An American astronaut experienced multiple biological changes in space but returned to normal – with some exceptions – after coming back to Earth, according to a study involving twin brothers that shed light on how space flight affects the human body.

In research published on Thursday, scientists compared Scott Kelly to identical twin Mark Kelly after Scott spent 340 days in orbit on the International Space Station and Mark remained Earth-bound.

Both are now retired as NASA astronauts, and Mark is running for a US Senate seat in Arizona.

Scott Kelly during the flight experienced thickening of the carotid artery and retina, weight loss, shifts in gut microbes, reductions in cognitive abilities, DNA damage, changes in gene expression, and a lengthening of the ends of chromosomes called telomeres, scientists said.

After returning home, the telomere elongation was replaced by accelerated shortening and loss, a potentially negative consequence for cellular health, they said.

“The return was much worse than the adaptation of getting up there, especially for the year flight,” Kelly told reporters on Thursday. “I felt like I had the flu after the first few days. I was tired for really long time.”

The study will help scientists better understand the changes astronauts undergo during long-term space travel, crucial knowledge as NASA contemplates human expeditions to the Moon and Mars.