Water metabolism may be non-invasively investigated using deuterium elimination from the body. Evaluation of elimination constants (kD) and pool sizes (N) becomes complicated when turnover rates vary periodically - a realistic scenario in free-living subjects. Unfortunately, phase and frequency of periodic variations are not a priori known and can therefore not be used for a sampling protocol. This study investigates the impact of periodical flux variations on the measurement of water turnover. For two models of periodic variation with identical apparent kD, data sets of tracer concentrations in body water were generated for two-hour intervals using a monoexponential decay equation. Data were analyzed using the two- and the multi-point method increasing observation periods stepwise. Apparent values for kD, N and water turnover (RH₂O) were compared with values originally used in the model. Periodically varying turnover rates introduce considerable errors for kD and RH₂O when the two-point method is used. The multi-point method gives a more robust estimate of RH₂O already after short observation periods, however, due to slope-intercept correlations, still tends to overestimate RH₂O. Errors are more pronounced the more uneven flux rates are distributed. Stepwise analysis of tracer enrichments using the multi-point approach identifies periodical variation of flux rates.
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