Method and rationale for recalculating dilution spaces to a single, common time point in doubly labeled water studies

The doubly labeled water (DLW) method has become widely used in studies of energy expenditure and body composition. Researchers differ in the analytical methods used to calculate the dilution spaces for deuterium and oxygen-18. Some determine dilution spaces using isotope enrichments extrapolated to the instant of dosing with DLW (slope-intercept method), but others use measured enrichments from body water samples obtained 3-10 h after dosing (plateau method). These differences limit the comparability of analyses across labs.

I derive a simple mathematical approach for recalculating reported dilution spaces to any time point post dosing, using reported dilution spaces and rates of isotope depletion. Simulated data are used to examine the effects of different dilution space protocols.

Recalculating dilution spaces enables researchers to determine outcome variables of interest (e.g., total body water, energy expenditure, and water throughput) from different labs under a unified protocol for determining dilution spaces, and improves comparisons among studies.

Differences between dilution space protocols can lead to substantial differences in outcome variables of interest in DLW studies. When comparing results of DLW studies that employ different dilution space protocols, dilution spaces should be recalculated for a common time point, and outcome variables recalculated as needed, prior to comparison across studies.