Astronaut Scott Kelly and his identical twin Mark participated in NASA's twin study to assess what effect space travel had on the body. Indeed, going to space increased inflammation, created shifts in nutrient levels and was stressful due to oxygen deprivation. All of that acted to change the expression of many different genes. Telomeres, the caps on the ends of chromosomes, also lengthened during Scott's time in space.
Now that Scott has been back on Earth for two years, the study has found that there was no "fundamental change" to his DNA, according to NASA. However, seven percent of the genes that were altered during spaceflight have not returned to their typical expression levels, although the remaining 93 percent did. That change is comparable to the effects of some other strenuous activities that people might undertake, said NASA, like diving or mountain climbing.