Practical aspects of indirect calorimetry in laboratory animals

Oxidation of the energetic substrates by the body is associated with oxygen consumption, carbon dioxide production, and heat release specific to the nature of the energetic substrates being oxidized. Therefore, measurement of respiratory exchanges (indirect calorimetry) is a powerful method to investigate heat production of a living organism. In this article, we review the elementary principles of indirect calorimetry and describe the operating principle of the two most typical devices used to perform indirect measurements of energy expenditure in the laboratory animal: the closed-circuit and the open-circuit. We then discuss some practical aspects of the day-to-day use of these devices: respective advantages and limitations of each technique, data processing, calibration, correction for body-size, and computation of the energy expended for activity. In the second part, we review some of the standard formulas of indirect calorimetry that offer the possibility to obtain more precise information such as the rate of oxidation of carbohydrates (CHO), lipids and proteins if some hypotheses are made on the intensity of lipogenic, ketogenic, and gluconeogenic processes. Finally, a practical example of the measurement of energetic cost of activity and thermic effect of food in the rat is given.