They survived a "mission to Mars" that helped us understand the challenges of long-term isolation. Now the crew who participated in a mock trip to the Red Planet may have helped overturn a long-standing assumption of how our body deals with dietary salt.
The Mars 500 mission, which ended in November 2011, was a dry run for a real Mars mission. It saw six volunteers confined for 520 days in a mocked-up spacecraft at the State Research Centre Institute for Biomedical Problems in Moscow, Russia. It also provided a chance to perform otherwise difficult experiments on diet.
It is known that dietary salt is rapidly excreted in urine. As it passes through the kidneys it helps them to eliminate unwanted fluids. But too much salt in the bloodstream can hamper the ability of kidneys to remove water, increasing blood pressure and adversely affecting health.
Salt cycle
It is a challenge to measure the long-term effects of salt because of difficulties in regulating dietary intake. However, Mars 500 "mission controllers" were able to dictate the amount of salt that the crew received each day.
During this mission, and a preliminary mission with a different crew that lasted 105 days, 12 crew members consumed either 6, 9 or 12 grams of salt per day for 29 to 60 days. Previous research suggests that there should be roughly the same amount of salt in urine as has been eaten that day.
Instead, the crew excreted different amounts each day, revealing a six to seven-day cycle of salt retention and excretion. The patterns varied between crew members, but were consistent within individuals. For example, on a 12-gram daily salt diet a crew member might excrete as little as 6 grams on day three of the cycle, and peak at 18 grams on day five.
Sleepy time
The fluctuations tallied with levels of the hormones aldosterone and cortisol. It is not yet known why the cycles lasted six to seven days. Jens Titze of Vanderbilt University School of Medicine in Nashville, Tennessee, who headed the study, suspects it could be influenced by sleep.
In the 105-day mission, crew members performed a night shift every six days. Blood pressure on the morning after a night shift was higher compared with day shifts. No night shifts were performed during the experiment on the longer mission, however.
Graham MacGregor of the Wolfson Institute of Preventive Medicine in London says the results are fascinating, but adds that they don't compromise previous studies. "It doesn't matter if there are [cyclic] fluctuations, because if you average single results across a large number of people, you'll still get a close correlation between the salt going in and out," he says.
Salt stores
Together with previous animal studies, the fluctuations indicate that salt might be stored away from the kidneys, which might otherwise sustain damage through salt overload. Subsequent MRI scans performed by Titze suggest that muscle and skin may also store salt. These results will be published in March, but more work will be needed to identify how such storage might affect health.
In another study published this week from the Mars 500 experiment, researchers report that the "cosmonauts" slept for longer periods, and became less physically active, the longer the mission went on. In the first quarter of the 520-day mission, the six crew members averaged 7.12 hours of sleep a day, which increased to 7.7 hours during the final quarter (PNAS, DOI: 10.1073/pnas.1212646110).
More sophisticated lighting could improve acclimatisation to the required daily sleep cycle in future experiments or indeed aboard long-haul space modules. Blue light intensity is particularly important, says Mathias Basner of Baylor College of Medicine in Houston, Texas, for setting day-night cycles, so lighting could be more carefully attuned to this intensity in future.
Journal reference: Cell Metabolism, DOI: 10.1016/j.cemet.2012.11.013
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