To date, 559 humans have been flown into space, but long-duration (>300 days) missions are rare (n = 8 total). Long-duration missions that will take humans to Mars and beyond are planned for the2020s and 2030s; therefore, comprehensive studies are needed. The selection of one of a pair of monozygotic (identical) twin astronauts for NASA’s first 1-year mission has enabled  to compare the impact of the spaceflight environment on one twin to the simultaneous impact of the Earth environment on a genetically matched subject.
The conclusion is as following: «Given that the majority of the biological and human health variables remained stable, or returned to baseline, after a 340-day space mission, these data suggest that human health can be mostly sustained over this duration of spacefligh»
Do not miss the whole article published very recently in Science
The NASA  Twins Study :A multidimensional analysis of a year long human spaceflight
Garret-Bakelman et al / Science 364 / April 2019