Martian metabolism: The rise and fall of salt

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  • Published: Feb 1, 2013
  • Author: David Bradley
  • Channels: MRI Spectroscopy
thumbnail image: Martian metabolism: The rise and fall of salt

Martian MRI

Titze and colleagues also found that total body sodium levels fluctuated on monthly and longer cycles, with this longer-term storage process seemingly independent of salt intake and not linked to weight gain that would be associated with water retention.

An MRI study of members of the Russian "Mars500" mission by Jens Titze and colleagues has looked at how the volunteers' sodium levels change in cycles lasting a week or even a month. The study published in the journal Cell Metabolism, suggests that the conventional wisdom regarding excretion of sodium keeping levels constant in the body ought to be revised.

Clinical pharmacologist Jens Titze and colleagues have exploited a unique opportunity to study human metabolism by monitoring the Russian cosmonauts who took part in a simulated flight to Mars. Titze, now at Vanderbilt University was convinced that the physiology textbooks were wrong about how humans maintain their long-term salt, or more specifically, sodium balance. The conventional explanation is that any sodium ingested from food is rapidly processed by the kidneys and excreted in urine to help maintain a relatively constant level of sodium in the body. Titze reasoned that the evidence pointed to a much more complicated process and that the "Mars500" cosmonauts with their strictly controlled diet and so standardised salt intake would allow him to prove his case.

Writing in the journal Cell Metabolism in January, Titze and his colleagues report how they have now demonstrated that, in contrast to that prevailing dogma, sodium levels can fluctuate rhythmically with weekly and monthly cycles. Ultimately, the research shows that sodium is not simply flushed through the body but can be stored, a discovery that has important connotations for the control of high blood pressure and the risk of cardiovascular disease associated with salt levels.

Salt is skin deep

Earlier simulations of the conditions of space flight carried out in the 1990s had intrigued Titze because they had shown rhythmic variations in sodium urine excretion. "It was so clear to me that sodium must be stored in the body, but no one wanted to hear about that because it was so different from the textbook view," he says. But, he and his colleagues continued with animal studies and showed that it is the skin that acts as the sodium store with the immune system regulating release of sodium from the skin, presumably in sweat. The team has recently used a newly developed magnetic resonance imaging (MRI) technique to image sodium and have shown that humans store sodium in skin and in muscle.

The plans for the Mars500 program were laid down in 2005 when Russia, the European Union and China were hoping to simulate a manned mission to Mars. A facility was established at the Russian Academy of Sciences' Institute of Biomedical Problems (IBMP) in Moscow in which three different crews of volunteers lived and worked in a mock-up spacecraft between 2007 and 2011 in three phases: a 15-day phase to test the equipment, a 105-day phase, and a 520-day phase to simulate a full-length manned mission. The mission has been described as a "psychosocial isolation experiment" and, of course, much of the testing involved monitoring the behaviour and well being of the all-male crew of "cosmonauts". However, it also provided a unique opportunity for carrying out biomedical and physiological tests where diet and water were well-controlled.

Titze and his colleagues organized the food for the mission, which would all be consumed and collected their urine each day. The team studied twelve men: six for the full 105-day phase of the program, and six for the first 205 days of the 520-day phase.

"It was the participants’ stamina to precisely adhere to the daily menu plans and to accurately collect their urine for months that allowed scientific discovery," Titze explains. This allowed them to reveal that 95 percent of the ingested salt was excreted in the urine, but not on a daily basis. Instead, at constant salt intake, sodium excretion fluctuated with a weekly rhythm, resulting in sodium storage. The levels of the hormones aldosterone (a regulator of sodium excretion) and cortisol (no known major role in sodium balance but a well-known stress hormone) also fluctuated weekly.

Cyclic sodium

Titze and colleagues also found that total body sodium levels fluctuated on monthly and longer cycles, with this longer-term storage process seemingly independent of salt intake and not linked to weight gain that would be associated with water retention. The results have one rather immediate implication for medical research and diagnostics: they suggest that current medical practice, which utilises 24-hour urine samples to determine salt intake, might paint a wholly inaccurate picture of a person's salt balance.

"We understand now that there are 7-day and monthly sodium clocks that are ticking, so a one-day snapshot shouldn't be used to determine salt intake," Titze asserts. The team suspects that the same genes that control our circadian rhythms, the so-called body clock may also underpin the sodium storage and release cycles. "We find these long rhythms of sodium storage in the body particularly intriguing," Titze explains. "The observations open up entirely new avenues for research."

Related Links

Cell Metabolism, 2013, 17, 125-131: "Long-Term Space Flight Simulation Reveals Infradian Rhythmicity in Human Na+ Balance"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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