When Astronaut Scott Kelly returned after nearly a year in space in March of 2016, his heart had shrunk in mass by more than one-quarter. The spacefarer’s heart shrank at a rate of 1/40th of an ounce a week despite Kelly working out almost every day, with a workout regimen that involved a mix of running, cycling, and resistance training to mimic lifting weights.
Researchers who studied this effect say that, without Earth’s pull of gravity, Kelly’s heart didn’t have to pump as hard to send oxygen to the rest of his body, despite the exercise. As a result, his heart shrank. The findings were published in a new Circulation paper.
“His heart didn’t become dysfunctional, the excess capacity didn’t get reduced to a critical level. He remained reasonably fit,” Benjamin Levine, the senior author of the paper and a professor of internal medicine at the University of Texas Southwestern Medical Center and Texas Health Presbyterian Dallas, told The New York Times. “His heart shrank and atrophied kind of as you’d expect from going into space.”
Despite his heart shrinking by 27 percent in mass, Kelly is just as fit as ever, and he has not experienced any ill effects or symptoms. And in fact, the researchers documented this phenomenon in another person, though not another astronaut: Long-distance swimmer, Benoît Lecomte.
In 2018, Lecomte, attempted to swim across the Pacific ocean. Over 159 days, Lecomte was mostly horizontal, and he traversed 2,821 kilometers. He didn’t make it all the way across the ocean—his swim had to be cut short after his support boat was damaged in a storm, but he still spent a significant amount of time in water, where he also experienced a sort of weightlessness. The authors of the Circulation paper found that Lecomte’s heart shrank at a similar rate to Kelly’s. Notably, his left ventricle, the chamber of the heart most important for pumping oxygen-rich blood to the rest of the body, dropped from an estimated six ounces to five ounces in mass. This shocked researchers, who expected that such high volume exercise would bulk up the organ rather than slim it down. Levine also noted that the shift in heart size also reflects that person’s fitness level: The hearts of previously athletic individuals shrank in zero-gravity despite them still exercising in space, but the hearts of those who were less active on Earth and more so in space, grew in size.
Another study examining the hearts of 13 other astronauts is currently underway, though not yet published. Both Kelly and Lecomte seem to be doing fine with their slimmer hearts, but this could prove a concern for future space travelers, especially for Mars missions. A weaker heart could be a health risk when stepping on to a new planet after months of zero gravity, especially if not everyone can regularly exercise. But scientists are hoping that, by studying people like Kelly and Lecomte, NASA will be able to design more effective exercise programs for astronauts to keep them all optimally healthy.