https://phys.org/news/2018-05-scientists-key-fake-news.html

Over the course of the one-year mission, NASA extensively examined the twins' physiology, gut bacteria and even their genetic code – sure enough, NASA saw the toll of space stress on Scott.

"Physical stress can certainly bring about molecular changes," said Western Biology professor David Smith, a genetic researcher. "If I sat on the couch and ate chips, while my twin brother ran the Boston Marathon, you would certainly see differences in our molecular makeup. In the case of Scott and Mark, what NASA found was significant, but not a paradigm shift."