First of all we may think that changing from one gravity to another is harder than you think, in Mars there is approximately one-third of the earth’s gravity, coming back from that gravity can affect your spatial orientation, head-eye and hand-eye coordination, balance, locomotion, and you’re likely to experience motion sickness.
NASA has learned that without gravity working on your body, your bones lose minerals, with density dropping at over 1% per month. By comparison, the rate of bone loss for elderly men and women on Earth is from 1% to 1.5% per year. Even after returning to Earth, your bone loss might not be corrected by rehabilitation, so you could be at greater risk of osteoporosis-related fractures later in life. If you don’t exercise and eat properly, you will lose muscle strength, endurance, and experience cardiovascular deconditioning since it does not take effort to float through space. The fluids in your body will shift upwards to your head, which could put pressure on your eyes causing vision problems.