Implications of the variability in time to isotopic equilibrium in the deuterium dilution technique

To investigate the variability in isotopic equilibrium time under field conditions, and the impact of this variability on estimates of total body water (TBW) and body composition.

Following collection of a fasting baseline urine sample, 10 women and 10 men were dosed with deuterium oxide (0.05 g/kg body weight). Urine samples were collected every hour for 8 h. The samples were analysed using isotope ratio mass spectrometry. Time to equilibration was determined using three commonly employed data analysis approaches.

Isotopic equilibrium was reached by 50, 80 and 100% of participants at 4, 6 and 8 h, respectively. The mean group equilibration determined using the three different plateau determination methods were 4.8+/-1.5, 3.8+/-0.8 and 4.9+/-1.4 h. Isotopic enrichment, TBW, and percent body fat estimates differed between early (3-5 h), but not later sampling times (5-8 h).

Although the three different plateau determination approaches resulted in differences in equilibration time, all suggest that sampling at 6 h or later will decrease the likelihood of error in body composition estimates resultant from incomplete isotopic equilibration in a small proportion of individuals.