With the development of several different commercial systems, measures of energy expenditure in mice are now much more common than 10–15 years ago, but results are sometimes misinterpreted2 –4 (link). Using direct calorimetry, energy expenditure is assessed by the direct measurement of the body’s heat production in a calorimeter21 (link)–23 (link). Despite high reproducibility and measurement errors of only 1–3%, these calorimeters are expensive, have slow response time22 (link) and do not provide information about the nature of the oxidized substrates. In indirect calorimetry, energy expenditure is calculated based on the amount of oxygen consumed and carbon dioxide produced (Supplementary Note 1). The most common indirect calorimeter types are ventilated, open-circuit systems, in which the animals are placed in gas-tight metabolic cages through which a flow of fresh air is passed. The system collects and mixes the expired air, measures the flow rate and analyzes the gas concentration of the incoming and outgoing air for both O2 and CO2 (ref. 22 (link)). Another indirect method of calorimetry is the doubly labeled water method, an isotope-elimination technique developed in the 1950s (refs. 24 (link)–26 ). This method has been traditionally used to measure the metabolic rate of small free-living animals, which are released in the field between two time points: it is often referred to as field metabolic rate27 (link). In the laboratory, the main advantage of the method is that it allows the measurement of energy demands of an animal embedded in a social environment28 (link). However, the time intervals between blood sampling are often too long to permit measurements of short-term or diurnal changes of the metabolic rate.