Accuracy and Practical Considerations for Doubly Labeled Water Analysis in Nutrition Studies Using a Laser-Based Isotope Instrument (Off-Axis Integrated Cavity Output Spectroscopy)

Background: Given the utility of the doubly labeled water (DLW) method for determination of energy expenditure, additional techniques for isotope analysis of the samples are welcome. Laser-based instruments are one such new analytical tool, but their accuracy and feasibility for DLW studies are grossly understudied. Objectives: We assessed the accuracy of laser-based isotope ratio measurements as part of the DLW method for estimation of carbon dioxide production rate (rCO(2)) and total energy expenditure (TEE), in between-group comparison study designs. Methods: Urine samples from a previous study were analyzed with a laser-based instrument [off-axis integrated cavity output spectroscopy (OA-ICOS)]. In that study, participants consumed a high-, moderate-, or low-carbohydrate diet for 20 wk; urine samples were obtained in weeks 18-20 before and after a H-2- and O-18-enriched water dose. Isotope ratios (delta H-2 and delta O-18), rCO(2), and TEE calculated by standard methods were compared to results previously obtained with the standard technique of isotope ratio mass spectrometry (IRMS). Bias, SD, and bias +/- 1.96SD bands between IRMS and OA-ICOS were computed. Results: The between OA-ICOS and IRMS rCO(2) and TEE trends were equivalent (within 1.2% and 4.1%, respectively), in spite of the differences in measured delta O-18 values at high enrichment levels. The OA-ICOS delta O-18 values displayed an increasing offset from the IRMS results as the O-18 enrichment increased (mean +/- SD 4.6-5.7 parts per thousand +/- 2 parts per thousand offset at the time point with highest O-18 enrichment, similar to 135 parts per thousand), whereas the hydrogen isotope ratio (delta H-2) differed only slightly between the methods (mean offset -4.9 parts per thousand for all time points). The between-diet differences in TEE from the previous study were recapitulated with a smaller subset of participants and time points. Conclusions: OA-ICOS analysis is an accurate and feasible technique for the DLW method. Given the delta O-18 offset observed at high enrichment, validation of each OA-ICOS instrumental setup against established methods (e.g., IRMS) is recommended.