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Spending very long periods of time in space has something in common with extreme endurance swimming – both can make your heart shrink.
This is the conclusion of a study which compared the effects of the year of astronaut Scott Kelly in space with a marathon swim of the athlete Benoît Lecomte.
Both remove loads on the heart that are usually applied by gravity, causing the organ to atrophy.
The exercise was not enough in either case to counteract the changes in the heart.
The study was led by Dr. Benjamin Levine, professor of internal medicine at the University of Texas Southwestern Medical Center in Dallas, and is published in the journal Circulation.
The research has implications for very long-term space travel – such as the expeditions to Mars that NASA plans to organize in the decades to come.
“One of the things we’ve learned over many years of study is that the heart is remarkably plastic. So the heart adapts to the load placed on it, ”Professor Levine, also director of the Institute for Exercise and Environmental Medicine, a collaboration between UT Southwestern and Texas Health Presbyterian Hospital Dallas, told BBC News.
“In spaceflight, one of the things that happens is you don’t have to pump blood upwards anymore, because you’re not pumping against gravity.”
Scott Kelly spent 340 days aboard the International Space Station (ISS) to allow scientists to study the effects of long-duration flights on the human body.
On June 5, 2018, Benoît Lecomte embarked on a swimming effort in the Pacific Ocean, after having already crossed the Atlantic.
He swam 2,821 km for 159 days, eventually giving up the attempt.
Swimming for very long periods of time also changes the loads placed on the heart by gravity because the person is in a horizontal position rather than vertical.
Lecomte swam an average of 5.8 hours a day, sleeping about eight hours a night. This meant he spent between nine and five o’clock each day in a recumbent state.
Scientists sometimes use bed rest studies to simulate space flight, because lying down eliminates the head-to-toe gradient that places a load on the heart. But Prof Levine said immersion in water for long periods of time in a horizontal position is an even better model for time spent in orbit.
“Now you take the gradient from head to toe, and then you put the person in the water, so you adjust that gradient as well. It’s pretty much like being in space, ”Professor Levine said.
Because the two men were no longer pumping blood upwards, as Professor Levine says, their hearts started to lose mass.
“When we look at the left ventricle [of the heart] we’re seeing a loss of about 20-25% of total mass over the four or five months that Mr. Lecomte was swimming, ”said co-author Dr. James MacNamara, also of UT Southwestern Medical Center .
“We specifically saw 19% and 27% of the mass lost for Captain Kelly during the year.”
Exercise, however, thwarts the process of mass loss. Astronauts on the International Space Station (ISS) are already subjected to a regime of intense exercise to alleviate muscle and bone wastage that also occurs in orbit.
Even so, this exercise regimen was not enough to prevent the cardiac atrophy seen in Captain Kelly.
At the start of the study, the researchers wondered whether the amount of physical exercise Mr. Lecomte was doing in the water could be enough to prevent heart tissue from melting.
“I absolutely thought that Ben’s heart wouldn’t atrophy. That’s one of the beautiful things in science – you learn the most when you find things you didn’t expect,” Professor Levine said. .
“It turns out that when you swim that many hours a day, it’s not like Michael Phelps, he doesn’t swim as hard as he can.”
Instead, Mr. Lecomte hit his legs relatively lightly throughout. “It’s just not a lot of activity. Low levels of physical activity do not protect the heart from adapting to the absence of gravity, ”said Professor Levine.
The cardiac adaptations, however, are not long-term – both men’s hearts returned to normal when they returned to dry land.
But the chambers of the heart called the atria expand in space, in part because of changes in the way fluid passes. This could lead to a condition called atrial fibrillation, where the heart beats quickly and irregularly. It can interfere with exercise, but can also increase the risk of stroke.
There is also another risk to this vital organ of space travel. Higher radiation levels in space could accelerate coronary heart disease. Astronauts are screened for atherosclerosis, but they’re usually middle-aged when they go to space, and scientists know it’s a problem that develops with age.
This is important because having a heart attack in space could be catastrophic.
Professor Levine is part of a NASA program called Cipher that will send 10 more astronauts into space on long-duration missions. Researchers will subject the hearts of crew members to a number of different tests and high-tech scanning methods for a more detailed picture of heart function in space.
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