7-DAY VALIDATION OF DOUBLY LABELED WATER METHOD USING INDIRECT ROOM CALORIMETRY

Carbon dioxide production, water production, and energy expenditure (EE) determined with (H2O)-H-2-O-18 were validated using a room calorimeter. Nine human subjects (5 males, 4 females) resided in the calorimeter for five to seven consecutive 23.5-h days. Daily carbon dioxide and water production and EE were averaged from calorimeter data and compared with (H2O)-H-2-O-18 results for the same period and for the following week. Isotope concentrations were measured by isotope ratio mass spectroscopy (O-18) and infrared spectrophotometry (H-2). Isotope clearance rates and pool sizes were determined by two multipoint methods and the two-point method. No significant differences were measured between calorimetry and (H2O)-H-2-O-18 results for the period within the chamber [multipoint A.M. urine 1.55% +/- 2.57 (SD) %, multipoint A.M. + P.m. urine 0.98% +/- 8.19%, two-point 1.59% +/- 4.50%]. Free-living EE was 13% +/- 7% greater. The results indicate that the (H2O)-H-2-O-18 method as applied is a valid technique for measuring EE, that multipoint calculation methods can be as precise as the two-point method, and that EE was greater in free-living than in calorimeter-confined conditions.