Physical activity assessment – Indirect calorimetry
Indirect calorimetry provides an estimation of energy expenditure. The experimental protocol used determines which components of energy expenditure are captured, which also depends upon the definition of these components. Basal Metabolic Rate (BMR) is the largest component of total energy expenditure (TEE), typically 60-75% when measured over 24 hours and the thermic effect of food is the smallest at 10%. The measurement of BMR requires strict (and generally impractical), conditions and protocol adherence. Therefore, the more loosely defined Sleeping Metabolic Rate (SMR) and Resting Metabolic Rate (RMR) are usually used as measures of an individuals’ lowest level of energy expenditure, acknowledging they are marginally different from BMR. The difference and the direction of the difference between RMR and BMR depend upon the population of interest (Wouters-Adriaens, 2006). The remaining component of TEE is energy expenditure due to physical activity (physical activity energy expenditure) and this component is the most variable between individuals but typically constitutes 15-30% of TEE when measured over 24 hours. However, in some extreme cases (e.g. elite cyclists during Tour De France), this could be as high as 70%. Indirect calorimeters use standard equations to calculate TEE (Weir 1949, Elia 1988, Elia 1992).
The laws of thermodynamics state that energy is neither created nor destroyed; rather it is transformed from one form (higher) to another (lower). Chemical energy from food (macronutrients, i.e. carbohydrates, proteins, fat and alcohol) is liberated in the presence of oxygen (O2) to produce mechanical work thus enabling the activities of daily living to be undertaken. All energy expended ends up as heat and the heat produced in this conversion is equal to the liberated chemical energy. Indirect calorimetry measures energy expenditure or the capacity of our bodies to do work, by assessing oxygen consumption; direct calorimetry in comparison, assesses heat production.
The biochemical principle behind the conversion of O2 consumption to heat equivalents i.e. kilojoules or kilocalories is reliant on knowing the composition of foods being metabolised. The macronutrients are each metabolised in the presence of oxygen to produce CO2, H20 and heat; usually protein does not contribute significantly to energy production except in periods of prolonged negative energy balance, e.g., starvation. The ratio of C02 produced to O2 consumed (i.e. VCO2/VO2) is known as the respiratory exchange ratio and will determine the kJ or kcal equivalent value for each litre of oxygen consumed. When 100% carbohydrate is metabolised the respiratory exchange ratio is 1.0, and when 100% fat is metabolised the respiratory exchange ratio is 0.7; a mixture of the two 50:50 is equivalent to a respiratory exchange ratio of 0.85.
Based on individual’s respiratory exchange i.e. oxygen consumed and carbon dioxide produced, energy expenditure is calculated by Weir’s equation (Weir 1949):
REE = [VO2 (3.94) + VCO2 (1.11)] 1440 min/day
The standard Weir equation therefore defines the relationship between VO2 and CO2 production and energy expenditure. Weir also showed that for a specific measurement technique, energy expenditure could be reasonably well approximated without CO2 measurement. However, during specific conditions, when the body covers a more significant proportion of the work from anaerobic (non-oxygen requiring) sources (e.g. maximal work for a few minutes, such as 400 to 800 meters running) of chemical energy, the proton concentration of the blood (from lactic acid) will shift the carbon dioxide – carbonic acid equilibrium towards an excess release of CO2, the measurement of CO2 will enhance the precision of the estimated energy expenditure.
There are three principal approaches to the measurement of energy expenditure using indirect calorimetry.
In closed collection systems expired air is collected in either an airtight rigid structure (i.e. a Tissot Gasometer) or a portable flexible bag (i.e. a Douglas bag). The Douglas bag method is an accurate method, but is prone to measurement error resulting from conditions such as the maintenance status of the equipment and the skills of the operator.
Open-circuit indirect calorimeter systems can be used to record energy expenditure over several hours or days depending upon the configuration selected and the experimental requirements. In an open-circuit system, the person under study breathes either normal room air or a gas mixture, following which expired gases are then analysed.
There are two types of open-circuit systems: 1) ventilated open-circuit systems where a subject breathes into a container through which air is sampled and analysed (e.g. transparent hood, canopy, or a chamber) and; 2) an expiratory collection system where the person under study inspires room air and expired air is collected via a non-return valve into a measurement unit. The expiratory collection open-circuit system can be designed as a portable device so that energy expenditure can be measured in free-living individuals for shorter periods of time (up to several hours). In general these devices comprise a mouth-piece or a mask connected to a one-way valve whereby expired air enters the instrument. The accuracy of portable devices are generally lower compared to stationary devices.
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