NASA scientists in an unprecedented study have found that space travel may alter gene expression.
The study involved astronaut Scott Kelly, who spent a year in space and his identical twin Mark who stayed on Earth.
From the lengths of the twins' chromosomes to the microbiomes in their guts, "almost everyone is reporting that we see differences", Christopher Mason, a geneticist at Weill Cornell Medical College in New York City, was quoted as saying to scientificamerican.com.
The changes that are likely attributable to Scott's time in orbit include alterations to gene expression, DNA methylation - the reversible addition of a chemical marker that can affect gene expression - and other biological markers.
DNA methylation decreased in Scott during flight and increased in Mark over the same period.
Levels for both men returned close to pre-flight levels after Scott came back to Earth, according to the preliminary results published in the journal Nature.
"What this means isn't yet clear," said Andrew Feinberg, a geneticist at Johns Hopkins University in Maryland, US.
The researchers also reported changes in gene-expression signatures between the twins. Such changes happen in earthbound people all the time, associated with environmental shifts such as changes in diet and sleep habits.
However, the changes in Scott seemed to be larger than normal - perhaps due to the stress of eating frozen food and trying to sleep while floating in space, Mason said.
But, because the Kelly twins are just two people, the results may not be generalised, the study said.
Scott spent 340 days in space during 2015-2016, giving him a lifetime total of 520 days, while Mark, also an astronaut, had previously flown in space for a total of 54 days over four space-shuttle missions between 2001 and 2011.