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  • Title: Sodium intake does not influence bioimpedance-derived extracellular volume loss in head-down rest.
    Author: Hinghofer-Szalkay HG, László Z, Passath H, Pilz K, Rössler A, Jezova D, Scharfetter H.
    Journal: Aviat Space Environ Med; 2004 Dec; 75(12):1036-41. PubMed ID: 15619857.
    Abstract:
    INTRODUCTION: There is disagreement regarding the impact of dietary sodium on alterations in extracellular volume during head-down bed rest (HDBR). The primary purpose of this study was to assess the effects of salt intake on extracellular volume (ECV) during HDBR. METHODS: We performed whole-body bioimpedance spectroscopy with controlled sodium intake during 4 d of ambulation and 8 d of -6 degrees HDBR in 10 normotensive men. Each subject performed an initial 12-d familiarization run with moderate sodium (246 +/- 12 mmol x L(-1) x d(-1) excreted) during which no measurements were made. They then participated in treatment runs involving low sodium (LS: 143 +/- 10 mmol x L(-1) x d(-1) Na+ excreted) and high sodium (HS: 434 +/- 17 mmol x L(-1) x d(-1) Na+ excreted). The different treatments were separated by > or =1 mo and the order of LS and HS was balanced among the subjects. These treatments were based on controlled food and drink supplies as prepared by a dietitian. We monitored sodium output and measured aldosterone, plasma renin activity (PRA), and vasopressin. Bioimpedance was measured every second day in supine position using tetrapolar electrodes. RESULTS: Based on exponential data fitting, we calculated an ECV decrease of 0.79 +/- 0.32 L (-5.8%; p = 0.018) in LS, and 1.21 +/- 0.31 L (-4.0%; p = 0.002) in HS during HDBR. LS and HS were not different (p > 0.1); 4 d pre-HDBR sodium adjustment produced a fall in ECV in the LS group only (-3.7%, p = 0.023). Hormone levels were not changed by HDBR. Plasma aldosterone was lower in HS (69 +/- 7 pg x ml(-1)) than in LS (180 +/- 24 pg x ml(-1)). DISCUSSION: Our bioimpedance data confirm that low sodium intake decreases ECV in ambulatory conditions and indicate that 8 d of HDBR produce a loss of ECV of about 5% (p < 0.05). The loss did not seem to be influenced by sodium intake between approximately 3 and approximately 10 g x d(-1).
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