Measurement of water turnover using deuterium dilution: Impact of periodically varying turnover rates on precision and accuracy

Water metabolism may be non-invasively investigated using deuterium elimina tion from the body. Evaluation of elimination constants (k(D)) and pool siz es (N) becomes complicated when turnover rates vary periodically - a realis tic scenario in free-living subjects. Unfortunately, phase and frequency of periodic variations are not a priori known and can therefore not be used f or a sampling protocol. This study investigates the impact of periodical fl ux variations on the measurement of water turnover. For two models of perio dic variation with identical apparent k(D), data sets of tracer concentrati ons in body water were generated for two-hour intervals using a monoexponen tial decay equation. Data were analyzed using the two- and the multi-point method increasing observation periods stepwise. Apparent values for k(D), N and water turnover (R-H2O) were compared with values originally used in th e model. Periodically varying turnover rates introduce considerable errors for k(D) and R-H2O when the two-point method is used. The multi-point metho d gives a more robust estimate of R-H2O already after short observation per iods, however, due to slope-intercept correlations, still tends to overesti mate R-H2O. Errors are more pronounced the more uneven flux rates are distr ibuted. Stepwise analysis of tracer enrichments using the multi-point appro ach identifies periodical variation of flux rates.