https://www.bbc.com/future/article/20230927-what-a-long-term-mission-in-space-does-to-the-human-body**********
Nasa astronaut Frank Rubio has just returned from a record-breaking 371 days in space onboard the ISS, but the trip may have altered his muscles, brain and even the bacteria living in his gut.
Without the constant tug of gravity on our limbs, muscle and bone mass quickly begins to diminish in space. The most affected are those muscles that help to maintain our posture in our back, neck, calves and quadriceps – in microgravity they no longer have to work nearly as hard and begin to atrophy. After just two weeks muscle mass can fall by as much as 20% and on longer missions of three-to-six months it can fall by 30%.
On Earth, gravity helps to force the blood in our bodies downward while the heart pumps it up again. In space, however, this process becomes messed up (although the body does adapt somewhat), and blood can accumulate in the head more than it normally would. Some of this fluid can pool at the back of the eye and around the optic nerve, leading to oedema. This can lead to changes in vision such as decreased sharpness and structural changes in the eye itself. These changes can start to occur after just two weeks in space but as that time goes on, the risk increases. Some of the vision changes reverse within about a year of astronauts returning to Earth, but others can be permanent.
researchers did notice that the speed and accuracy of Kelly's cognitive performance did decrease for around six months after he landed, possibly as his brain readjusted to the Earth's gravity and his very different lifestyle back home.
A study on a Russian cosmonaut who spent 169 days on the ISS in 2014 also revealed some changes to the brain itself seem to occur while in orbit. It found there were changes in the levels of neural connectivity in parts of the brain relating to motor function
It is apparent from research in recent years that a significant key to good health is the make up and diversity of the microorgansims that live in and on our bodies. This microbiota can influence how we digest food, affect the levels of inflammation in our bodies and even alter the way our brains work.
Also unexpected was that telomere length shortened rapidly on return to Earth for all crewmembers. Of particular relevance to long-term health and ageing trajectories, astronauts in general had many more short telomeres after spaceflight than they did before."
There were also some changes in gene expression – the mechanism that reads the DNA to produce proteins in cells
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