A self-correcting indirect calorimeter system for the measurement of energy balance in small animals.

Indirect calorimetry involves measurement of CO(2) produced and O(2) consumed by an organism. These measurements are then used to calculate energy output, metabolic rate (MR), and respiratory quotient (RQ), a relative assessment of carbohydrate and lipid oxidation. By far the most difficult aspect of indirect calorimetry is measurement of O(2). Moreover, the abundance of O(2) (20.95%) relative to CO(2) (0.03%) in ambient conditions dictates that measurement errors of O(2) have greater implications on calculations of MR and RQ. Because compressed air is not feasible for use with animals in long-term experiments, changes in ambient conditions are nearly unavoidable. A self-correcting indirect calorimetry system was designed and constructed utilizing differential O(2) and CO(2) analyzers and a blank cage to monitor ambient conditions periodically. The system was validated by changing ambient O(2) and CO(2) concentrations by infusing N(2) into the system during a test butane burn. MR and RQ were largely unaffected by these changes in ambient conditions, and inclusion of a blank cage in the system accounted for slight calibration offsets. MR and RQ were measured in mice (n = 95) with and without correction for any small changes in ambient conditions measured in the blank cage. Coefficients of variation for MR and RQ were significantly decreased by taking into account ambient conditions measured in the blank cage (P < 0.001), which resulted in a 2.3% increase in precision for measurement of MR. This system will be used to more accurately assess long-term measurements of energy balance in the many murine models of leanness and obesity to gain better insights into pathophysiology and treatment of human obesity.