Increased salt consumption induces body water conservation and decreases fluid intake
2017
BACKGROUND. The idea that increasing salt intake increases drinking and urine volume is widely accepted. We tested
the hypothesis that an increase in salt intake of 6 g/d would change fluid balance in men living under ultra-long-term
controlled conditions.
METHODS. Over the course of 2 separate space flight simulation studies of 105 and 205 days’ duration, we exposed 10 healthy
men to 3 salt intake levels (12, 9, or 6 g/d). All other nutrients were maintained constant. We studied the effect of salt-driven
changes in mineralocorticoid and glucocorticoid urinary excretion on day-to-day osmolyte and water balance.
RESULTS. A 6-g/d increase in salt intake increased urine osmolyte excretion, but reduced free-water clearance, indicating
endogenous free water accrual by urine concentration. The resulting endogenous water surplus reduced fluid intake at
the 12-g/d salt intake level. Across all 3 levels of salt intake, half-weekly and weekly rhythmical mineralocorticoid release
promoted free water reabsorption via the renal concentration mechanism. Mineralocorticoid-coupled increases in free water
reabsorption were counterbalanced by rhythmical glucocorticoid release, with excretion of endogenous osmolyte and water
surplus by relative urine dilution. A 6-g/d increase in salt intake decreased the level of rhythmical mineralocorticoid release
and elevated rhythmical glucocorticoid release. The projected effect of salt-driven hormone rhythm modulation corresponded
well with the measured decrease in water intake and an increase in urine volume with surplus osmolyte excretion.
CONCLUSION. Humans regulate osmolyte and water balance by rhythmical mineralocorticoid and glucocorticoid release,
endogenous accrual of surplus body water, and precise surplus excretion.
FUNDING. Federal Ministry for Economics and Technology/DLR; the Interdisciplinary Centre for Clinical Research; the NIH; the
American Heart Association (AHA); the Renal Research Institute; and the TOYOBO Biotechnology Foundation. Food products
were donated by APETITO, Coppenrath und Wiese, ENERVIT, HIPP, Katadyn, Kellogg, Molda, and Unilever.
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