Microgravity has various negative impacts on health. While bone and muscle loss are the most well known and dangerous, they have recently been resolved a combination of medicine, diet, and exercise.
One remaining issue is the fluid shift from the spinal column to the eyes and optic nerves over time. Roughly two out of three astronauts have negative impacts on vision that linger even after returning to Earth.
What this means for Space Exploration
The impact of this does not seem to be treatable with diet or exercise. As noted, roughly a third of astronauts do not have this impact. We may have to be selective with early Mars missions to find astronauts whose anatomy minimizes the effect. The only way to test for this may be an extended trip to a space station. Alternatively, it may be possible to use an MRI of the nervous system to determine the capacity of the individual to deal with microgravity effects on the optic nerves.
Another option would be to spin spacecraft for Mars missions to create artificial gravity. That begs the question then, at what gravity level does the impact go away?
Work on spinning space stations should be a higher priority in the near future. I will be announcing some near-term designs to help isolate and qualify solutions for this issue. These are parts of my talk at the 2017 Mars Society Conference in Irvine, California.