Maintaining the right sodium levels in the body is crucial for controlling blood pressure and ensuring proper muscle function. Conventional wisdom has suggested that constant sodium levels are achieved through the balance of sodium intake and urinary excretion, but a new study in humans published by Cell Press on January 9th in the journal Cell Metabolism reveals that sodium levels actually fluctuate rhythmically over the course of weeks, independent of salt intake. This one-of-a-kind study, which examined cosmonauts participating in space-flight simulation studies, challenges widely accepted assumptions that sodium levels are maintained within very narrow limits.
"The study highlights the importance of measuring salt excretion in urine over a longer time period to accurately estimate salt intake," says senior study author Jens Titze of Vanderbilt University School of Medicine. "This information is very important, given the emphasis on salt intake in terms of risk for cardiovascular disease and healthcare outcomes."
Past studies in humans have shown that when dietary salt intake increases, a steady state of sodium levels in the body is achieved through rapid urinary excretion. This process is under the control of a hormone called aldosterone, which causes sodium to be retained in the kidneys. However, most of these studies were short-term and did not examine fluctuations in sodium levels in response to constant salt intake.
To address these limitations, Titze and his team took advantage of a unique opportunity to control salt intake and study salt balance over the course of nearly seven months in 12 male participants in the Mars105 and Mars520 studies. These men spent 105 and 520 days, respectively, in an enclosed habitat consisting of hermetically sealed interconnecting modules at a spaceship simulation facility in Moscow, where they lived and worked as if they were cosmonauts at the international space station. As expected, their aldosterone levels increased when they consumed less salt. But surprisingly, a decrease in salt intake led to a reduction in levels of the "stress hormone" cortisol.
When the researchers kept salt intake constant, sodium excretion and the levels of aldosterone and cortisol fluctuated together in weekly cycles. On the other hand, sodium levels in the body exhibited longer-term rhythmic changes that were independent of salt intake. Over this longer timescale, elevated sodium levels were associated with high aldosterone levels and low cortisol levels, suggesting that the two hormones work in opposite ways to control sodium storage and release.
"To the best of our knowledge, the long-term rhythm we observed in sodium levels in the body has not been previously reported, and it was not known that cortisol and aldosterone work in opposite directions to regulate sodium metabolism," Titze says.
Rakova et al.: "Long-term space-flight simulation reveals infradian rhythmicity in human Na+ balance."