In urological practice it’s often noticed that patients show a different response on (anti)diuretic medication, have a different diuresis rate, develop edema more easily, etc… During this study, the potential of 2 techniques to investigate the human water shift and the role of the third space, was evaluated. The use of bio-impedance to estimate the body water content of different body parts and deuterium as a biological marker can probably improve knowledge about the water shift, diuresis, polyuria and edema formation and the role of the interstitium.
The aim of this pilot study was threefold: Evaluate the relevance of using deuterium and bio-impedance as added value to the third space research; examine water handling in the interstitium of healthy volunteers; examine the impact of body position on the water shift between body parts.

This prospective interventional pilot study was performed between July 2018 and October 2018. All participants were aged between 20 and 30 years and had a Body Mass Index (BMI) between 20 kg/m2 and 24 kg/m2. Volunteers with chronic diseases, implanted devices (pacemaker, IPG), on medication (except for anti-conception) and pregnant women were excluded. 
All tests started in the morning and volunteers had to be fasted. In total 17 volunteers were tested. Before starting the test protocol, a baseline urine-, blood- and saliva sample was collected. In the first part of the protocol the participants stood up for one BIA-measurement, thereafter they went to lay down for the next 70 minutes, followed by 5 minutes in a seated position and ending with 25 minutes in the standing position. 15 minutes after the start of the test, participants drunk 330 ml of deuterium (D) enriched water of which the deuterium-oxide volume was calculated as 0,25% of the individual’s total body water (TBW) content. Bio-electrical impedance (BIA) (Inbody S10, Inbody Co, Seoul) measurement was done every 5 minutes. Saliva samples and serum samples were collected every 10 minutes, starting after the intake of the deuterium enriched water. At the end of the test a second urine sample was collected.
After a cryogenic distillation of the samples, the D/H fraction was analyzed using cavity ring down spectrometry (WS-CRDS, L2120-I, Picarro, USA). Statistical analysis was performed using SPSS v.25 for Windows and SAS 9.4. The median, interquartile range (IQR) and frequency were recorded as descriptive parameters. A Friedman test and Wilcoxon signed-rank test was used to evaluate the differences in impedance between the standing and supine position. A Friedman test was used to evaluate the differences in impedance over time and between man and women. A two-sided p-value <0.05 was considered statistically significant. This study was approved by the Ghent University Hospital review board (EC 2017/1636) and all volunteers signed an informed consent.

The bio-impedance (at 50 kHz) in the trunk was significantly different between the standing and the supine position, for both man and women (Friedman test; p <0.001). The BIA significantly increased between minute 0 and 5, median and IQR respectively 24.3 Ω (22.2 - 26.0) and 25.3 Ω (24.1- 28.2) (Wilcoxon signed-rank test; p <0.001). BIA significantly decreased between minute 70 and 80, median and IQR respectively 25.5 Ω (24.1 - 28.65) and 23.4 Ω (22.05 - 25.45) (Fig 1).

The D/H fraction in the blood changed significantly over time. This happened different in women and man (Friedman test; p< 0.001). In man the maximum blood deuterium fraction was reached after ±40 min. while in women it increased almost linearly to a maximum after ±80 min. (Fig 2).

Significant differences in impedance between the standing and supine position were found. This suggests that the water content in the trunk increases within 5 minutes when people are going to lie down, and stays quite constant while the person stays in the supine position. When a person stands up again, the body water in the trunk immediately decreased within 5 minutes to a new equilibrium. This pattern was the same for women and man. This result can help to explain which mechanism is causing a decrease in edema when people go to lie down with their legs up. It also suggests a direct link between gravity and the body water distribution in the different body parts. 
The Deuterium concentration in the water pool of the blood increased significantly after drinking the deuterium enriched water. This happened different in women and man. In man the maximum blood deuterium concentration was reached faster than in women. These difference in the blood D/H fraction between genders can have different possible explanations. One is that there is link to the fact that the vasopressine-V2-receptor-gen (V2R-gen) is on the X chromosome and that women therefore respond different on the water uptake. Another possibility is that the women in this study had ingested lower deuterium oxide volumes compared to the man, as these volumes were calculated based on their length and height in order to get a D2O-fraction of 0.25% of their total body water. 
These first results of the data suggest the potential and usefulness of these two techniques in this research area. When the results of the analysis of the D/H fraction in the saliva and urine samples are available it will be interesting to study how it is changing over time compared to the fraction in the blood. It will then also be possible to better link the water shift in BIA with the deuterium shift in the different water pools.

Not all data were analyzed at this time but the first results were promising. There is a clear water shift in the human body when people change their body position. The results also suggest that there is a difference in water handling between man and women. Both BIA and Deuterium proved to be a useful technique in urological research