In our lab we measure energy expenditure and endogenous substrate source utilization by indirect calorimetry. The term ‘indirect’ refers to the fact that energy production is determined by measuring O2 consumption and CO2 production rather than directly measuring heat transfer (direct calorimetry). We make use of 3 different systems:
To measure subjects’ energy expenditure over a longer period (24-72 h), the respiration chamber is used. The respiration chamber is a 14 m3 room, furnished with a bed, chair, television, radio, telephone, computer, washbowl and toilet. Air locks provide passage for exchange of food and/or blood sampling. Energy expenditure can be determined from the subjects’ O2 consumption, CO2 production and urine nitrogen excretion. The respiration chamber is ventilated with fresh air at a rate of 70-80 L/min. Ventilation rate is measured by a dry gas meter. A paramagnetic O2 analyzer and an infrared CO2 analyzer are used to analyze the in- and outgoing air. 24 h urine can be collected to measure nitrogen balance. Room temperature and humidity can be controlled. And physical activity can be monitored by means of a radar system, based on the Doppler principle. Respiration chamber can also be used as climate chambers to study acclimatization and/or performance in the heat or cold.
Open circuit indirect calorimetry: ventilated hood system
To measure subjects’ resting energy expenditure and substrate use in a more acute setting, the ventilated hood system is used (Omnical and Oxycon Pro).
A clear plastic hood, with a volume of 30 L, is placed over the subject’s head and neck. Indoor air is used to ventilate the hood with a flow of 25-50 L/min. The flow will be adjusted if the CO2 concentration reaches a certain threshold. By measuring the O2 and CO2 concentrations of in- and outgoing air, VO2 and VCO2 are being measured. The latter are used to calculate energy expenditure and total fat and carbohydrate oxidation rates. This setting is generally applied to study the impact of nutritional and/or exercise interventions on resting energy expenditure and whole-body fat oxidation rate.
Open circuit indirect calorimetry: exercise tests
To measure subjects’ energy expenditure and substrate use in a more acute exercise setting we use the same systems (Omnical and Oxycon Pro) in an exercise mode. Subjects breathe trough a mask that is placed over their nose and mouth. Inspired and expired breath are sampled by means of a 3-way respiratory valve or non-rebreathing mask. By measuring ventilation and O2 and CO2 concentration in the in- and outgoing breath, energy expenditure and whole-body fat and carbohydrate oxidation rates can be measured.
Indirect calorimetry forms the basis of many human in vivo metabolic studies. However, in this setting indirect calorimetry only provides information on whole-body energy expenditure and total fat and carbohydrate use. To obtain more information on endogenous and/or exogenous substrate source utilization other methodologies are warranted. As such, we often combine the use of indirect calorimetry with the application of contemporary stable isotope methodology.