Should we abandon indirect calorimetry as a tool to diagnose energy expenditure? Not yet. Perhaps not ever. Commentary on Burnett and Grobe (2014)

Inference in science depends on us having the right tools to measure with sufficient accuracy and precision the phenomena we are attempting to understand. In this issue of Molecular Metabolism, Burnett and Grobe call into question the accuracy of respirometry, the standard method for measuring energy expenditure in animals and humans [1]. This is an important contribution because in the field of obesity, almost without exception, scientists are agreed that the problem is due to energy imbalance. Energy intake is too high, expenditure is too low, or both. This energy balance framework provides a useful starting point for any discussions about fat storage and obesity [2]. However, the level of energy imbalance that can drive fat storage is rather small. An example is given in Ref. [3] of a typical 45 year old man who might accumulate 0.5 kg of fat over the course of a year, containing around 13.8 MJ of energy, which is only 0.27% of the estimated 5180 MJ of energy expenditure over the same period. Clearly the tools necessary to detect this sort of level of variation in energy expenditure would need to be spectacularly good, and it is widely known that none of the available methods comes close to providing us with this level of accuracy or precision. However, other questions in the field may require less stringent methods. Take for example the characterisation of a mutant mouse, or the impact of a given dietary exposure, or a drug. Here we might expect the impacts on expenditure to be larger, and hence the methodological requirements less taxing. What Burnett and Grobe show is that the techniques we currently have available may not be up to even this less demanding task.