Validation of the doubly labeled water method using integrated cavity output spectroscopy against isotope ratio mass spectrometry and whole‐room indirect calorimetry

Abstract

BackgroundThe doubly‐labeled water (DLW) method is the gold standard methodology for determination of free‐living energy expenditure. Isotope enrichments have traditionally been determined by isotope ratio mass spectrometry (IRMS), but new technologies are emerging, such as off‐axis integrated cavity output spectroscopy (OA‐ICOS). We have previously shown that compared to IRMS, OA‐ICOS provides comparable isotope enrichments of standard waters and of human urine samples, and to carbon dioxide (CO2) production measured using whole‐room indirect calorimetry (IC). In this study, we compared CO2 production in humans measured using OA‐ICOS and IRMS to simultaneous whole‐room indirect calorimetry (IC) measurements over a period of 7 days.Methods12 subjects (5 male, 7 female; age=24 to 63 yrs.; body mass index=19.4 to 46.4 kg/m2) were studied for 7 consecutive days in the IC. Prior to entry on the first day, subjects consumed an oral dose of 0.25 g H218O (95% APE) and 0.14 g 2H2O (99.8% APE) per kg of total body water (estimated as 73% of fat free mass, measured using Dual X‐ray Absorptiometry). Urine samples were obtained daily, and the samples on day 1 and 8 were used to measure average CO2 production over the 7 days using both OA‐ICOS and IRMS.ResultsAverage estimated CO2 production by IC, TIWA, and IRMS was 413.6±69.4, 400.6±74.5, and 385.5±71.1 L/day (P=0.03), respectively (mean±SD). Post‐hoc comparisons indicated significant differences in CO2 production between IRMS and IC (95% confidence interval of difference = +3.7 to +52.5 L/min), but not between TIWA and IC (−11.4 to 37.5 L/min) or TIWA and IRMS (−9.3 to 39.5 L/min). CO2 production measured with the different instruments were significantly correlated: TIWA vs. IC (r=0.89), IRMS vs. IC (r=0.93), IRMS vs. TIWA (r=0.86).ConclusionsOA‐ICOS produces valid measurements of CO2 production compared to IC, and comparable results to IRMS. OA‐ICOS provides a valid and viable alternative to IRMS for DLW studies in humans.Support or Funding InformationThis work was supported by NIH SBIR 1R43DK093362‐01, The Colorado Nutrition and Obesity Research Center (P30 DK048520) and Colorado Clinical and Translational Science Institute (UL1 TR000154).