They naturally took the opportunity to study the effects of one in space for a year versus the other:

It made me smarter and more handsome than my brother Mark [laughs]. With the twin study, particularly, there was a bunch of experiments that were genetic-based. And there are these things called telomeres, which are the ends of our chromosomes, and the length and quality of them are indications of our physical age. The hypothesis was that with me being in space, and the radiation, and the microgravity, maybe the stress of living there for a long period of time, my telomeres would get shorter and basically older compared to Mark’s. What they found is that mine got better. So physically I got a little younger than he is, and then once I got back they kind of went back to their normal, preflight condition.

So, basically the guy didn't age while on the ISS. Amazing. Interesting clue:

So when the CO2 is at our normal low levels on the space station, it’s 10 times what it is on Earth. ...The Russians say that it should be high because it helps protect you from radiation

Interesting, reducing oxidative stress may give one time to recover.