While astronaut Scott Kelly spent 340 days on the International Space Station, the mass of his heart shrank by about 27 percent, according to new research in the journal Circulation.
That might sound alarming, but it’s a reflection of how adaptable the human heart is, explains study author Benjamin Levine, a professor of internal medicine at the University of Texas Southwestern Medical Center and Texas Health Presbyterian Dallas, to the New York Times’ Kenneth Chang. On Earth, the heart has to pump blood hard enough to move it upward while gravity pulls it down. But in the weightless conditions of orbit, gravity is no longer a factor, and the heart shrinks to an appropriate size.
The researchers also measured the changes in long-distance swimmer Benoît Lecomte’s heart as he attempted to cross the Pacific Ocean. Between swimming and sleeping, Lecompte spent the majority of his time horizontal, which, like spaceflight, reduced the time that his heart had to spend pumping blood against the pull of gravity. After 159 days, Lecomte’s heart had also shrunk by about 25 percent.
"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 that's placed on it," says Levine to Paul Rincon at BBC News.
Kelly spent nearly a year in space over the course of 2015 and 2016 specifically to study the health impacts of long-duration space travel. The results have already illuminated details about how space travel affected his DNA, his eyes, his gut microbiome and his arteries, Catherine Zuckerman reported for National Geographic in 2019. To combat health risks like weakening bones and muscles, astronauts stick to a strict, six-days-a-week exercise regimen of stationary bike, treadmill and resistance training.
“It’s pretty strenuous,” says Kelly to the New York Times. “You push it pretty hard, more weight than I would lift at home here certainly.”
Kelly was physically fit before undertaking the long ISS mission, so when he arrived and continued to exercise, his heart no longer had to accommodate the pull of gravity. When it adjusted by shrinking, it did not cause him harm.
"The heart gets smaller and shrinks and atrophies, but it doesn't become weaker — it's just fine," says Levine to Ashley Strickland at CNN. "The function is normal, but because the body is used to pumping blood uphill against gravity in the upright position, when you remove that gravitational stimulus, particularly in someone who is pretty active and fit beforehand, the heart adapts to that new load."
Levine tells the New York Times an upcoming study, which has not yet been published, analyzes space travel’s effects on the hearts of several astronauts who had different fitness levels before their ISS missions.
“What’s really interesting,” says Levine to the Times, “is that it kind of depended on what they did before they flew.”
Athletic astronauts tended to lose heart mass during their trips, while the hearts of less-fit astronauts saw gains.
The Circulation study compared Kelly’s results to Lecomte, who attempted to swim across the Pacific Ocean in 2018. (He’d crossed the Atlantic in 1998.) During the 159 day project, during which he made it about a third of the way across the Pacific, Lecomte spent an average of 5.8 hours per day in the water, and slept for about eight hours each night, per BBC News. Lecomte’s left ventricle, which is the biggest chamber of the heart, shrunk by about 20 to 25 percent during the study.
"I absolutely thought that Ben's heart would not atrophy. That's one of the nice things about science - you learn the most when you find things you didn't expect," says Levine to BBC News. "It turns out when you swim for that many hours a day, it's not like Michael Phelps, he's not swimming as hard as he can."
Instead, Levine tells BBC News, the low-intensity strategy of light kicking that Lecomte utilizes is “just not that much activity. Low levels of physical activity do not protect the heart from adapting to the absence of gravity.”
Since returning to Earth and retiring from NASA, Kelly tells the New York Times his body has recovered from the changes he experienced because of space travel. Next, NASA has funded further research on ten more astronauts’ year-long trips to the ISS, as well as studies of shorter duration trips, as a way to prepare for lengthy, possibly interplanetary, travel in the future.
Levine tells CNN that the studies are being conducted "so we'll be ready when we're going to go to Mars.”