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Sodium Intake Does Not Influence Bioimpedance-Derived Extracellular Volume Loss in Head-Down Rest

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Hinghofer-Szalkay HG, Lásló Z, Passath H, Pilz K, Rössler A, Jezova D, Scharfetter H. Sodium intake does not influence bioimpedance-derived extracellular volume loss in head-down rest. Aviat Space Environ Med 2004; 75:1036–1041.

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° HDBR in 10 normotensive men. Each subject performed an initial 12-d familiarization run with moderate sodium (246 ± 12 mmol · L−1 · d−1 excreted) during which no measurements were made. They then participated in treatment runs involving low sodium (LS: 143 ± 10 mmol · L−1 · d−1 Na+ excreted) and high sodium (HS: 434 ± 17 mmol · L−1 · d−1 Na+ excreted). The different treatments were separated by ≥ 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 · ml−1) than in LS (180 ± 24 pg · 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 ≈ 3 and ≈ 10 g · d−1.
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Keywords: aldosterone; bioelectrical impedance spectroscopy; plasma renin activity; renal function; vasopressin

Document Type: Research Article

Publication date: 01 December 2004

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