Precise Measurement of Metabolic Rates

The airtight metabolic chamber measured 2.00 × 3.45 × 2.10 m (FHC-15S, Fuji Medical Science Co., Ltd., Chiba, Japan), and air in the chamber was pumped out at a rate of 80 L/min. The temperature and relative humidity of the incoming fresh air were controlled at 25 °C and 55%, respectively. The chamber was furnished with an adjustable hospital bed, desk, chair, and toilet. Concentrations of oxygen (O₂) and carbon dioxide (CO₂) in the outgoing air were measured with high precision by online process mass spectrometry (VG Prima δB; Thermo Electron Co., Winsford, UK). The precision of the mass spectrometry, defined as the standard deviation for continuous measurement of the calibrated gas mixture (O₂, 15%; CO₂, 5%), was 0.0016% for O₂ and 0.0011% for CO₂. Every minute, O₂ consumption ( $\dot{V}{\text{O}}_2$ ) and CO₂ production ( $\dot{V}{\text{CO}}_2$ ) rates were calculated using an algorithm for improved transient response^{40 (link)}. Energy expenditure was calculated from $\dot{V}{\text{O}}_2$ , $\dot{V}{\text{CO}}_2$ , and urinary nitrogen excretion (N), as described previously^{22 (link),39 (link),41 (link)}.