If I recall correctly, one of the hardest problems about getting humans to Mars is how to shield them from cosmic radiation. Usually stuff that shields radiation, like lead, is very heavy and very heavy stuff doesn't go very well with space travel.
The key conclusion of the study:
> They found the Apollo astronauts’ mortality rate from cardiovascular disease was as much as five times higher than for astronauts who never flew, or for those who flew low-altitude missions aboard the space shuttle that orbited a few hundred miles above Earth.
[0] https://www.rt.com/usa/353865-apollo-study-heart-disease/
[1] http://www.reuters.com/article/us-space-health-astronauts-id...