For the purpose of most energy-expenditure studies, indirect calorimetry consists of measuring the volume of expired air per unit of time and determining the percentage of oxygen utilized. This data are then used for estimating the metabolic rate (energy expenditure) and respiratory exchange ratio (relative measurement of fat, carbohydrate and protein oxidation). Indirect calorimetry studies are generally accomplished by utilizing sophisticated systems whereby the amount of oxygen consumed and carbon dioxide produced by an animal is precisely measured over a period of time. Theses systems consists of cages (chambers), air pumps, air flow controllers, valves, and gas analyzers and are controlled by potent software for the acquisition, storage and analysis of the data. Indirect calorimetry can be evaluated together with food & drink intake and activity in metabolism studies.