WASHINGTON (TIP): Space travel may cause changes in gene expression and other biological markers in astronauts, a NASA study of twins has found.

Scientists studied the genetic differences between astronaut Scott Kelly, who spent nearly a year in space, and his identical twin Mark.

Measurements taken before, during and after Scott Kelly’s mission show changes in gene expression, DNA methylation and other biological markers that are likely to be attributable to his time in orbit.

From the lengths of the twins’ chromosomes to the microbiomes in their guts, “almost everyone is reporting that we see differences,” said Christopher Mason, a geneticist at Weill Cornell Medicine in New York City.

The challenge now is to untangle how many of the observed changes are specific to the physical demands of spaceflight – and how many might be simply due to natural variations.

Since the Kelly twins are just two people, the results may not be generalisable to others, researchers said.

Still, the work is some of the most detailed molecular profiling ever done, involving some of the most physically demanding environments.

Scott Kelly spent 340 days in space in 2015-16, giving him a lifetime total of 520 days.