Though an attempt was made to keep diet, exercise, and as many other factors as possible equal between the two brothers, spaceflight definitely had its effects. Scott Kelly actually consumed less and lost about 7 percent of his body mass compared to the ground-based Mark, and the level of gene expression in Scott’s body increased (Note: His genetic code did not change, it was just which genes that were kicked on that altered—something that changes even among those not headed for space). It’s thought that this later may represent his immune system reacting to the novel environment, and perhaps even directly responding to the level of radiation in the ISS and the resulting increase in the amount of DNA damage. That wasn’t all. Scott’s gut bacteria balance changed. His vision altered for the fuzzier.
Perhaps the most unexpected effect was that the telomeres in Scott’s cells, areas at the ends of chromosomes which help to protect against errors in DNA duplication and which generally shorten with age, actually got longer. Then, within days of landing, his telomeres returned to previous length. Something similar had been seen in experiments on lab animals, but the cause is still not understood.
When it comes to gene expression, this is something that changes normally over time even for the earth-bound. However, Scott Kelly demonstrated a higher level of changes while in space. Again, most of these changes seem to rapidly revert once he was back on the ground, but about nine percent of the changes remained — possibly as a result of damage caused by radiation exposure.
Another somewhat unexpected result was a kind of post-spaceflight fugue. While Scott’s mental alertness and other tests of brain functions remained at high levels while he was in flight, there was a pronounced decrease in both speed and accuracy of his responses for up to six months after landing. This may have been due to changes in blood flow to the brain as he readjusted to gravity—but it may have also had something to do with the amount of attention the high-flying brother had to deal with on his return. Also, Scott Kelly reported feeling flu-like symptoms and fatigue over this period.
Bone loss, which had been a serious concern based on previous flights, definitely did occur … but it didn’t appear to be as bad as some expected. Bone loss was greatest in the first six months that Scott was in space, but slowed during the remainder of his flight and density seemed to be restored in the months after landing.
There were some areas of definite concern, and not just the persistent change in gene expression. Researchers found evidence that of inflammation and thickening of the carotid artery in Scott when compared to Mark, and this was one change that didn’t seem to reverse over the period of the study.
But for those who were concerned that a year in space might leave astronauts too weakened to carry out tasks at the other end … that is not what the study showed. At the end of the study, twins Mark and Scott Kelly didn’t show much more difference than would have been experienced by two twins who had both been living their normal lives over that period—maybe even less than might have been expected when the two were getting different food, different exercise, and different conditions. For the most part, the changes that Scott Kelly demonstrated in flight, were reversed on his return.
Now NASA needs to sponsor twin study part two—and see just how stressful a Senate campaign is on brother Mark.