(CNN)When astronaut Scott Kelly spent nearly a year in space, his heart shrank despite the fact that he worked out six days a week over his 340-day stay, according to a new study.

Surprisingly, researchers observed the same change in Benoît Lecomte after he completed his 159-day swim across the Pacific Ocean in 2018. The findings suggest that long-term weightlessness alters the structure of the heart, causing shrinkage and atrophy, and low-intensity exercise is not enough to keep that from happening. The study published Monday in the American Heart Association’s journal Circulation.The gravity we experience on Earth is what helps the heart to maintain both its size and function as it keeps blood pumping through our veins. Even something as simple as standing up and walking around helps pull blood down into our legs. When the element of gravity is replaced with weightlessness, the heart shrinks in response.Read MoreKelly lived in the absence of gravity aboard the International Space Station from March 27, 2015, to March 1, 2016. He worked out on a stationary bike and treadmill and incorporated resistance activities into his routine six days a week for two hours each day.Lecomte swam from June 5 to November 11, 2018, covering 1,753 miles and averaging about six hours a day swimming. That sustained activity may sound extreme, but each day of swimming was considered to be low-intensity activity.He swam from Japan to Hawaii and at times saw a piece of plastic in the ocean every three minutesHe swam from Japan to Hawaii and at times saw a piece of plastic in the ocean every three minutesHe swam from Japan to Hawaii and at times saw a piece of plastic in the ocean every three minutesEven though Lecomte was on Earth, he was spending hours a day in the water, which offsets the effects of gravity. Long-distance swimmers use the prone technique, a horizontal facedown position, for these endurance swims. Researchers expected that the activities performed by both men would keep their hearts from experiencing any shrinkage or weakening. Data collected from tests of their hearts before, during and after these extreme events showed otherwise. Kelly and Lecomte both experienced a loss of mass and initial drop in diameter in the left ventricles of the heart during their experiences.Both long-duration spaceflight and prolonged water immersion led to a very specific adaptation of the heart, said senior study author Dr. Benjamin Levine, a professor of internal medicine/cardiology at the University of Texas Southwestern Medical Center. While the authors point out that they only studied two men who both performed extraordinary things, further study is needed to understand how the human body reacts in extreme situations. No negative impactIn this case, researchers saw that the heart adapted, but the shrinkage did not cause any ill effects, present or long-term. “The heart gets smaller and shrinks and atrophies, but it doesn’t become weaker — it’s just fine,” said Levine, who is also director of the Institute for Exercise and Environmental Medicine, a collaboration between UT Southwestern and Texas Health Presbyterian Hospital Dallas. “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.”Human health can be 'mostly sustained' for a year in space, NASA Twins Study concludesHuman health can be 'mostly sustained' for a year in space, NASA Twins Study concludesHuman health can be 'mostly sustained' for a year in space, NASA Twins Study concludesLevine noted the plasticity and adaptability of the heart’s muscle mass, nearly three-quarters of which is responsive to physical activity. “If there’s one thing that I’ve learned over 25 years of studying how the heart adapts to spaceflight, exercise training and high altitude, it’s that it’s a remarkably adaptive organ and it responds to the demands that are placed on it.”The larger the load that’s placed on the heart, the bigger it gets; the same happens in reverse. Currently, astronauts stick with the same exercise regiment Kelly used while on the station. Looking ahead to missions to the moon and Mars, the exercise countermeasures to prevent muscle and bone loss may need to shift. Levine believes the current countermeasures work, but limits will be placed due to the space allowed for exercise equipment on future vehicles. Rowers have the biggest heart of any athletes, Levine said, so a combination of rowing and strength training may be the best strategy for astronauts moving forward. Rowing is a dynamic exercise because it loads the heart in a way that feels like strength and endurance training simultaneously, Levine said. The effects of space radiationFuture long-term spaceflight missions will return humans to the moon and send them on to Mars, so understanding how spaceflight impacts all aspects of the heart is crucial. Astronauts on a Mars mission will need to be 'conscientious' to work well togetherAstronauts on a Mars mission will need to be 'conscientious' to work well togetherAstronauts on a Mars mission will need to be 'conscientious' to work well togetherAstronauts are largely middle-aged men and women, so the main concern is that they may experience a heart attack. These space explorers are highly screened before selection, but they deal with the same things everyone else does, including hypertension and elevated cholesterol. While NASA and medical experts can work with these known parameters as they quantify risk and choose the healthiest people, there is one large unknown: radiation exposure. What happens to the heart arteries after long-term exposure to weightlessness and radiation? That’s a question Levine and his fellow researchers want to answer in the future. They will look at the coronary arteries of astronauts before and after flight using a computed tomography angiogram, an X-ray test that can reveal the overall structure and lining of the heart arteries. Atrial fibrillation, or a fast, irregular heartbeat, is the most common form of arrhythmia — and astronauts are getting it about a decade earlier than the rest of the population, Levine said. That may be because the atria, the two upper chambers of the heart, get dilated in space. Radiation on moon's surface measured for the first time, study saysRadiation on moon's surface measured for the first time, study saysRadiation on moon's surface measured for the first time, study saysLevine is concerned that astronauts could be at risk of developing this during long-duration spaceflight. While it’s not life-threatening, atrial fibrillation can cause discomfort, reduce exercise tolerance and increase the risk of stroke in people who are otherwise healthy, he said. Having access to cardiac MRIs of astronauts before and after their flight in the future could provide researchers with a better and more detailed understanding of what is happening in the right and left ventricles of the heart, said first study author Dr. James MacNamara, an advanced echocardiography fellow with UT Southwestern who works with Levine. Levine and his colleagues will study 10 more astronauts who plan to spend a year in space over the next decade, focusing on the most intensive look at the heart arteries and muscle itself. The study will also include astronauts spending six months on the space station, as well as shorter duration flights. “So we’ll be ready when we’re going to go to Mars,” Levine said.

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