Note to future space travellers: Reduced gravity will shrink your heart

In space, your heart gets smaller.
 
In a study published on Monday in the journal Circulation, scientists reported that the largest chamber of the heart of Scott Kelly, who spent nearly a year on the International Space Station in 2015 and 2016, shrank in mass by more than one-quarter by the time he returned to Earth.
 
That just adds to the litany of transformations that the human body undergoes without the steady downward pull of gravity. Astronauts also tend to have swelled heads, squashed eyeballs, shriveled legs and bones that become more brittle.
 

But a smaller heart did not appear to have any ill effects on Kelly. “He did remarkably well over one year,” said Benjamin D Levine, the senior author of the Circulation paper.

 
“His heart adapted to the reduced gravity,” Levine said. “It didn’t become dysfunctional, the excess capacity didn’t get reduced to a critical level. He remained reasonably fit. His heart shrank and atrophied kind of as you’d expect from going into space.”
 
Without the pull of gravity, the heart does not have to pump as hard, and like any other muscle, it loses some fitness from less strenuous use. For Kelly, the shrinkage occurred even though he exercised almost every day on the space station, a regimen that has proved effective at limiting the brittling of bone and loss of muscle overall.

 
But a smaller heart could be a concern for future missions to Mars.
 
Based on the experience of Kelly and other astronauts on the space station, “They’ll probably be OK,” Levine said. But problems could arise if an astronaut were injured or fell sick and could not exercise. Or if the exercise equipment broke. With weaker hearts, they could become lightheaded and faint when stepping foot on the red planet after months of weightless travel.
 
In the paper, Levine and his colleagues also compared Kelly’s heart to that of Benoît Lecomte, a long-distance endurance swimmer, when he attempted to cross the Pacific in 2018. Buoyancy in water has many of the same effects on the body as weightlessness. Lecomte was horizontal most of the time — up to eight hours of swimming and eight hours of sleeping on an accompanying support boat.

 
Scientists thought that the hours of swimming would be strenuous enough to maintain Lecomte’s heart, which was observed by periodic echocardiograms. Instead it shrank, almost as quickly as Kelly’s had in space.
 
Over 159 days — Lecomte had to abandon the swim less than a third of the way into a planned 5,650-mile journey after the boat was damaged in a storm — the left ventricle of his heart lightened from an estimated six ounces to five ounces. The left ventricle is the biggest and strongest chamber of the heart, pumping blood into the aorta and through the body.

 
In an interview, Lecomte estimated that his heart rate was “maybe in the low hundreds” as he swam and described the intensity of long-distance swimming as “more like a fast walking, maybe, or a very slow running.”
 
NASA may now be able to design better exercise programmes for astronauts. “There’s a big question as to the appropriate intensity and duration of exercise,” said James MacNamara, a cardiology fellow at the University of Texas South­western Medical Center.
 
On the space station, Kelly exercised six days a week, jogging on a treadmill for about 30 to 40 minutes or working out on a stationary bicycle. In addition, he used a resistance machine that mimicked the lifting of weights.

 
“It’s pretty strenuous,” Kelly, now retired from NASA, said in an interview. “You push it pretty hard, more weight than I would lift at home here certainly.”
 
And yet, over his 340 days in space, Kelly’s heart mass shrank to 4.9 ounces from 6.7 ounces, a decline of about 27 per cent.
 
The hearts of both Kelly and Lecomte slimmed at a rate of about 1/40th of an ounce a week.
 
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