Metabolic rate refers to the total energy expenditure of a person per unit time. It is expressed in calories per square meter of body surface area per hour. The body derives energy through the oxidation of food, and this process releases heat. Therefore, metabolic rate can be measured indirectly by estimating heat production over time.

Metabolic rate varies depending on physiological conditions and is commonly expressed as:

• Total Metabolic Rate
• Basal Metabolic Rate (BMR)

   What is Basal Metabolic Rate (BMR)?

Basal Metabolic Rate (BMR) is the minimum energy required by the body to maintain essential life processes under resting conditions.

It supports:

• Respiration
• Circulation
• Cellular metabolism
• Organ function

Standard Conditions for Measuring BMR

• Subject in supine position (complete physical & mental rest)
• Post-absorptive state (12–14 hours after last meal)
• Thermoneutral environment (no heat or cold stress)
• No recent physical activity or emotional stress

Units of BMR

• kcal/m²/hour
• kcal/day

   Determination of Metabolic Rate

Metabolic rate can be determined using two main methods:

1. Direct Calorimetry

Direct calorimetry measures heat production directly using an insulated chamber called the Atwater–Rosa–Benedict calorimeter.

Principle

Energy expenditure is calculated by measuring:

• Heat released by the body
• Evaporation loss (sweat and respiration)

Process

• Subject performs a specific activity inside the chamber
• Water circulates through the chamber and absorbs heat
• Temperature difference of incoming and outgoing water is measured
• Evaporated water is absorbed using chemical absorbents
• 1 gram of water evaporation = 0.58 kcal energy loss

Calculation

Total heat loss = heat gained by water + evaporative loss
Metabolic rate = kcal/hour or kcal/m²/hour

Limitation

• Expensive and complex
• Not suitable for routine clinical use

2. Indirect Calorimetry

Indirect calorimetry estimates energy expenditure by measuring oxygen consumption (VO₂) and respiratory quotient (RQ).

Principle

Energy production is proportional to oxygen used during metabolism.

Steps

• Measure oxygen consumption (VO₂)
• Determine respiratory quotient (RQ)
• Calculate energy expenditure using standard values

Energy Values (Approx.)

• Sitting at rest: 100 kcal/hour
• Typing: 140 kcal/hour
• Walking slowly: 200 kcal/hour
• Walking fast: 650 kcal/hour
• Swimming: 500 kcal/hour
• Walking upstairs: 1100 kcal/hour

   Example Calculation

A person consumes 60 liters of O₂ in 1 hour with RQ = 0.85.
At RQ 0.85:

• 1 liter O₂ = 4.862 kcal

Energy expenditure =
60 × 4.862 = 291.72 kcal/hour

   Modern Method (Advanced Physiology)

In modern physiology, metabolic rate is more accurately calculated using:

• Oxygen consumption (VO₂)
• Carbon dioxide production (VCO₂)

Weir’s Equation

This method provides a more precise estimation of energy expenditure than oxygen alone.

   Factors Affecting Metabolic Rate

Metabolic rate varies among individuals due to several factors:

1. Age

• Higher in children
• Lower in elderly

2. Sex

• Higher in males due to greater muscle mass

3. Body Composition

• Muscle increases metabolic rate
• Fat tissue has lower metabolic activity

4. Hormonal Status

• Thyroid hormones significantly increase metabolic rate

5. Temperature

• Cold environment increases metabolism

6. Physical Activity

• Exercise and stress increase energy expenditure

   Clinical Importance

Understanding metabolic rate is important in:

• Obesity management
• Thyroid disorders
• Malnutrition assessment
• Critical care nutrition planning

Conclusion

Metabolic rate is a key physiological parameter that reflects energy expenditure in the human body. It can be measured using direct and indirect calorimetry, while BMR represents the baseline energy requirement. Multiple physiological and environmental factors influence metabolic rate, making it an essential concept in both physiology and clinical medicine.

   FAQs

1. What is metabolic rate?

It is the rate at which the body consumes energy to maintain vital functions.

2. Which method is most accurate?

Indirect calorimetry using VO₂ and VCO₂ is most widely used in modern practice.

3. What is BMR?

Basal Metabolic Rate is the energy required by the body at complete rest.

4. Why is oxygen used to measure metabolism?

Because oxygen is directly involved in energy production through oxidation of food.

5. What increases metabolic rate the most?

Thyroid hormones and physical activity significantly increase metabolic rate.