Indirect calorimetry is used to calculate the heat produced by living organisms by either measuring their oxygen consumption or calculating the production of CO2 and wasted nitrogen.
Indirect calorimetry helps estimate the rate and type of energy metabolism and substrate consumption in the body beginning with measurements of gas exchange, i.e., consumption of oxygen and CO2 during exercise and rest.
This method provides noninvasive and unique information that can be integrated with various experimental methods to investigate several aspects of thermogenesis, nutrient assimilation, and metabolic diseases.
It’s used in taking care of patients with chronic and acute diseases. Indirect calorimetry helps in assessing nutrition support for such patients. Using indirect calorimetry in assessing accurate resting energy expenditure (REE) and resting metabolic rate (RMR) give a better knowledge of patients’ calorie requirements and their further treatment during a particular illness.
Indirect calorimetry assumptions
Indirect calorimetry methods assume the following when measuring energy expenditure:
- Any fuel consumption by the body has an energy content that is intrinsic on which metabolic modifications in a living organism are dependent and lead to energy production or heat.
- The synthesis or combustion of protein, fat, or carbohydrates results from every biochemical reaction that occurs in a body.
- The oxidation of protein, glucose, or fat leads to a fixed ratio between the level of carbon dioxide produced and oxygen.
- Substrates lost in urine and feces are negligible.
Gas measurement and analysis of flow rate and volume
Flowrate and gas measurements are crucial components for precise indirect calorimetry measurements. Some of the most widely used analyzers include paramagnetic oxygen analyzers, infra-red carbon dioxide analyzers, and galvanic oxygen sensors. Flow rate measurement can be calculated either by integrating the rate of flow’s continuous measurement in four different forms (hot-wire anemometers, pitot tubes, turbines, or pressure-differential) or by measuring expired gas volume during a certain time.
One of the most important requirements of calorimetry is the precise calibration of analyzers. The energy production must always be calculated with utmost precision and accuracy if a full-energy balance of a body is required or metabolic response to a certain treatment is needed.
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