Original Investigations: Pathogenesis and Treatment of Kidney Disease and Hypertension
Long-term sodium balance in humans in a terrestrial space station simulation study*,**,,★★

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Abstract

Background: Sodium accumulation has been considered to take place in the extracellular space, leading to water retention and weight gain. This traditional view has been questioned by recent studies that showed sodium accumulation in humans without expansion of the extracellular volume. We investigated sodium balance and its impact on body weight (BW) during a long-term balance study. Methods: Three healthy subjects were confined to a terrestrial MIR simulator for 135 days under conditions simulating a long-term spaceflight. During the entire isolation period, we meticulously measured daily sodium balance and its contribution to BW. Results: During the study period, subjects accumulated between 2,973 and 7,324 mmol of sodium and gained between 5.1 and 9.3 kg in weight. In all subjects, there was a positive correlation between changes in total-body sodium (ΔTBS) content and BW, reflecting sodium-associated volume expansion. However, toward the end of isolation, sodium gain exceeded weight gain, suggesting that sodium accumulated in an osmotically inactive form. Especially at the onset of the experiment, two subjects showed inverse correlations between ΔTBS and BW. Conclusion: The finding of sodium gain without weight gain is in contradiction to the widely accepted theory that changes in TBS levels are accompanied by changes in extracellular volume. We suggest the existence of a sodium reservoir with the ability to store significant amounts of sodium in an osmotically inactive form. This reservoir might be located in bone, dense connective tissue, or cartilage. © 2002 by the National Kidney Foundation, Inc.

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Subjects and environmental conditions

During HUBES, three male subjects were isolated over a period of 135 days. Two subjects (no. 002 and 003) were trained cosmonauts, and successful participation in the isolation study would qualify them for a space flight. According to body mass index, subject 004 was marginally underweight, whereas subjects 002 and 003 were of normal body mass index (Table 1).

The isolation study was conducted at the Institute for Biomedical Problems in Moscow and approved by several ethical boards of the

BW and body composition

Figure 1 shows changes in BW over the entire isolation period.

. Changes (Δ) in BW during 135 days of confinement.

Descriptives are listed in Table 1. BW increased substantially in all subjects, at least initially, but eventually tended to remain stable in all subjects. Figure 2 shows changes in body composition.

. Body composition during confinement estimated using the BIA method. Changes in body fat content coincide mostly with changes in BW (Fig 1). *P < 0.05. **P < 0.01. ***P <

Discussion

We determined long-term sodium balances to investigate whether infradian oscillations in TBS content exist in humans. In addition to such hypothetical oscillations, subjects showed an unexpected excessive sodium accumulation. TBS content in humans is estimated at 55 to 60 mmol/kg BW.14, 15 If this held true, sodium accumulation in subject 004 almost tripled his TBS content from presumed normal values to far beyond the norm. Excessive positive sodium balances toward the end of isolation give

Acknowledgements

The authors thank the three confined crew members for their cooperation during confinement, training, and control periods; R. Ziegler, for curve fitting; J. Nurrish-Weiß, for help with the text; and F.C. Luft, for advice and comments on the manuscript.

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    *

    This experiment was performed in the MIR simulator of the Institute for Biomedical Problems, Moscow, Russia.

    **

    Supported in part by the European Space Agency-Long Term Program Office; and Else Kröner-Fresenius-Stiftung (J.T.). The Institute of Physiology, Free University of Berlin, receives financial support from DLR Germany, grant no. 50WB9622; and the Laboratory of Environmental Physiology (Lyon, France) receives financial support from CNES and GIP Exercise for its space-related research.

    Address reprint requests to Jens Titze, MD, Department of Nephrology, Friedrich-Alexander-University Erlangen-Nürnberg, Loschgestraße 8, 91054 Erlangen, Germany. E-mail: [email protected]

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