Spacemen upset sodium balance theories

By Eleanor McDermid, Senior medwireNews Reporter

Levels of salt in the body and sodium in the urine fluctuate over fixed cycles that are independent of salt intake, shows a space simulation study.

"The study highlights the importance of measuring salt excretion in urine over a longer time period to accurately estimate salt intake," said lead researcher Jens Titze (Vanderbilt University School of Medicine, Nashville, Tennessee, USA) in a press statement. "This information is very important, given the emphasis on salt intake in terms of risk for cardiovascular disease and healthcare outcomes."

There were two space simulation studies, which involved six male cosmonauts spending 105 days and six spending 520 days in a sealed, controlled environment, living and working as if they were on the international space station. Thus, the conditions provided the opportunity for precise control of the men's sodium intake.

The studies "provide a unique and detailed profile of the characteristics of long-term sodium balance in humans," say David Ortiz-Melo and Thomas Coffman (Duke University and Durham VA Medical Centers, North Carolina, USA) in a commentary accompanying the report in Cell Metabolism.

They consider it "unlikely that a study with this scope, duration, and level of environmental control will ever be repeated."

The cosmonauts collected all of their daily urine output, from which Titze et al ascertained that 95% of dietary sodium was excreted via the urine. In line with expectations, increasing the cosmonauts' daily salt intake from 6 to 12 g/day resulted in increases in total-body sodium levels (measured with bioimpedance spectroscopy), extracellular water, a reduction in aldosterone excretion, and increased urinary excretion of sodium. There was also a delayed modest increase in blood pressure.

But unexpectedly, the team also saw short-term increases in urinary free cortisol and cortisone in response to increased salt intake.

The researchers say that had they terminated their study 1 week after increasing the cosmonauts' salt intake, they would have largely "confirmed preconceived notions." However, the opportunity for longer-term analysis produced very different results, showing that parameters linked to salt ingestion had rhythmic fluctuations with a peak about every 6 days, despite the fixed salt intake.

"We uncovered complex rhythms and suggest unappreciated functions for corticoid hormones," write Titze et al.

During the longer-term analysis, urinary sodium, aldosterone, and free cortisol and cortisone fluctuated over a 6-day cycle, with cortisol and cortisone also having a prominent monthly peak in levels. When urinary sodium excretion rose, it was accompanied by decreased aldosterone excretion, which "confirmed the antinatriuretic action of aldosterone." But free cortisol and cortisone excretion matched patterns of sodium excretion.

Total-body sodium levels showed a monthly pattern, independent of salt intake, adding to previous evidence that salt can be stored within the body. Changes in total-body sodium levels correlated positively with aldosterone excretion and inversely with free cortisol excretion.

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