The energy expenditure is evaluated through indirect calorimetry by measuring oxygen consumption with an open flow respirometric system.
Very precise CO2 and O2 sensors measure the difference in CO2 and O2 concentrations in air volumes flowing through control or animal cages. The amount of oxygen consumed over a given period of time can thus be calculated, as far as the air flow through the cage is known. Data are expressed as ml O2 h-1animal-1. The system also monitors CO2 production, therefore, the respiratory exchange ratio (RER) and finally, heat production can be calculated. An activity and food intake monitoring system can also be integrated to the set up for the measurement of activity e.g. to ensure that periods of resting can be identified.