Limitations and requirements for measuring metabolic rates: a mini review

Metabolic measurement of humans and model animals is an important aspect of biomedicine. Particularly, in the case of model animals, the limitations of currently widely used metabolic measurement methods are not widely understood. In this mini-review, I explain the theoretical underpinnings of flow-through respirometry as a linear time-invariant system, and the (usually serious) distortions of metabolic data caused by the interaction of chamber volume and flow rate. These can be ameliorated by increasing the flow rate through the chamber, though this is at the expense of the magnitude of the O₂ depletion and CO₂ enhancement signals from which metabolic rates are calculated. If achieved, however, the improvement in temporal response that follows higher flow rates can be marked, and allows confident and accurate measurement of resting and active energy expenditure. Applications of this approach in multiplexing gas signals from multiple cages, and in human room calorimetry, are also discussed.