Basal Metabolism (BMR)
Introduction
Basal Metabolism, often expressed as Basal Metabolic Rate (BMR) or Basal
Energy Expenditure (BEE), refers to the minimum amount of energy required by
the body to perform its most vital physiological functions when at complete rest, in
a thermoneutral environment, and in the post-absorptive state (usually 12–14 hours
after last meal). These functions include breathing, blood circulation, maintenance
of body temperature, nerve activity, hormone secretion, and cell repair.
In simple terms, BMR represents the energy cost of living — the energy needed to
keep the body alive when no physical activity is being performed. It is the largest
component of total energy expenditure, accounting for about 60–75% of daily
energy use in sedentary adults.
For example, even when we are lying quietly, our heart keeps beating, lungs exchange
oxygen and carbon dioxide, kidneys filter blood, and body cells continue their
metabolic processes — all these functions need energy. Hence, basal metabolism
forms the foundation of human energy requirements.
Concept and Measurement Conditions
Basal metabolism is measured under very specific and controlled conditions to ensure
accuracy. To obtain the true BMR, the following conditions must be fulfilled:
The person should be awake but completely at rest, lying in a supine
position.
Measurement should be done in the morning, after a full night’s sleep, and in
a post-absorptive state (no food intake for 12 hours).
The surrounding temperature should be thermoneutral (neither too hot nor
too cold).
The person should be mentally relaxed and free from stress or anxiety.
There should be no recent physical activity, fever, or illness that can
influence metabolism.
Under these conditions, the energy used by the body represents the true basal
metabolism — the minimum level of energy necessary for survival.
, Methods for Measuring BMR
Measurement of BMR can be performed using scientific techniques similar to energy
expenditure methods:
A. Direct Calorimetry
In this method, the person is placed in an insulated chamber (calorimeter), and the
amount of heat produced by the body is directly measured. The total heat output
corresponds to the basal energy expenditure. Although accurate, this method is
expensive and not used routinely in dietetic practice.
B. Indirect Calorimetry
This is the most commonly used method for measuring BMR. It estimates energy
expenditure by measuring oxygen consumption (O₂) and carbon dioxide output
(CO₂). Since oxidation of food in the body requires oxygen, the amount of O₂ used
can be converted into energy in kilocalories.
The relationship is based on the Respiratory Quotient (RQ) — which depends on
which macronutrient (carbohydrate, fat, or protein) is being oxidized.
C. Predictive Equations
In clinical and dietetic settings, instead of direct measurement, predictive equations
are used to estimate BMR. Common formulas include:
Harris–Benedict Equation (Revised):
For men:
BMR = 88.362 + (13.397 × weight in kg) + (4.799 × height in cm) −
(5.677 × age in years)
For women:
BMR = 447.593 + (9.247 × weight in kg) + (3.098 × height in cm) −
(4.330 × age in years)
Mifflin–St Jeor Equation:
For men:
BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years)
+5
For women:
BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years)
− 161
These equations are practical tools for dietitians to estimate an individual’s basal
energy needs and total energy requirements.
Introduction
Basal Metabolism, often expressed as Basal Metabolic Rate (BMR) or Basal
Energy Expenditure (BEE), refers to the minimum amount of energy required by
the body to perform its most vital physiological functions when at complete rest, in
a thermoneutral environment, and in the post-absorptive state (usually 12–14 hours
after last meal). These functions include breathing, blood circulation, maintenance
of body temperature, nerve activity, hormone secretion, and cell repair.
In simple terms, BMR represents the energy cost of living — the energy needed to
keep the body alive when no physical activity is being performed. It is the largest
component of total energy expenditure, accounting for about 60–75% of daily
energy use in sedentary adults.
For example, even when we are lying quietly, our heart keeps beating, lungs exchange
oxygen and carbon dioxide, kidneys filter blood, and body cells continue their
metabolic processes — all these functions need energy. Hence, basal metabolism
forms the foundation of human energy requirements.
Concept and Measurement Conditions
Basal metabolism is measured under very specific and controlled conditions to ensure
accuracy. To obtain the true BMR, the following conditions must be fulfilled:
The person should be awake but completely at rest, lying in a supine
position.
Measurement should be done in the morning, after a full night’s sleep, and in
a post-absorptive state (no food intake for 12 hours).
The surrounding temperature should be thermoneutral (neither too hot nor
too cold).
The person should be mentally relaxed and free from stress or anxiety.
There should be no recent physical activity, fever, or illness that can
influence metabolism.
Under these conditions, the energy used by the body represents the true basal
metabolism — the minimum level of energy necessary for survival.
, Methods for Measuring BMR
Measurement of BMR can be performed using scientific techniques similar to energy
expenditure methods:
A. Direct Calorimetry
In this method, the person is placed in an insulated chamber (calorimeter), and the
amount of heat produced by the body is directly measured. The total heat output
corresponds to the basal energy expenditure. Although accurate, this method is
expensive and not used routinely in dietetic practice.
B. Indirect Calorimetry
This is the most commonly used method for measuring BMR. It estimates energy
expenditure by measuring oxygen consumption (O₂) and carbon dioxide output
(CO₂). Since oxidation of food in the body requires oxygen, the amount of O₂ used
can be converted into energy in kilocalories.
The relationship is based on the Respiratory Quotient (RQ) — which depends on
which macronutrient (carbohydrate, fat, or protein) is being oxidized.
C. Predictive Equations
In clinical and dietetic settings, instead of direct measurement, predictive equations
are used to estimate BMR. Common formulas include:
Harris–Benedict Equation (Revised):
For men:
BMR = 88.362 + (13.397 × weight in kg) + (4.799 × height in cm) −
(5.677 × age in years)
For women:
BMR = 447.593 + (9.247 × weight in kg) + (3.098 × height in cm) −
(4.330 × age in years)
Mifflin–St Jeor Equation:
For men:
BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years)
+5
For women:
BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years)
− 161
These equations are practical tools for dietitians to estimate an individual’s basal
energy needs and total energy requirements.
