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- The human heart shrinks when not exposed to gravity for prolonged periods, a study found.
- One case study was astronaut Scott Kelly, who spent 340 days in space.
- Exercise can't counteract the effect, though his heart went back to normal after returning.
- See more stories on Insider's business page.
The human heart shrinks while in space, and exercise couldn't stop the effect, according to a study that analyzed an astronaut who spent almost a year in orbit.
Captain Scott Kelly of NASA spent 340 days on the International Space Station in 2015 and 2016 to test the effects on the human body of long periods in space. He was 51 when he left and turned 52 in orbit.
An analysis of the results, published Monday in the science journal Circulation, found that even a heavy program of exercise could not defend the heart from the effects of lower gravity.
However, his heart returned to normal after getting back to earth, the BBC reported.
Learning how to counteract the effects of space on people is vital for the development of long-haul space travel, and the findings make clear the scale of the challenge.
The heart usually has to pump hard to fight the effect of gravity and move blood around the body. But in space, without gravity, the heart pumps less hard and gets steadily weaker, the study said.
Scientists had hoped that exercising could counteract this effect, but Monday's results contradicted that.
The scientists found that Kelly's heart shrank even though he did cycling, treadmill runs, and resistive exercise 6 days a week.
The scientists also looked at another extreme example in which the heart was subjected to lower gravity.
Benoît Lecomte attempted to swim across the Pacific from Japan to California, before he had to cancel his attempt because of stormy weather and kit failures.
He swam on average 5.8 hours and slept around 8 hours a day, which means he spent between 9 and 17 hours horizontally.
Here, again, the heart has less gravity to contend with, which reduces the pressure gravity puts on the heart.
Even though Lecomte went through extreme physical exertion, swimming for 159 days over 1,700 miles, his heart also shrank.
As with Kelly's, his heart later returned to normal size.
The scientists analyzing the results said it was "surprising" that exercise did not counteract the effects of low gravity.
The results could prove a setback for long-flight space travel and attempts to put people on Mars.
Micro-gravity causes a host of problems, such as bone and muscle loss. Another study had shown that losing gravity in space causes the brain to shift upwards, causing coordination problems and blurry vision.
Elon Musk said in December that he is "highly confident" that Space X will put humans on Mars by 2026.
This is not the first time that scientists have looked at Kelly. His year-long mission to the International Space station was specifically designed to test the effects of long spaceflight on the human body.
Because his identical twin, Mark Kelly, now a State Senator for Arizona, remained on Earth during his space trip, scientists were able to compare biological characteristics of the twins.
When Scott Kelly returned to Earth, he was 2 inches taller than his twin. 7% of his genes were expressed differently and he hosted different gut bacteria.