How long should washout be to reliably measure hepatic de novo lipogenesis using deuterated water? Exploratory analysis from a randomized crossover trial: Exploratory analysis from a randomised crossover trial.

Deuterated water ( ²H ₂O) can be used to measure de novo lipogenesis (DNL) with minimal participant burden, but implications of tracer washout on repeated measures in humans are unclear. This study is an exploratory analysis of data from a crossover trial to determine the impact of duration between repeated ²H ₂O dosing for sequential assessments of DNL, alongside day-to-day variability in DNL. A total of 22 nonobese adults (11 men and 11 women) completed three laboratory visits in a randomized, crossover design (35±13-day washout). Participants consumed 3g·kg ^-1 body water of ²H ₂O the evening before laboratory visits. Blood was sampled before ²H ₂O ingestion, and the following morning in a fasted state. Deuterium ( ²H) enrichment of plasma water and very-low-density lipoprotein-triacylglycerol (VLDL-TG)-palmitate were used to determine DNL. Although predosing plasma water ²H enrichments (mole percent) increased across visits from 0.017 ± 0.003% to 0.022 ± 0.008% and 0.027 ± 0.014%, respectively (P < 0.05), postdose enrichments did not display a systematic bias, and nor did ²H enrichments of VLDL-TG-palmitate or measures of DNL [largest mean difference = -1.2%; 95% confidence interval (95% CI) = -12.7% to 9.1%, P = 0.45]. The day-to-day standard deviation and coefficient of variation of fractional hepatic DNL was 2.39% (95% CI = 1.35%-3.42%) and 27% (95% CI = 19%-35%), respectively. Repeated ²H ₂O dosing does not systematically bias measures of fasting hepatic DNL when using a washout duration of ~4 wk. We also found no evidence that DNL is biased by washout durations of 3 wk. Therefore, ²H ₂O can be used to reliably assess human hepatic DNL in repeated measures designs with at least 3 wk between sequential measures. NEW & NOTEWORTHY Repeated deuterated water dosing in humans does not systematically bias measures of fasting hepatic DNL when using a washout duration of ~4 wk. Fasting fractional hepatic DNL can be reliably assessed using deuterated water over sequential visits with as little as 3 wk washout between measures. At the group level, the within-participant variability in fasting hepatic DNL was relatively small and permits efficient detection of minimal clinically important differences in hepatic DNL with crossover studies.