed, licensed under CC-BY-SA 4.0

Formulae

Various formulae are also used to estimate BMR such as the Harris-Benedict equation. The result is often multiplied by a 'stress factor' to account for critical illness. Some populations require a bigger stress factor than others (e.g. sepsis, burns, trauma).

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