Indirect Calorimetry: Measuring Energy Expenditure & Metabolism

Indirect Calorimetry Basics

Overall Energy Consumption

Indirect calorimetry determines energy consumption by measuring O₂ consumption and CO₂ production. It uses the equation: Food + O₂ → CO₂ + H₂O, and also accounts for urinary nitrogen (~1g urinary N represents the oxidation of 6.25g protein).

The amount of calories produced per liter of O₂ in metabolism is relatively constant, regardless of whether the fuel used is carbohydrates, lipids, or proteins.

Limitations and Applications

Various physiological situations can alter the body's gas exchange and affect energy expenditure estimations. These include changes in acid-base balance due to acidosis or hyperventilation (including hiccups).

Respiratory Quotient (RQ)

RQ = VCO₂ / VO₂. This ratio indicates the proportion of carbohydrates, lipids, protein, and alcohol consumed in the diet.

Doubly Labeled Water Method

This method involves oral administration of ²H₂¹⁸O, followed by fluid collection (> 7 days) and analysis by mass spectrometry. It allows for the measurement of Total Metabolic Rate (TMR) and considers energy intake (TID), aiming for equivalence in energy balance.

Determining Metabolic Rates

Standard Metabolic Rate (SMR)

Determining basal metabolism (SMR) via calorimetry requires the subject to be awake and resting after an 8-12 hour fast, under ambient conditions. Resting basal metabolism refers to energy expenditure under normal feeding conditions without prior fasting.

Estimating Total Metabolic Rate (TMR)

Directly measuring TMR is not always feasible. Mathematical approximations are used:

Harris-Benedict Equations

• Female: REE (kcal) = 655.5 + (9.53 x weight in kg) + (1.85 x height in cm) - (4.67 x age in years)
• Male: REE (kcal) = 66.5 + (13.5 x weight in kg) + (5 x height in cm) - (6.77 x age in years)

Brody-Klieber Equation

• Women and Men: REE (kcal) = 70 x (weight in kg)^3/4

Quick Estimation (Fasting)

• Women: 0.95 kcal/kg/h
• Males: 1 kcal/kg/h

Specific Energy Components

Thermic Effect of Food (TID)

TID is the increase in energy expenditure above basal levels in response to food intake. It's challenging to measure reproducibly due to individual factors (age, genetics, insulin sensitivity), food characteristics (quantity, composition, palatability), and methodological considerations (time of day, duration, environment). Indirect calorimetry is used for its determination.

Energy Expenditure of Physical Activity

Calorimetry can determine the energy cost of physical activity, although it's difficult to measure free and voluntary movement. The doubly labeled water method allows for studies in normal living conditions but doesn't identify isolated activity components.

Estimating Physical Activity: METs

Metabolic Equivalent Task (MET): A resting person's oxygen consumption is estimated at 3.5 ml/min/kg, defined as one MET. Various activities are measured in multiples of METs.

Introduction to Balanced Nutrition

A balanced diet allows individuals (adults, growing individuals, or those in special situations) to maintain optimal health and perform daily activities. All cultural dietary patterns are beneficial if they provide the necessary nutrients throughout the life cycle.