https://www.scientificamerican.com/article/tests-on-astronaut-and-twin-brother-highlight-spaceflights-human-impact/

In a landmark study, NASA scientists conducted exhaustive tests on Scott Kelly during his grueling 342-day tour aboard the International Space Station in 2015–2016. And they simultaneously monitored his identical twin brother Mark (a retired astronaut who was on Earth at the time), hoping the comparison would reveal new information about the effects of long-term spaceflight on the human body.

The results of the NASA Twins Study, reported Thursday in Science, show Scott Kelly underwent a number of physiological changes—most of which returned to pre-flight status after he returned to Earth. The findings could help inform preparations for future long-term missions, such as travel to Mars or stints on a moon-orbiting space station.