Your heart gets smaller in space.

In a study published Monday in Circulation magazine, scientists reported that the largest chamber of Scott Kelly’s heart, who spent nearly a year on the International Space Station in 2015 and 2016, shrank by more than a quarter times he returned to earth.

This only adds to the litany of transformations the human body goes through without the steady downward movement of gravity. Astronauts also tend to have swollen heads, bruised eyeballs, shrunken legs, and bones that become more brittle.

But having a smaller heart didn’t seem to have any negative effects on Mr. Kelly.

“He has done remarkably well for over a year,” said Dr. Benjamin D. Levine, senior circulator writer and professor of internal medicine at the University of Texas Southwestern Medical Center and Texas Health Presbyterian Dallas.

“His heart adapted to reduced gravity,” said Dr. Levine. “It did not become inoperable, the overcapacity was not reduced to a critical level. He stayed reasonably fit. His heart shrank and withered, as one would expect from space. “

Without gravity, the heart doesn’t have to pump as hard and, like any other muscle, loses fitness with less strenuous use. For Mr. Kelly, the shrinkage occurred despite exercising on the space station almost every day, a regimen that has been shown to be effective in limiting bone embrittlement and overall muscle loss.

However, a smaller heart could be a problem for future missions to Mars.

Based on the experiences of Mr. Kelly and other astronauts on the space station, “You will likely be fine,” said Dr. Levine. However, problems can arise when an astronaut becomes injured or sick and unable to exercise. Or if the training device is broken. With weaker hearts, they could become dazed and passed out when they set foot on the red planet after months of weightless travel.

In the newspaper, Dr. Levine and his colleagues also shared Mr Kelly’s heart with that of Benoît Lecomte, a long-distance endurance swimmer, when he tried to cross the Pacific in 2018. Buoyancy in water has many of the same effects on the body as weightlessness. Mr. Lecomte was horizontal most of the time – swimming up to eight hours and sleeping eight hours on an accompanying support boat.

Scientists believed the hours of swimming would be strenuous enough to preserve Mr. Lecomte’s heart, which was observed through regular echocardiograms. Instead, it shrank almost as quickly as Mr. Kelly’s did in space.

Over 159 days – Mr Lecomte had to stop swimming less than a third of the way to a planned 5,650 mile trip after the boat was damaged in a storm – the left ventricle of his heart brightened from an estimated six ounces to five ounces . The left ventricle is the largest and most powerful chamber in the heart and pumps blood into the aorta and around the body.

“I was just shocked,” said Dr. Levine. “I really thought his heart was going to get bigger. That was a lot of practice he did. “

In an interview, Mr. Lecomte estimated that his heart rate was “maybe in the low hundreds” when he was swimming and described the intensity of long-distance swimming as “more like walking fast, maybe or running very slowly”.

NASA may now be able to develop better training programs for astronauts. “There is a big question about the appropriate intensity and duration of exercise,” said Dr. James MacNamara, a cardiologist at the University of Texas Southwestern Medical Center and another author on the paper. “Mister. The Lecomte swimming gave us the opportunity to look at someone who was doing a fair amount of low-intensity exercises.

At the space station, Mr. Kelly exercised six days a week, jogging on a treadmill for about 30 to 40 minutes, or exercising on a stationary bike. He also used a resistance machine that mimicked lifting weights.

“It’s pretty exhausting,” said Mr. Kelly, who has now retired from NASA, in an interview. “You push it pretty hard, more weight than I would certainly lift here at home.”

And yet, during his 340 days in space, Mr. Kelly’s heart mass shrank from 6.7 ounces to 4.9 ounces, a decrease of about 27 percent.

Mr. Kelly and Mr. Lecomte’s hearts decreased at the rate of about 1/40 ounce per week.

Mr Kelly joked that he found the study interesting because it found that “my heart looked like a professional athlete”.

Dr. Levine said another study looked at the hearts of 13 astronauts before and after spending six months on the space station. This as-yet-unpublished study offers a wider range of data that seems reassuring.

“Which is really interesting,” said Dr. Levine, “is that it kind of depends on what they did before they flew.”

For the sportiest astronauts, their hearts lost mass in space, just like Mr. Kelly did. But for those who were couch potatoes on Earth but then had to exercise regularly on the space station, their hearts grew, like those of the Grinch in the Dr. Seuss story, in size.

This was not because they were experiencing newfound friendliness and generosity, but simply because of increased exertion.

“The heart is like any other muscle and responds to the stress that is placed on it,” said Dr. Levine.

NASA has provided funding to study the heart health of the next 10 astronauts who will spend a year in space.

Mr Kelly said that his body, having experienced other changes including bone loss, has almost returned to normal.

“I don’t have any symptoms from space, at least no physical,” he said. “If you let me today, I would do it all over again.”