What happens to the human heart after 1 year in space?

• American astronaut Mark Kelly spent 340 days in space, and changes to his body help researchers understand the effects of living in low gravity.
• Over the course of his mission, Kelly lost heart mass despite a rigorous exercise regimen.
• A new research paper says studying extreme swimmers can help understand the impact low gravity has on human bodies.
• The study reports on a similar loss in heart size in a man who attempted to swim across the Pacific Ocean in 2018.

Earth’s gravity was undoubtedly a factor in the way human muscles evolved. The heart, for example, has to be strong enough to pump blood upward from the feet, where it would otherwise collect.

In low gravity, however, such muscles do not have to work so hard.

Since muscles must be exercised to remain healthy, and as preparations for extended missions are underway, scientists are concerned about the effect living in space can have on the human body.

A research letter from cardiovascular experts at the University of Texas Southwestern (UT Southwestern) examines the physiological effects spending 340 days in space had on Kelly.

Kelly’s heart shrank by an average of 0.74 grams per week during the year he spent in space, although the muscle continued to perform well.

The senior author of the letter is Dr. Benjamin Levine. Of Kelly’s heart, Dr. Levine says:

“It did shrink a little bit. It did atrophy, and it did get a little smaller, but the function remained good. I think this is encouraging for long-duration space flight. It shows that even after a year in space, the heart adapts relatively well.”

The research, published in the journal Circulation, compares the changes to Kelly’s heart size with an unexpected yet similar change in the heart size of an athlete who attempted to swim across the Pacific Ocean.

Tracking the human heart in space

Dr. Levine’s experience with space and cardiological health extends back to the 1980s, when he implanted the first catheter in an astronaut, which allowed NASA to monitor heart pressure while in space.

Kelly was in space between March 27, 2015, and March 1, 2016. The loss in left-ventricle mass he experienced was the equivalent of three-tenths of an ounce per week.

Kelly’s heart mass decreased in spite of a rigorous regimen of cycling, treadmill, or resistance work 6 days per week.

Dr. Levine recently completed a study of 13 astronauts who had been aboard the International Space Station for 6 months. The effects of the time spent in space varied from person to person.

Dr. Levine found that the astronauts who were the fittest at launch lost heart muscle during their missions, whereas the less fit flyers gained heart mass while in space.

“It all depended on how much work the astronaut’s heart did in space relative to how much it regularly did on the ground,” he says.

A marathon swim and a smaller heart

In addition to his work with astronauts, Dr. Levine, who holds the Distinguished Professorship in Exercise Sciences at UT Southwestern, also studies the effects of exercise on the human body here on Earth. He is particularly interested in extreme exercise.

He believes that the cardiovascular experiences of extreme swimmers may be a predictor of what human travelers can expect in space.

As the study says of swimming and life in low gravity, “Both are associated with removal of gravitational loading of the musculoskeletal system and the absence of weight-bearing activities.”

The new study compares the reduction in Kelly’s heart size to a similar effect recorded for long-distance swimmer Benoit Lecomte. Although Lecomte did not complete his 2018 swim across the Pacific due to equipment issues, he swam over 1,750 miles over 159 days.

Lecomte lost 0.72 g of left-ventricle muscle per week during his swim, just slightly less than Kelly lost in space.

Dr. Levine says he was more surprised by Lecomte’s heart-mass reduction than Kelly’s. He says that while swimming is not considered a high-impact exercise, Lecomte was nonetheless averaging 6 hours of exercise per day compared with Kelly’s 2 hours.

The reason for Lecomte’s loss of heart mass, Dr. Levine asserts, is likely the fact that the heart muscle does not need to pump blood uphill when the body is suspended horizontally in water during swimming.

Dr. Levine’s research paper seeks to answer the question of whether low-intensity, long-duration exercise, such a Lecomte’s, can effectively counteract the effects of weightlessness on the cardiovascular system. For now, the swimmer’s experience suggests the answer to that question may be “no.”

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