Indirect Calorimetry

Indirect calorimetry calculates heat that living organisms produce by measuring either their production of carbon dioxide and nitrogen waste (frequently ammonia in aquatic organisms, or urea in terrestrial ones), or from their consumption of oxygen. Indirect calorimetry is the method by which the type and rate of substrate utilization, and energy metabolism are estimated in vivo starting from gas exchange measurements (carbon dioxide production and oxygen consumption during rest and steady-state exercise). This technique provides unique information, is noninvasive, and can be advantageously combined with other experimental methods to investigate numerous aspects of nutrient assimilation, thermogenesis, the energetics of physical exercise, and the pathogenesis of metabolic diseases.

Indirect calorimetry measures O₂ consumption and CO₂ production. On the assumption that all the oxygen is used to oxidize degradable fuels and all the CO, thereby evolved is recovered, it is possible to calculate the total amount of energy produced. It should be clear that “energy production” means conversion of the chemical free-energy of nutrients into the chemical energy of ATP plus loss of some energy during the oxidation process. Respiratory indirect calorimetry, or only indirect calorimetry (IC) as it is often known by most authors, is a noninvasive and highly accurate method of metabolic rate which has an error rate lower than 1%. It has high reproducibility and has been considered a gold standard method. This method allows estimating BEE and REE, and also allows for identification of energy substrates that are being predominantly metabolized by the body at a specific moment. It is based on the indirect measure of the heat produced by oxidation of macronutrients, which is estimated by monitoring oxygen consumption (O₂) and carbon dioxide production (CO₂) for a certain period of time. The calorimeter has a gas collector that adapts to the subject and a system that measures the volume and concentrations of O₂ and CO₂ minute by minute. Through a unidirectional valve, the calorimeter collects and quantifies the volume and concentration of O₂ inspired and CO₂ expired by the subject. After a volume is met, Resting Energy Expenditure is calculated by the Weir formula and results are displayed in software attached to the system.

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