What we learned from a year in space

Some interesting insights into human space flight have arisen out of the ongoing study of Scott Kelly, the American astronaut who has logged the longest continuous stay in space, spending 340 days on the International Space Station in 2015 and 2016 (Russian astronaut Valeri Polyakov holds the world record of 438 days).
Kelly, now 53 years old, was specifically being monitored for data on the physiological effects of space travel, and he will continue to provide data even since his return to earth and his retirement from NASA last year. Much of the data involves comparisons with his earthbound identical twin, Mark (actually, Mark too made a space trip in 2011, but only of a modest 54 days). Mr. Kelly has recently published a book, Endurance: A Year In Space, A Lifetime Of Discovery, detailing some of the findings, or at least those that NASA are willing to release.
One of the most important factors in the study is his reaction to the radiation he experienced in space (more than 30 times the exposure we receive on earth, although apparently this still represents less of a cancer risk than a terrestrial smoker exposes himself to). Scott remains cancer-free for now, and is remarkably sanguine about the risks for the future.
Loss of bone density is another big one. It is unavoidable in the zero gravity of space, although interestingly the loss appears to level off, and his bone density loss is no more after 340 days than it was after less than half that time. Kelly muses that future generations that live permanently in space may evolve without a skeleton, and live perfectly serviceable lives as invertebrates.
One interesting, and perhaps unexpected, corollary of life in space is the toll it takes on vision. The reasons are not entirely clear, but time in space appears to cause permanent folds in the choroid (the blood-filled layer between the retina and the white of the eye), and Kelly has had to increase his eyeglass prescription several times since returning from space. Interestingly, women astonauts do not seem to be affected in the same way, and Kelly envisions a female-only flight to Mars unless more progress can be made on this problem.
Breathing much higher levels of carbon dioxide while living in the confines of the Space Station is another big impediment to space travel - it causes headaches, congestion, burning eyes, irritability, and trouble thinking straight (the latter in particular has distinct and obvious drawbacks in a space flight context). It remains to be seen what long-term effects this might have, but Kelly is quick to point out that NASA could have allowed much cleaner air using additional CO2 scrubbers available on the ISS, but chose not to. NASA has since agreed to improve CO2 conditions by up to a third for the future.
In fact, Mr. Kelly has some pretty harsh words to direct at NASA, particularly regarding its secrecy and its authoritarian attitudes. I guess when your life is effectively in their hands from day to day for nearly a year, it leads to a pretty fraught relationship.