Bone loss in weightlessness
No matter how much astronauts exercize, they keep losing bone mass especially at the hip.
A simple movement like walking on Earth is a great exercize and that cannot be recreated as systematically in space.
A solution for Mars: a rotating structure....Never tested (since Gemini 11); It will have to be very fast in order to recreate 1G...Lower than 1G? Will that help demineralization? Besides, Mars is an unavoidable 38% of a G.