The Relationship Between Training Status and Circadian Rhythm of Water Turnover as Measured by the Deuterium Oxide Dilution Method in Athletes

I performed a fundamental study to establish a method for measuring short-term or short day energy expenditure using the double-labeled water (DLW) method, which is the gold standard for this type of measurement. Because the DLW method measures the total energy expenditure based on a one-day average during a one-week or two-week period, it is not capable of measuring changes in daily energy expenditure or the circadian rhythm of energy expenditure. However, I considered that this approach would be insufficient with only average energy expenditure for working with specific duty regimes or athletes.

Therefore, using the deuterium oxide (D₂O) dilution method to measure water turnover (WT), the principle of which is similar to the double-labeled water method, I examined WT within a short period (12 hours) or during one day.  The subjects were two members of a university swimming team, aged 22-23_yr, in whom WT was measured for seven consecutive days.

The results revealed that daily water turnover in the subjects varied considerably, irrespective of whether they were training or not. For example, the water turnover in female subject B was high on the days with training, but low value on days without training. This indicated that the athlete’s metabolism is changed considerably day by day, and also that daily total energy expenditure is also changed considerably too.

Moreover, in addition to changes in WT from day to day, the WT of female subject B in the daytime (9:00-21:00) on days that included training was higher than at night (21:00-9:00) on days that did not include training. Male subject A showed a change in WT that was completely contrary to that in female subject B on both a daily and short-term basis.

In addition, since the time for the stable isotope (²H) administered orally to reach equilibrium in subject A was clearly late longer in comparison with data obtained previously, the possibility that metabolism was delayed in subject A can be considered.

Therefore, the D₂O method seems applicable for measuring daily or short-term changes in metabolism, and the DLW method seems applicable for determination of short-term or daily energy expenditure.