January 9, 2013
Researcher Uses Space Simulation To Find Sodium Rhythms In The Body
April Flowers for redOrbit.com - Your Universe Online
The Russians were simulating a flight to Mars, and clinical pharmacologist Jenz Titze, M.D., knew that it would be a one-of-a-kind opportunity to study the participating cosmonauts. Titze wanted to explore long-term sodium balance in humans because he didn't believe that the standard textbook view was entirely correct — namely, that the salt we eat is rapidly excreted in urine to maintain relatively constant body sodium levels.The Russian's "Mars500" live simulation allowed Titze, now an associate professor of Medicine at Vanderbilt University, the opportunity to keep salt intake constant and monitor urine sodium levels in human participants over a long period.
In contrast to prior thought, Titze found that sodium levels fluctuate rhythmically with both 7-day and monthly cycles. The study demonstrated that sodium is stored in the body, which has implications for blood pressure control, hypertension and salt-associated cardiovascular risk.
This study is a continuation of Titze's earlier work in the 1990's which found rhythmic variations in sodium urine excretion during human space flight simulation studies.
"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 said.
After the early space flight simulation studies, Titze and his team continued with animal studies, demonstrating that the skin stores sodium and that the immune system regulates sodium release from the skin.
The planning for Mars500 — a collaboration between Russia, the European Union and China to prepare for manned spaceflight to Mars — began in 2005, but the final simulation was run at a Moscow research facility between 2007 and 2011. The experiment had 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. Participants were healthy male cosmonauts who volunteered to live and work in an enclosed habitat of sealed interconnecting modules as if they were on an international space station.
Participants committed to eating all the food provided by Titze and his team and to collecting all of their urine each day. The team studied 12 men in two different portions of the experiment: 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 said in praise of the Mars500 cosmonauts.
Although the team found that nearly all — 95 percent — of the ingested salt was excreted in the participant's urine, this did not occur on a regular daily basis. At a constant level of salt intake, Titze found that there was a weekly rhythm to sodium excretion that resulted in sodium storage within the body. The hormones aldosterone (a regulator of sodium excretion) and cortisol (no known major role in sodium balance) also fluctuated weekly.
Total body sodium level changes fluctuate on both monthly and longer cycles, with sodium storage on this longer cycle being independent of salt intake. The storage did not include weight gain, supporting the idea that sodium is stored without accompanying increases in water, suggesting that current medical practice and studies relying on 24-hour urine samples are not accurate.
"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 and his colleagues have found that humans store sodium in skin and in muscle by using newly developed MRI technology to view sodium in the body.
Titze´s team suspects that circadian "clock" genes, which regulate daily rhythms, may also be involved in sodium storage and release.
"We find these long rhythms of sodium storage in the body particularly intriguing," Titze said. "The observations open up entirely new avenues for research."
"The study highlights the importance of measuring salt excretion in urine over a longer time period to accurately estimate salt intake," Titze said. "This information is very important, given the emphasis on salt intake in terms of risk for cardiovascular disease and healthcare outcomes."
The findings of this study, titled "Long-Term Space Flight Simulation Reveals Infradian Rhythmicity in Human Na+ Balance," were recently published in the journal Cell Metabolism.