Indirect calorimetry

Indirect calorimetry estimates the type and rate of substrate utilization and energy metabolism in vivo starting from gas exchange measurements (oxygen consumption and carbon dioxide production 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.

It is based on the indirect measurement of the heat produced by oxidation of macronutrients, which is estimated by monitoring O2 consumption and CO2 production for a certain period of time.

Antoine Lavoisier noted in 1780 that heat production, in some cases, can be predicted from oxygen consumption[citation needed], using multiple regression.

Studies of indirect calorimetry over the past 20 years have led to the characterization of the hypermetabolic stress response to injury and the design of nutritional regimens whose substrates are most efficiently assimilated in different disease processes and organ failure states.