Abstract
SummaryThe doubly labeled water (DLW) method measures total energy expenditure (TEE) in free-living subjects. Several equations are used to convert isotopic data into TEE. Using the International Atomic Energy Agency (IAEA) DLW database (5,756 measurements of adults and children), we show considerable variability is introduced by different equations. The estimated rCO2 is sensitive to the dilution space ratio (DSR) of the two isotopes. Based on performance in validation studies, we propose a new equation based on a new estimate of the mean DSR. The DSR is lower at low body masses (<10 kg). Using data for 1,021 babies and infants, we show that the DSR varies non-linearly with body mass between 0 and 10 kg. Using this relationship to predict DSR from weight provides an equation for rCO2 over this size range that agrees well with indirect calorimetry (average difference 0.64%; SD = 12.2%). We propose adoption of these equations in future studies.
Highlights
The doubly labeled water (DLW) method[1,2] is an isotope-based technique for measuring rCO2 in free-living animals and humans.[3]
Adolescents, and adults We have compiled in the International Atomic Energy Agency (IAEA) DLW database (v3.1; https://www.dlwdatabase.org) individual data from 119 DLW studies comprising a total of 6,246 measurements of individuals aged 2–96 years.[8]
To choose the best equation for the common calculation, we compiled data from six validation studies involving 61 adult humans, where rCO2 by DLW has been compared with simultaneous indirect calorimetry (Table 1).[12,13,14,15,16,17]
Summary
The doubly labeled water (DLW) method[1,2] is an isotope-based technique for measuring rCO2 in free-living animals and humans.[3] The method is based on the observation that the oxygen in respiratory CO2 is in complete isotopic equilibrium with the oxygen in body water. Isotopically labeled oxygen introduced into the body water is eliminated as both water and CO2. A simultaneously introduced label of hydrogen (such as deuterium) will be predominantly eliminated as water. The difference in elimination rates of the two isotopes ( ‘‘doubly labeled’’ water) gives a measure of rCO2.
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