To the list of health-related problems caused by exposure to microgravity, including bone loss and the effects of radiation, we must now add deterioration of visual acuity. The problem may be related to increased cerebrospinal fluid pressure, an issue not easily resolved by stationary biking and resistance training. How serious the vision loss might be, as well as whether it stabilizes over time, remains to be determined.
Artificial gravity (e.g., attained by rotating the habitation portion of the interplanetary vehicle) might help address this, but the current plans for the Mars One transit vehicle do not involve such an arrangement. Then again, that could change. I guess we’ll see.