Spaceflight and long-distance swimming shrink the heart

An illustration of a heart.
(Image credit: Shutterstock)

What do extreme long-distance swimming and spaceflight have in common? They can both shrink the heart, according to a new study.

Both activities reduce the pressure of gravity on the heart, making it so that it doesn't have to work as hard to pump blood upwards through the body. The heart is a muscle, and just like any other muscle in the body, if it's not used as much as it used to be, it will shrink.

To understand what effect weightlessness has on the heart, a group of researchers analyzed health data from retired astronaut Scott Kelly's year aboard the International Space Station from 2015 to 2016, and health data from elite endurance swimmer Benoît Lecomte, who swam 1,753 miles (2,821 kilometers) across the Pacific Ocean in 2018.

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Lecomte swam for 159 days between June 5 and Nov. 11 of 2018, taking 7-day and 32-day breaks due to unfavorable weather (which was a limitation in data gathering but was for his own safety), according to the study. He swam about 5.8 hours per day, on average. 

Kelly spent 340 days up in space and exercised a couple of hours a day, 6 days a week by cycling, using the treadmill and doing resistance exercises. Doctors analyzed both men's hearts before, during and after their respective journeys.

The researchers found that during his year out in space, Kelly lost about 0.74 grams of heart mass per week in his left ventricle, the heart's main pumping chamber. Lecomte, during his swim across the Pacific, lost 0.72 greams of heart mass a week in his left ventricle. The researchers also found that when Kelly and Lecomte first began their journeys, both men experienced an initial drop in the diameter of their left ventricle. The average heart is about 280 to 340 grams in men and 230 to 280 grams in women, according to Live Science.

Overall, Kelly had a 19% to 27% left ventricle mass loss over his year out in space and Lecomte had a 20 to 25% loss in the five months he was swimming, co-author Dr. James MacNamara, a cardiology fellow at the University of Texas  Southwestern Medical Center in Dallas told the BBC.

In other words, despite exercising in a weightless environment (water and space), both mens' hearts shrank during their journeys. That was surprising to the scientists, as high-intensity swim training of 1 to 3 hours a day has previously been found to be linked with an increased left ventricle size and mass. "We anticipated that a long duration of swimming exercise would have been enough of a stimulus," to increase the left ventricle mass, the authors wrote.

"The heart is remarkably plastic and especially responsive to gravity or its absence," senior author Dr. Benjamin Levine, a professor of internal medicine at UT Southwestern Medical Center said in a statement. "Both the impact of gravity as well as the adaptive response to exercise play a role, and we were surprised that even extremely long periods of low-intensity exercise did not keep the heart muscle from shrinking." 

But after their journeys, both mens' hearts returned to their normal sizes once they returned to walking on the ground, according to the BBC. The researchers still plan to analyze magnetic resonance imaging (MRI) scans of Lecomte's heart from before and after his swim to further understand if the long-term effects of weightlessness can be fully reversed, according to the statement. 

The findings were published March 29 in the journal Circulation.

Originally published on Live Science.

Yasemin Saplakoglu
Staff Writer

Yasemin is a staff writer at Live Science, covering health, neuroscience and biology. Her work has appeared in Scientific American, Science and the San Jose Mercury News. She has a bachelor's degree in biomedical engineering from the University of Connecticut and a graduate certificate in science communication from the University of California, Santa Cruz.